Cellular immune responses to platelet factor 4 and heparin complexes in patients with heparin-induced thrombocytopenia

Essentials The immunogenesis of Heparin-induced thrombocytopenia (HIT) is not well understood. Immunization to platelet factor 4 (PF4)-heparin occurs early in life, before any heparin exposure. PF4 and PF4-heparin complexes induce the proliferation of CD14+ cells. Reduced levels of regulatory cytokines contribute to immune dysregulation in HIT.

SUMMARY

Background Heparin-induced thrombocytopenia (HIT) is an adverse reaction to heparin characterized by thrombocytopenia and thrombotic complications. HIT is caused by pathogenic antibodies that bind to complexes of platelet factor 4 (PF4) and heparin, leading to platelet activation and inducing a hypercoagulable state. Previous studies have shown immunity to PF4-heparin complexes occurs early in life, even before heparin exposure; however, the immunogenesis of HIT is not well characterized. Objectives To investigate cellular proliferation in response to PF4-heparin complexes in patients with HIT. Patients/Methods Peripheral blood mononuclear cells (PBMCs) from healthy controls (n = 30), postoperative cardiac surgery patients who had undergone cardiopulmonary bypass (CPB) (n = 17) and patients with confirmed HIT (n = 41) were cultured with PF4 and PF4-heparin complexes. Cellular proliferation was assessed by [3 H]thymidine uptake and 5-ethynyl-2'-deoxyuridine detection. Results and Conclusions PBMCs proliferated in the presence of PF4, and this was enhanced by the addition of heparin in all study groups. CPB and HIT patients showed significantly greater proliferative responses than healthy controls. PBMC proliferation was antigen-specific, depended on the presence of platelets, and only CD14+ cells were identified as proliferating cells. Culture supernatants were tested for the levels of regulatory cytokines, and both CPB and HIT patients produced significantly lower levels of interleukin-10 and transforming growth factor-β1 than healthy controls. These findings further demonstrate cellular immune sensitization to PF4-heparin complexes occurs before heparin exposure, and suggests immune dysregulation can contribute to HIT.

Distinguishing between anti-platelet factor 4/heparin antibodies that can and cannot cause heparin-induced thrombocytopenia

BACKGROUND

Many patients exposed to heparin develop antibodies against platelet factor 4 (PF4) and heparin, yet only those antibodies that activate platelets cause heparin-induced thrombocytopenia (HIT). Patients who produce anti-PF4/heparin antibodies without developing HIT either have antibodies that do not cause platelet activation or produce pathogenic antibodies at levels that are insufficient to cause HIT. Understanding the differences between anti-PF4/heparin antibodies with and without HIT will improve test methods and reduce overdiagnosis.

AIMS

To investigate the presence of low levels of platelet-activating antibodies in patients investigated for HIT who had anti-PF4/heparin antibodies but failed to cause platelet activation in the (14) C-serotonin release assay (SRA).

MATERIALS/METHODS

We developed a platelet activation assay similar to the SRA using exogenous PF4 without added heparin (PF4-SRA). This assay was able to detect low levels of platelet-activating antibodies. We used this PF4-SRA to test for platelet-activating antibodies in patients investigated for HIT.

RESULTS

The PF4-SRA detected platelet-activating antibodies in seven (100%) of seven SRA-positive sera even after the samples were diluted until they were no longer positive in the standard SRA. Platelet-activating antibodies were detected in 14 (36%) of 39 patients who had anti-PF4/heparin antibodies but tested negative in the SRA and did not have clinical HIT. The clinical diagnosis of HIT was confirmed by chart review and concordant with the SRA results.

CONCLUSIONS

A subset of heparin-treated patients produce subthreshold levels of platelet-activating anti-PF4/heparin antibodies that do not cause HIT. An increase in the titer of these pathogenic antibodies, along with permissive clinical conditions, could lead to HIT.

Severe bleeding events in adults and children with primary immune thrombocytopenia: a systematic review

BACKGROUND

The burden of severe bleeding in adults and children with immune thrombocytopenia (ITP) has not been established.

OBJECTIVES

To describe the frequency and severity of bleeding events in patients with ITP, and the methods used to measure bleeding in ITP studies.

PATIENTS/METHODS

We performed a systematic review of all prospective ITP studies that enrolled 20 or more patients. Two reviewers searched Medline, Embase, CINAHL and the Cochrane registry up to May 2014. Overall weighted proportions were estimated using a random effects model. Measurement properties of bleeding assessment tools were evaluated.

RESULTS

We identified 118 studies that reported bleeding (n = 10 908 patients). Weighted proportions for intracerebral hemorrhage (ICH) were 1.4% for adults (95% confidence interval [CI], 0.9-2.1%) and 0.4% for children (95% CI, 0.2-0.7%; P < 0.01), most of whom had chronic ITP. The weighted proportion for severe (non-ICH) bleeding was 9.6% for adults (95% CI, 4.1-17.1%) and 20.2% for children (95% CI, 10.0-32.9%; P < 0.01) with newly-diagnosed or chronic ITP. Methods of reporting and definitions of severe bleeding were highly variable in primary studies. Two bleeding assessment tools (Buchanan 2002 for children; Page 2007 for adults) demonstrated adequate inter-rater reliability and validity in independent assessments.

CONCLUSIONS

ICH was more common in adults and tended to occur during chronic ITP; other severe bleeds were more common in children and occurred at all stages of disease. Reporting of non-ICH bleeding was variable across studies. Further attention to ITP-specific bleeding measurement in clinical trials is needed to improve standardization of this important outcome for patients.

FcγRIIa proteolysis as a diagnostic biomarker for heparin-induced thrombocytopenia

BACKGROUND

A significant challenge in the management of heparin-induced thrombocytopenia (HIT) patients is making a timely and accurate diagnosis. The readily available enzyme immunoassays (EIAs) have low specificities. In contrast, platelet activation assays have higher specificities, but they are technically demanding and not widely available. In addition, ~ 10% of samples referred for HIT testing are initially classified as indeterminate by the serotonin release assay (SRA), which further delays accurate diagnosis. HIT is characterized by platelet activation, which leads to FcγRIIa proteolysis. This raises the possibility that identification of the proteolytic fragment of FcγRIIa could serve as a surrogate marker for HIT.

OBJECTIVES

To determine the specificity of platelet FcγRIIa proteolysis induced by sera from patients with HIT, and to correlate the results with those of the SRA.

METHODS/PATIENTS

Sera from HIT patients and control patients with other thrombocytopenic/prothrombotic disorders were tested for their ability to proteolyse FcγRIIa. The results were correlated with anti-platelet factor 4 (PF4)/heparin antibodies (EIA), and heparin-dependent platelet activation (SRA).

RESULTS

Only HIT patient samples (20/20) caused heparin-dependent FcγRIIa proteolysis, similar to what was shown by the SRA. None of the samples from the other patient groups or hospital controls caused FcγRIIa proteolysis. Among nine additional samples that tested indeterminate in the SRA, FcγRIIa proteolysis resolved five samples that had a positive anti-PF4/heparin EIA result; three had no FcγRIIa proteolysis, and two were shown to have heparin-dependent FcγRIIa proteolysis

CONCLUSIONS

This study suggests that heparin-dependent FcγRIIa proteolysis is at least as specific as the SRA for the diagnosis of HIT.

A systematic evaluation of laboratory testing for drug-induced immune thrombocytopenia

BACKGROUND

Drug-induced immune thrombocytopenia (DITP) can be confirmed by the demonstration of drug-dependent platelet antibodies in vitro; however, laboratory testing is not readily accessible and test methods are not standardized.

OBJECTIVE

To identify drugs with the strongest evidence for causing DITP based on clinical and laboratory criteria.

PATIENTS/METHODS

We developed a grading system to evaluate the quality of DITP laboratory testing. The 'DITP criteria' were: (i) Drug (or metabolite) was required for the reaction in vitro; (ii) Immunoglobulin binding was demonstrated; (iii) Two or more laboratories obtained positive results; and (iv) Platelets were the target of immunoglobulin binding. Laboratory diagnosis of DITP was considered definite when all criteria were met and probable when positive results were reported by only one laboratory. Two authors applied the DITP criteria to published reports of each drug identified by systematic review. Discrepancies were independently adjudicated.

RESULTS

Of 153 drugs that were clinically implicated in thrombocytopenic reactions, 72 (47%) were associated with positive laboratory testing. Of those, 16 drugs met criteria for a definite laboratory diagnosis of DITP and thus had the highest probability of causing DITP. Definite drugs were: quinine, quinidine, trimethoprim/sulfamethoxazole, vancomycin, penicillin, rifampin, carbamazepine, ceftriaxone, ibuprofen, mirtazapine, oxaliplatin and suramin; the glycoprotein IIbIIIa inhibitors abciximab, tirofiban and eptifibatide; and heparin.

CONCLUSIONS

We identified drugs with the strongest evidence for an association with immune thrombocytopenia. This list may be helpful for ranking potential causes of thrombocytopenia in a given patient.

Quantitative interpretation of optical density measurements using PF4-dependent enzyme-immunoassays

BACKGROUND

Many laboratories test for heparin-induced thrombocytopenia (HIT) using a PF4-dependent enzyme-immunoassay (EIA). An advantage of the EIA is its simplicity; a disadvantage is that it only indirectly detects heparin-dependent, platelet-activating antibodies ('HIT antibodies').

OBJECTIVES

To determine whether the magnitude of a positive EIA result, expressed in optical density (OD) units, predicts risk of HIT antibodies, defined as a strong-positive platelet serotonin-release assay (SRA) result (>or=50% serotonin release).

PATIENTS/METHODS

We determined the risk of a strong-positive SRA result for five categories of OD reactivity (<0.40, 0.40-<1.00, 1.00-<1.40, 1.40-<2.00, and >or=2.00 OD units) using two EIAs (commercial anti-PF4/polyanion IgG/A/M and in-house anti-PF4/heparin-IgG).

RESULTS

For patient sera investigated for HIT antibodies, a weak-positive result (0.40-<1.00 OD units) in either EIA indicated a low probability (or= 2.00 units. Quantifying the EIA-SRA relationship for 1553 referred patient sera, we found that for every increase of 0.50 OD units in the EIA-IgG, the risk of a strong-positive SRA result increased by OR = 6.39 [95% confidence interval (CI), 5.13, 7.95; P < 0.0001]. For every increase of 1.00 OD units in the EIA-IgG, the risk increased by OR = 40.81 (95% CI, 26.35, 63.20; P < 0.0001).

CONCLUSIONS

The probability of HIT antibodies (strong-positive SRA result) inferred by a positive PF4-dependent EIA varies considerably in relation to the magnitude of the EIA result, expressed as OD values. In our laboratory, the probability of HIT antibodies being present reached >or=50% only when the OD level was >or=1.40 units.

The interaction of ancrod with human platelets

Ancrod, a serine protease purified from the venom of Agkistrodon rhodostoma, has been used as a therapeutic anticoagulant for a number of indications, including replacement of heparin in patients with heparin-induced thrombocytopenia. Ancrod has similar fibrinolytic activity to thrombin, but ancrod specifically cleaves only the alpha chain of fibrinogen, producing the characteristic fibrinopeptides A, AP and AY. Because ancrod has been used in patients with heparin-induced thrombocytopenia, it is important to ensure that ancrod does not directly affect the platelets and potentially increase the hemostatic effect. The effect of ancrod on platelets has not been well established, and there is not agreement in published studies. Additionally, some of the studies are over 15 years old and pre-date sensitive assays such as glycoprotein analysis. For these reasons, we investigated the interaction of ancrod with human platelets using direct and indirect, functional and biochemical techniques. Incubation of platelets with ancrod alone did not induce platelet aggregation or the release of dense-granule contents. Pre-incubation of platelets with ancrod did not augment or inhibit the maximal aggregation achieved with thrombin, nor did it affect the amount of serotonin release from dense granules caused by activation by thrombin. Studies of ancrod-treated platelets using monoclonal antibody-mediated radioimmunoprecipitation demonstrated that high concentrations of ancrod did not cause measurable cleavage of either the glycoproteins Ib-IX or IIb-IIIa. Incubation of radiolabeled platelets with ancrod-treated plasma also had no effect on the platelet glycoproteins, indicating that ancrod does not indirectly affect the major surface receptors. Direct binding studies using radiolabeled ancrod did not demonstrate specific binding to the platelet surface. Together these studies indicate that ancrod does not directly affect nor bind to platelets in vitro. The hypo-coagulant state and subsequent platelet function defect resulting from the use of ancrod appears to be limited to the removal of fibrinogen from the circulation.

Intravenous immunoglobulin as an adjunct to plasma exchange for the treatment of chronic thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease. Therapeutic plasma exchange (TPE) is the most effective therapy; however, despite TPE, about one-third of TTP patients will relapse. A subset of patients with TTP has antibodies to ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) and may become resistant to conventional treatments. We describe a patient with TTP and high-titre anti-ADAMTS13 antibodies who developed a chronic, relapsing course of TTP despite frequent TPE. Once adjuvant treatment with intravenous immunoglobulin (IVIG) was added, remission was achieved. Even during remission, anti-ADAMTS13 antibodies remained elevated. We conclude that IVIG may sustain remission in some patients with chronic, relapsing TTP.

In vitro evaluation of prestorage pooled leukoreduced whole blood-derived platelets stored for up to 7 days

BACKGROUND

Advantages to storing whole blood-derived platelets (PLTs) as a pool for 7 days would include operational efficiencies and facilitation of bacterial testing and pathogen inactivation. The in vitro quality of pre-storage pooled PLTs stored for up to 7 days was assessed.

STUDY DESIGN AND METHODS

Leukoreduced PLTs were pooled before storage (5 units/pool) and stored for either 5 or 7 days. Samples were collected at the time of pooling and either on Day 5 (n=16-29) or on Day 7 (n=4-30) and tested for biochemical and activation markers and morphology and/or shape change. Control PLTs were stored individually for 5 or 7 days and then tested as indicated above.

RESULTS

The mean PLT counts (x10(9)/L) were similar: control PLTs, 1344 (464 SD); and prestorage pooled PLTs, 1327 (220 SD; p=0.93). On Day 5, the pH value was significantly lower (p<or=0.0001) for the prestorage pooled PLTs. Lactate and partial pressure of O2 (pO2) were significantly higher (p<or=0.003). On Day 7 of storage, significant differences were noted with pH, pCO2, and hypotonic shock all being lower with the prestorage pooled product, whereas pO2, lactate, and morphology scores were higher (p values all <0.03). Although significant differences were found, the magnitude of the difference would not be expected to impact on transfusion outcomes.

CONCLUSION

These results suggest that prestorage pooled PLTs can be stored for up to 7 days; however, studies are needed to ensure that the clinical benefit of PLTs stored as a pool is not inferior to those stored individually.

Assessing the effectiveness of whole blood-derived platelets stored as a pool: a randomized block noninferiority trial

BACKGROUND

Prestorage pooling of whole blood-derived platelets (PLTs) would simplify bacterial detection. This study evaluated the in vivo effect of the prestorage pooling of PLTs stored for up to 5 days, by assessing the corrected count increment (CCI) 18 to 24 hours after transfusion of the product.

STUDY DESIGN AND METHODS

A randomized block noninferiority design was used. Eligible patients had chemotherapy-induced thrombocytopenia and were considered likely to need at least six PLT transfusions. For every block of two transfusion events, one consisted of PLTs stored individually and then pooled before transfusion, and the other was a product pooled before storage. The primary outcome was categorized as a successful (>4.5) or unsuccessful (<or=4.5) 18- to 24-hour posttransfusion CCI analyzed with a matched pair score test. A mixed-effect linear model estimated the mean difference in CCI between the two types of storage.

RESULTS

Twenty-three eligible patients received a total of 189 PLT transfusions. The median number of PLT transfusions was 7 (range, 0-27). Eighty-five complete transfusion pairs were used in the primary analysis. The proportions of transfusions leading to a CCI of greater than 4.5 was identical for both routine and PLTs pooled before storage (45/85=52.9%; relative risk, 1.00; lower limit of the one-sided 95% confidence interval [CI], 0.83). The estimate of the mean difference in CCI between pooled and routine storage (pooled-routine) was -0.45 (95% CI, -2.23 to 1.33; p=0.63).

CONCLUSION

These results provide evidence that storage of PLTs as a pool for up to 5 days results in posttransfusion CCIs that are not inferior to PLTs stored individually.

Heparin-induced thrombocytopenia: an overview

Heparin-induced thrombocytopenia (HIT) is the most important immunological drug reaction that patients face today. HIT typically develops in patients 5 days after starting heparin therapy, but can occur sooner with recent heparin exposure or rarely have a delayed onset. The platelet count typically drops below 150 x 10(9)/L (average 60 x 10(9)/L), and patients may experience a thrombotic episode simultaneously or shortly after the onset of thrombocytopenia. The thrombocytopenia and the associated thrombotic episodes are now considered to be overlapping outcomes of the same syndrome. The pathophysiology of HIT has been characterized: immune complexes of IgG and heparin in association with a small platelet peptide, platelet factor 4 (PF4), activate platelets by binding to the Fc receptors (FcR) and releasing procoagulant-active, platelet-derived microparticles. The recognition that HIT is characterized by intense thrombin generation dictates the use of antithrombin agents in HIT therapy. Therapeutic approaches that are currently prevalent in the management of HIT will be discussed.

Delayed-onset heparin-induced thrombocytopenia and thrombosis

BACKGROUND

Heparin-induced thrombocytopenia is a prothrombotic drug reaction caused by platelet-activating antibodies that recognize complexes of platelet factor 4 and heparin.

OBJECTIVE

To describe a syndrome termed delayed-onset heparin-induced thrombocytopenia, in which thrombocytopenia and thrombotic events begin 5 or more days after withdrawal of heparin.

DESIGN

Case series.

SETTING

Secondary and tertiary care hospitals.

PATIENTS

12 patients who presented with serologically confirmed, delayed-onset heparin-induced thrombocytopenia, including 6 outpatients presenting after hospital discharge.

MEASUREMENTS

The platelet serotonin-release assay was used to measure IgG-induced heparin-dependent and heparin-independent platelet activation; an enzyme immunoassay that detects IgG against platelet factor 4-heparin complexes was also used.

RESULTS

Patients with delayed-onset heparin-induced thrombocytopenia presented with thrombocytopenia and associated thrombosis a mean of 9.2 days (range, 5 to 19 days) after stopping heparin therapy. Nine patients received additional heparin, with further decrease in platelet counts. Compared with controls, patients with delayed-onset heparin-induced thrombocytopenia had higher titers of IgG antibodies to platelet factor 4-heparin and greater IgG-induced heparin-dependent and heparin-independent platelet activation.

CONCLUSIONS

Delayed-onset heparin-induced thrombocytopenia should be suspected when patients present with thrombocytopenia and thrombosis up to 3 weeks after exposure to heparin. This syndrome could be caused by high titers of platelet-activating IgG induced by heparin.

Decreased von Willebrand factor protease activity associated with thrombocytopenic disorders

Recent studies investigating thrombotic thrombocytopenic purpura (TTP) have implicated abnormal plasma von Willebrand factor (vWF)-cleaving metalloprotease activity in this disorder. It has been proposed that a metalloprotease cleaves unusually large (UL) multimers of vWF, which enter the circulation from the endothelium. Abnormal metalloprotease activity could result in ULvWF, which could participate in TTP. However, the diagnostic specificity of abnormalities in the plasma metalloprotease activity has not been established. A prospective study of vWF protease activity was performed using samples from 20 healthy controls, 20 patients with acute TTP, 20 patients with immune idiopathic thrombocytopenic purpura (ITP), 10 patients with disseminated intravascular thrombocytopenia (DIC), 10 patients with systemic lupus erythematosus (SLE,) and 5 thrombocytopenic patients with leukemia. Studies were performed blinded to the diagnosis. Samples from hospitalized patients with normal platelet counts were also tested. The vWF digests and multimer analysis were done using previously described methods. Six laboratory personnel independently scored each of the multimer gels. Reduced protease activity was observed in 9 of 20 patients with TTP. Reduced activity was also observed in 6 of 20 patients with ITP, 6 of 10 patients with DIC, 5 of 10 patients with SLE, 1 of 5 patients with leukemia, 2 of 20 healthy controls, and 3 of 25 hospitalized patients. This study indicates that abnormalities of vWF protease activity are not restricted to patients with the diagnosis of TTP.

Temporal aspects of heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia is a relatively common antibody-mediated drug reaction. We studied the temporal relation between previous or current heparin therapy and the onset of heparin-induced thrombocytopenia.

METHODS

We examined the time between the start of heparin therapy and the onset of thrombocytopenia in 243 patients with serologically confirmed heparin-induced thrombocytopenia. We also investigated the persistence of circulating heparin-dependent antibodies by performing a platelet serotonin-release assay and an assay for antibodies against platelet factor 4. The outcome in seven patients who had previously had an episode of heparin-induced thrombocytopenia and were later treated again with heparin was also examined.

RESULTS

A fall in the platelet count beginning four or more days after the start of heparin therapy occurred in 170 of the 243 patients (70 percent); in these patients, a history of previous heparin treatment did not influence the timing of the onset of thrombocytopenia. In the remaining 73 patients (30 percent), the onset of thrombocytopenia was rapid (median time of onset, 10.5 hours after the start of heparin administration); all these patients had been treated with heparin within the previous 100 days. During recovery from thrombocytopenia, heparin-dependent antibodies in the serum fell to undetectable levels at a median of 50 to 85 days, depending on the assay performed. In the seven patients who were given heparin again after the disappearance of heparin-dependent antibodies, a new episode of heparin-induced thrombocytopenia did not occur.

CONCLUSIONS

Heparin-induced thrombocytopenia can begin rapidly in patients who have received heparin within the previous 100 days. Heparin-dependent antibodies do not invariably reappear with subsequent heparin use.

Argatroban anticoagulant therapy in patients with heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an immune-mediated syndrome caused by heparin. Complications range from thrombocytopenia to thrombocytopenia with thrombosis. We report a prospective, historical- controlled study evaluating the efficacy and safety of argatroban, a direct thrombin inhibitor, as anticoagulant therapy in patients with HIT or HIT with thrombosis syndrome (HITTS).

METHODS AND RESULTS

Patients with HIT (isolated thrombocytopenia, n=160) or HITTS (n=144) received 2 microgram. kg(-1). min(-1) IV argatroban, adjusted to maintain the activated partial thromboplastin time 1.5 to 3.0 times baseline value. Treatment was maintained for 6 days, on average. Clinical outcomes over 37 days were compared with those of 193 historical control subjects with HIT (n=147) or HITTS (n=46). The incidence of the primary efficacy end point, a composite of all-cause death, all-cause amputation, or new thrombosis, was reduced significantly in argatroban-treated patients versus control subjects with HIT (25.6% versus 38.8%, P=0.014). In HITTS, the composite incidence in argatroban-treated patients was 43.8% versus 56.5% in control subjects (P=0.13). Significant between-group differences by time-to-event analysis of the composite end point favored argatroban treatment in HIT (P=0.010) and HITTS (P=0.014). Argatroban therapy, relative to control subjects, also significantly reduced new thrombosis and death caused by thrombosis (P<0.05). Argatroban-treated patients achieved therapeutic activated partial thromboplastin times generally within 4 to 5 hours of starting therapy and, compared with control subjects, had a significantly more rapid rise in platelet counts (P=0.0001). Bleeding events were similar between groups.

CONCLUSIONS

Argatroban anticoagulation, compared with historical control subjects, improves clinical outcomes in patients who have heparin-induced thrombocytopenia, without increasing bleeding risk.

Impact of the patient population on the risk for heparin-induced thrombocytopenia

The frequency of immune heparin-induced thrombocytopenia (HIT) varies among prospective studies. It is unknown whether this is caused by differences in the heparin preparations, the patient populations, or the types of serologic assay used to confirm the diagnosis. Seven hundred forty-four patients were studied from 3 different clinical treatment settings, as follows: unfractionated heparin (UFH) during or after cardiac surgery (n = 100), UFH after orthopedic surgery (n = 205), and low-molecular-weight heparin (LMWH) after orthopedic surgery (n = 439). Both an activation assay and an antigen assay were used to detect heparin-dependent IgG (HIT-IgG) antibodies. By activation assay, the frequency of HIT-IgG formation ranged from a low of 3.2% in orthopedic patients receiving LMWH to a high of 20% in cardiac patients receiving UFH; by antigen assay, the corresponding frequencies ranged from 7.5% to 50%. Both UFH use (P =.002) and cardiac surgery (P =.01) were more likely to be associated with HIT-IgG formation. However, among patients in whom HIT-IgG formed and who were administered UFH, the probability for HIT was higher among orthopedic patients than among cardiac patients (by activation assay: 52.6% compared with 5%; odds ratio, 21.1 [95% CI, 2.2-962.8]; P =.001; by antigen assay: 34.5% compared with 2.0%; odds ratio, 25.8 [95% CI, 3.2-1141]; P <.001). It is concluded that there is an unexpected dissociation between the frequency of HIT-IgG formation and the risk for HIT that is dependent on the patient population. HIT-IgG antibodies are more likely to form in patients who undergo cardiac surgery than in orthopedic patients, but among patients in whom antibodies do form, orthopedic patients are more likely to develop HIT. (Blood. 2000;96:1703-1708)

The use of electron microscopy in the investigation of the ultrastructural morphology of immune thrombocytopenic purpura platelets

Ultrastructural studies have proven useful for the accurate identification and classification of certain lymphoid and hematopoietic disorders; however, the value of electron microscopy in the diagnosis and clinical management of platelet pathology is less well defined. Electron microscopy has been used to evaluate inherited platelet disorders. In these disorders, certain platelet structural defects can be characteristic. Recently, we investigated the ultrastructural morphology and immunogold localization of IgG in platelets from patients with idiopathic thrombocytopenic purpura (ITP). The accelerated platelet destruction of ITP is mediated by antiplatelet autoantibodies directed against platelet surface glycoproteins and by the functional capacity of the reticuloendothelial system. The basic dysregulation that occurs in these patients remains unexplained. Traditionally, laboratory investigation of ITP has focused on the development of serologic assays to measure the autoantibodies. As an alternative investigative approach, determination of the immunomorphologic characteristics of platelet-associated IgG (PAIgG) in ITP platelets may prove useful in the diagnosis or clinical management of patients with ITP. Our results showed that the ultrastructural morphology of ITP platelets is similar to that observed for normal platelets. No structural abnormalities are observed in ITP platelets. Immunogold labeling of IgG within alpha-granules of ITP platelets is significantly higher than that of normal platelets. Additionally, in some ITP platelets, immunogold labeling is also observed on the platelet surface and within channels of the open-cannalicular system. In comparison, immunogold labeling of these structures in normal platelets, is rare, or absent. In conclusion, electron microscopic studies should contribute to furthering our understanding of this common autoimmune disorder, and may provide possible biological explanations for the increased levels of PAIgG in platelets from patients with ITP.

Management of idiopathic thrombocytopenic purpura in pregnancy

Idiopathic thrombocytopenic purpura (ITP) is a relatively common autoimmune disorder among women of child-bearing age. It has a frequency of approximately one to two per 1,000 live births, accounting for about 3% of all cases of maternal thrombocytopenia at delivery. ITP in pregnancy necessitates the management of two patients, the mother and her baby; hence, the close collaboration of a multidisciplinary group composed of a hematologist, obstetrician, and pediatrician is essential. Our understanding of thrombocytopenia in pregnancy has evolved considerably over the last decade, yet the optimal diagnostic and treatment strategies for ITP in pregnancy continues to create controversy. In reviewing the recent literature, there is resurgence in the trend towards treating these patients in a more conservative fashion. This review will summarize the current approach to the diagnosis of ITP in pregnancy, as well as explore the pertinent and controversial issues of investigation and management.

Morphological analysis of microparticle generation in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) with thrombosis is a serious complication of heparin use. HIT sera can generate platelet-derived microparticles, which are produced in a heparin-dependent manner and are hypothesized to be important initial pathological participants because they promote vascular occlusion. To date, microparticles have been studied using flow cytometric techniques. However, it is uncertain whether the small-sized material seen in flow cytometric studies represents true platelet microparticles shed from activated platelets or whether they are platelets that have contracted after releasing their internal components. This report describes a morphological investigation of platelet-derived microparticles in HIT using, among other techniques, confocal, scanning electron, and transmission electron microscopy. Following incubation with HIT sera, the existence of small membrane-bound vesicles in the milieu of activated platelets was demonstrated. A population of microparticles, expressing platelet-specific glycoproteins, was separated from platelets by centrifugation over a sucrose layer. These microparticles had identical flow cytometric profiles, size heterogeneity, and GPIb(alpha) and GPIIb/IIIa staining intensity as the microparticle population in unfractionated samples. When microparticles were generated in situ and fixed onto grids, they were demonstrated to be distinct membrane-bound vesicles that originated near the platelet body and terminal ends of pseudopods on activated platelets. These microparticles appeared to be generated by localized swelling, budding, and release. Collectively, these morphological studies document the existence of true microparticles in platelets activated by HIT sera. The microparticles may play an important role in the pathogenesis of HIT.

Investigation of a platelet factor 4 polymorphism on the immune response in patients with heparin-induced thrombocytopenia

A small fraction of patients who receive heparin develop heparin-induced thrombocytopenia (HIT) and, of these patients, a still smaller proportion develop associated thrombotic complications. Heparin-induced thrombocytopenia is caused by the formation of antibodies that bind to specific complexes of platelet factor 4 (PF4) and heparin. However, it remains uncertain why certain patients form these antibodies and develop HIT or why certain patients have thrombotic events. In this report we describe studies on individuals with and without HIT to determined if a potential PF4 polymorphism could explain differences in susceptibility to HIT. In the 10 control individuals and the 10 patients we studied, we did not find a difference in the PF4 sequences. Genetic difference in the PF4 antigenic target does not explain the occurrence of HIT in susceptible patients.

Measurement of endogenous and exogenous alpha-granular platelet proteins in patients with immune and nonimmune thrombocytopenia

Idiopathic thrombocytopenic purpura (ITP) is caused by antiplatelet antibodies and is characterized by increased platelet destruction and elevated levels of IgG (platelet-associated IgG, PAIgG). Nonimmune thrombocytopenic patients also have elevated levels of PAIgG. In this study we investigated two possible biological explanations for the increased levels of PAIgG in these patients. The first hypothesis suggests that a thrombocytopenic stress causes increased thrombocytopoiesis with increased numbers and content of the platelet alpha granules. The second hypothesis is that for uncertain reasons (immunological or cytokine) there is increased absorption of plasma proteins by either megakaryocytes or by the platelets themselves. To address this issue, we compared the level of megakaryocyte synthesized alpha granular proteins [platelet factor 4 (PF4) and beta-thromboglobulin (beta-TG)] to plasma-absorbed alpha granular proteins (albumin, IgG and fibrinogen) in patients with immune (n = 39) and nonimmune (n = 60) thrombocytopenias. Plasma-absorbed alpha-granular proteins were elevated in both immune and nonimmune thrombocytopenia with no increase in megakaryocyte synthesized alpha-granular proteins. These plasma-derived protein elevations were not attributable to elevated mean platelet volumes or elevated plasma concentrations of the respective protein. We hypothesize that the increased IgG in these platelets is not the result of production of larger platelets, but reflects a selective increase in the endocytosis of plasma-absorbed alpha-granular proteins at the megakaryocyte and/or platelet level.

Calpain activity in bone marrow transplant-associated thrombotic thrombocytopenic purpura

The pathophysiology of thrombotic thrombocytopenic purpura (TTP) is not well understood. Recent studies have described a platelet aggregating factor which has been characterized as a calcium-dependent cysteine protease (calpain) in patients with TTP. A type of TTP, sometimes called secondary TTP, has been associated with bone marrow transplantation (BMT). However, unlike primary adult TTP, BMT-TTP has important differences and often does not respond well to plasma exchange. We describe the measurement of calpain activity in a group of BMT patients (with and without the clinical syndrome of transplant-associated TTP). Calpain was measured using a functional assay (14C-serotonin platelet release with inhibition by the cysteine protease inhibitor, leupeptin) in the sera of patients following autologous (auto) or allogeneic (allo) BMT. We also independently diagnosed and graded the BMT-TTP on the day of blood sampling using a scale that related to the percentage schistocytes and lactic dehydrogenase level. Calpain activity was detected in 1/8 (13%) grade 0-1 (6 auto, 2 allo); 6/16 (38%) grade 2 (3 auto, 13 allo) 9/16 (56%) grade 3 (2 auto, 14 allo) and 8/8 (100%) grade 4 BMT-TTP. Pre-BMT samples were tested in 10 allo-BMT patients who had positive calpain results post-BMT. One patient gave positive results before the transplant. This patient developed grade 4 BMT-TTP (day 24 post-BMT) and died despite apheresis. Positive calpain results were highly associated with neurologic symptoms, P < 0.001. Nineteen of 24 (79%) patients with positive results had neurologic symptoms compared to three of 21 (14%) patients with negative results. In conclusion, calpain was detected in half of the BMT patients with mild to moderate BMT-TTP (grades 2-3) and was uniformly found in those with severe (grade 4) BMT-TTP. Typically the calpain activity develops as TTP complicates the transplant process. It is unknown whether calpain contributes to the pathogenesis of this disorder, or is a secondary event.

RhD status of a fetus at risk for haemolytic disease with a discrepant maternal DNA-based RhD genotype

The cloning of the RHD gene has made it possible to determine the RhD status of fetuses at risk for haemolytic disease due to RhD iso-immunization using amniotic fluid or chorionic villi-derived DNA and the polymerase chain reaction. However, some Rh haplotypes are associated with false-positive or negative DNA-based results with the potential for an adverse outcome. We determined the RhD status of a fetus using amniotic fluid-derived DNA for an anti-D iso-immunized woman. Initially, we obtained the ethnic background and the complete RhD and RhCcEe phenotypes of both parents. The mother was RhD negative (Cde/cde) but her DNA was positive for exon 10 of the RHD gene. The fetus was positive for both exons 4 5 and exon 10. Southern analysis confirmed that the maternal DNA contained a portion of the RHD gene with a restriction pattern that was similar to RhD-positive individuals. This report illustrates that, in addition to fetal DNA genotyping, the same PCR assays, complete with RhD and RhCcEe phenotypes, and ethnic background of the parents should be obtained to alert the molecular diagnostic laboratory to the presence of rare Rh haplotypes that are associated with DNA genotyping errors.

A prospective study of protein-specific assays used to investigate idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP) is a disorder in which platelets, sensitized by autoantibodies, are destroyed by the reticuloendothelial system. The diagnosis of ITP has been a clinical one because assays measuring platelet-associated IgG (PAIgG) have low specificity. The recently introduced assays that measure antibodies against specific platelet glycoproteins (GP) offer the possibility of improved specificity. In this report we describe two prospective studies. In the first study we compared two protein-specific assays (AC and MAIPA) looking for the presence of autoantibodies against GP IIb/IIIa in 81 patient samples. These results were compared with an immunoradiometric assay for PAIgG. The second study investigated the enhanced sensitivity of measuring anti-GP Ib/IX autoantibodies in 76 patient samples. The protein-specific assays were able to differentiate immune from non-immune thrombocytopenia (specificity 91%, sensitivity 39%), whereas the PAIgG assay could not (specificity 19%, sensitivity 78%). The addition of the Ib/IX AC assay maintained a specificity of 92% while increasing the diagnostic sensitivity to 66%. In contrast to the PAIgG assay, there was no correlation between the platelet count and the likelihood or degree of positivity within the control samples using the glycoprotein assays. These studies confirm that glycoprotein assays can be used as diagnostic tests for ITP.

Serum-induced platelet procoagulant activity: an assay for the characterization of prothrombotic disorders

Platelets contribute to hemostasis by forming a platelet plug and by providing a procoagulant surface for the assembly and activation of the coagulation factors. The contribution of platelets to prothrombotic disorders has been difficult to analyze. Recently an assay was reported that measured the procoagulant activity of test platelets by making the platelet lipid surface the limiting factor in the production of thrombin. In this report we describe a novel technique, based on this assay, that we used to study patient serum factors that activate control platelets and in turn initiate measurable procoagulant activity. Using this assay we investigated a group of patients with prothrombotic disorders. The patient test serum was incubated with normal platelets in the presence of activated factor Xa. The resultant thrombin was measured in a chromogenic assay. The rate-limiting step was the presence of any potential platelet-activating factors, such as antibodies in the heat-treated test serum, that would allow the Xa to bind to the platelet phospholipid surface. Serum samples from patients with heparin-induced thrombocytopenia (HIT) and the anti-phospholipid antibody syndrome enhanced platelet procoagulant activity, while samples from patients with idiopathic thrombocytopenic purpura and disseminated intravascular coagulation (DIC) did not. HIT serum samples also induced platelet activation, as measured by platelet microparticle shedding, carbon 14-labeled serotonin release, and platelet aggregation. The measurement of serum-induced platelet procoagulant activity provides a method for the investigation of circulating platelet agonists in prothrombotic disorders.

The clinical management of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immunologic complication of drug therapy with potentially serious venous and arterial thrombotic sequelae. Recent studies have provided insight into the molecular and cellular basis of HIT underlying the diagnosis and treatment of this syndrome. Because various approaches to HIT treatment have resulted in mixed success, the use of heparin substitutes, other anticoagulant agents, and adjunctive therapy needs to be clarified. For patients with pre-existing conditions such as deep venous thrombosis or pulmonary embolism, new understanding of the clinicopathologic syndrome of HIT offers the promise of improved antithrombotic therapy. The participation of thrombin in HIT suggests that specific (hirudin and argatroban) thrombin inhibitors may be ideal agents for treating acute HIT.

Laboratory abnormalities in thrombotic thrombocytopenic purpura. Canadian Apheresis Group

Thrombotic thrombocytopenic purpura is an uncommon disorder that requires prompt recognition and intervention to prevent death. To date, information regarding the classic laboratory abnormalities in the disease has been derived from small numbers of patients whose laboratory tests have been done at many different sites. We report the laboratory findings in 135 patients who presented with thrombotic thrombocytopenic purpura to 17 Canadian centres. 50 men and 85 women had a mean platelet count of 25.3+/-19.4x10(9)/l. The initial platelet count correlated with mortality; 32% of patients with a platelet count of 20x10(9)/l or less died compared with 18% of patients with a platelet count >20x10(9)/l (P=0.058). The platelet-associated IgG was elevated in 88% at presentation whereas the indirect platelet suspension immunofluorescence test was positive in only 18%, 93% of the sera showed reactivity against platelets following protein blotting. All sera tested also showed reactivity against endothelial cells. Immune complexes were seen in all patients, whereas the platelet aggregating factor was detected in 59%. Although the von Willebrand factor was elevated in the majority of patients at entry, the multimer pattern was variable and showed no predictive pattern. Renal dysfunction was common (18%).

ABH antigens on human platelets: expression on the glycosyl phosphatidylinositol-anchored protein CD109

Platelets express alloantigens that are platelet specific (eg, the HPA antigens) and alloantigens that are shared with other blood cells (eg, the ABH antigens). The blood group A and B determinants are expressed on glycolipids and on some intrinsic platelet membrane glycoproteins. This report characterizes multiple platelet proteins reacting with blood group antibodies in serum samples from mothers of children born with neonatal alloimmune thrombocytopenia. ABH antigens on additional platelet proteins are identified, including the glycosyl phosphatidylinositol-anchored protein CD109. The proteins that carry ABH antigens were identified by using monoclonal antibodies to glycoproteins Ib, IIb/IIIa, Ia/IIa, CD31, and CD109 and immunoprecipitation/immunoblotting techniques with monoclonal antibodies to A and B antigens. The maternal serum samples and anti-A and anti-B monoclonal antibodies immunoprecipitated identical radiolabeled platelet proteins including proteins at 220 and 175 kd and proteins with mobilities corresponding to glycoproteins Ib, IIb/IIIa, IV, and V. Treatment of platelets with phosphatidylinositol-specific phospholipase C released into the supernatant a 175-kd protein that expressed the blood group determinants. This protein comigrated with the glycosyl phosphatidylinositol-anchored protein CD109. When platelet proteins were purified by immunoprecipitation with monoclonal antibodies and then tested by immunoblotting, anti-A reacted with the glycosyl phosphatidylinositol-anchored protein CD109 and to glycoproteins Ib, IIb, IIa, IIIa, and CD31 (PECAM). These results indicate that structures for modification by glycosyltransferases exist on platelet CD109, which also expresses the Gov alloantigen system. This study indicates that certain platelet proteins express both platelet-specific and blood group antigens that may contribute to platelet transfusion refractoriness and to neonatal alloimmune thrombocytopenia.

Dealing with drug-induced thrombocytopenia

Unpredictable and potentially life-threatening, immune thrombocytopenia can occur with any drug, at times even after long, uneventful use. Heparin is the most common cause; although the fall in platelet count tends to be modest, these cases are particularly dangerous because of associated thrombosis. Much has been learned about heparin-induced disease, but complex issues remain.

Heparin-induced thrombocytopenia. Diagnosis, natural history, and treatment options

Heparin-induced thrombocytopenia develops when patients given heparin as treatment for thrombosis have an immunologic reaction to the agent. Patients may have an underlying thrombotic disorder--for example, venous thrombosis or pulmonary embolism--that requires intervention. Alternative antithrombotic therapies that have been tried include defibrinogenating agents, LMWH and heparinoids, thrombin-specific inhibitors, antiplatelet agents, oral anticoagulants, and vena cava interruption.

Factor V Leiden and thrombotic complications in heparin-induced thrombocytopenia

To determine whether factor V Leiden is associated with thrombotic events in patients with heparin-induced thrombocytopenia (HIT), we evaluated 165 patients with serologically confirmed HIT for the presence of factor V Leiden and determined the incidence of venous or arterial thrombosis during the period of HIT. Factor V Leiden was detected in 16 of 165 HIT patients (9.7%). HIT-associated venous thrombosis occurred in 11 of 16 factor V Leiden positive subjects and 94 of 149 factor V Leiden negative subjects (69% vs. 63%; p = 0.79). Arterial thrombosis occurred in 1 of 16 factor V Leiden positive subjects and 21 of 149 factor V Leiden negative subjects (6% vs. 14%; p = 0.70). There was no difference in the incidence of proximal limb DVT, pulmonary embolism, venous limb gangrene, local skin reactions, hemorrhagic adrenal infarction, stroke, or myocardial infarction between the groups. No difference in the severity of venous thrombosis between Leiden positive and negative subjects was detected. Our data suggest that in the acute prothrombotic milieu of HIT, heterozygous factor V Leiden is not an important additional risk factor for thrombosis.

The pathogenesis of venous limb gangrene associated with heparin-induced thrombocytopenia

BACKGROUND

Platelet-mediated arterial occlusion is a well-recognized cause of limb loss in patients with heparin-induced thrombocytopenia. However, the syndrome of distal ischemic necrosis complicating the deep venous thrombosis (venous limb gangrene) sometimes associated with heparin-induced thrombocytopenia has not been well characterized.

OBJECTIVE

To study the pathogenesis of venous limb gangrene associated with heparin-induced thrombocytopenia.

DESIGN

Characterization (based on descriptive and case-control studies) of a novel syndrome of limb loss and hypothesis testing by analysis of plasma samples.

SETTING

Five university-associated hospitals in one medical community.

PATIENTS

Clinical and laboratory records of 158 patients with heparin-induced thrombocytopenia were reviewed to identify patients with venous limb gangrene (n = 8), limb arterial thrombosis (n = 10), and uncomplicated deep venous thrombosis (n = 58).

MEASUREMENTS

Clinical and laboratory factors associated with venous limb gangrene, including thrombin-antithrombin complexes and vitamin K-dependent procoagulant and anticoagulant factors.

RESULTS

Warfarin treatment was more frequently associated with venous limb gangrene than with limb arterial thrombosis (8 of 8 patients compared with 3 of 10 patients; P = 0.004). The anticoagulant effect of warfarin seemed greater in the 8 patients with venous limb gangrene than in the 58 patients who did not develop gangrene (median International normalized ratio, 5.8 compared with 3.1; P < 0.001). Compared with plasma from controls, plasma from patients with venous limb gangrene had a higher ratio of thrombin-antithrombin complex to protein C activity during warfarin treatment. No hereditable abnormalities of the protein C anticoagulant pathway were seen in any patient.

CONCLUSIONS

Warfarin treatment of deep venous thrombosis associated with heparin-induced thrombocytopenia is a possible cause of venous limb gangrene, perhaps because of acquired failure of the protein C anticoagulant pathway to regulate thrombin generation.

Activation of platelets by sera containing IgG1 heparin-dependent antibodies: an explanation for the predominance of the Fc gammaRIIa "low responder" (his131) gene in patients with heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder caused by heparin-dependent IgG (HIT-IgG) that recognizes a complex of heparin and platelet factor 4 (PF4), leading to platelet activation via the platelet Fc gammaIIa receptors (Fc gammaRIIa). Not all patients who generate HIT-IgG in response to heparin develop HIT, however, possibly because of observed differences in the ability of platelets from healthy individuals to be activated by HIT sera. It is known that a polymorphism in the platelet Fc gammaRIIa plays an important role in determining platelet reactivity to murine platelet-activating monoclonal antibodies of the IgG1 subclass: homozygous arg131 ("high responder" or HR) platelets respond well, and homozygous his131 ("low responder" or LR) platelets respond poorly, respectively, to these murine monoclonal antibodies. We sought to determine whether the differing risk for HIT among patients who receive heparin, as well as the variable platelet reactivity to HIT sera, could be explained by preferential activation by HIT-IgG of platelets bearing a particular Fc gammaRIIa phenotype. We found that the LR Fc gammaRIIa gene frequency was significantly overrepresented among 84 HIT patients, compared with that of 264 control subjects (0.565 versus 0.471; p = 0.03). We studied the subclass distribution of HIT-IgG against its major antigen, heparin/PF4 complexes, and found that 55 of 61 (90%) HIT sera expressed IgG1 antibodies either alone (n = 47) or in combination with IgG2 (n = 5) or IgG3 (n = 3). We then compared the platelet-activating profile of HIT sera with murine platelet-activating monoclonal antibodies. As expected, the murine IgG1 monoclonal antibodies preferentially activated platelets from homozygous HR individuals. In contrast, however, the LR homozygous platelets exhibited the greatest reactivity to HIT sera that contained predominantly anti-heparin/PF4 antibodies of the IgG1 subclass. We conclude that the significant overrepresentation of the LR (his131) gene among patients with HIT may be explained by the preferential activation of LR Fc gammaRIIa platelets by HIT antibodies of the IgG1 subclass, which is the predominant immunoglobulin subclass generated in HIT.

Maternal immunization to Gov system alloantigens on human platelets

BACKGROUND

Immunization to platelet alloantigens can occur during pregnancy or after the transfusion of blood components. Platelet alloantibodies can cause neonatal alloimmune thrombocytopenia and posttransfusion purpura. Transfusion-induced alloimmunization to a novel platelet alloantigen system, Gov, expressed on the 175-kDa glycosyl phosphatidylinositol-anchored platelet glycoprotein, CD109, was previously described. This report describes three unrelated patients who were alloimmunized to Gov(a) or Gov(b) during pregnancy.

STUDY DESIGN AND METHODS

Platelets were typed by using radioimmunoprecipitation for HPA-1a, -3a, -5a, -5b, Gov(a), and Gov(b) and by polymerase chain reaction-restriction fragment length polymorphism for HPA-1a, -1b, -3a, and -3b. Maternal sera were screened for platelet antibodies by using radioimmunoprecipitation and the antigen capture assay.

RESULTS

Patients 1 and 2 were investigated after the diagnosis of neonatal alloimmune thrombocytopenia in their children, and alloantibodies specific for Gov(b) and Gov(a), respectively, were detected in maternal serum. Serum from patient 3, who had mild idiopathic thrombocytopenia purpura with no detectable autoantibody, was found to contain alloantibodies to Gov(b) and to HPA-5b, presumably as a result of immunization during pregnancy. Platelet typings confirmed that the patients were at risk for alloimmunization to the respective antigen.

CONCLUSION

This report of three cases of maternal alloimmunization to antigens in the Gov system indicates that immunization can occur via placental transfer of antigen and that Gov system alloantibodies may be associated with neonatal alloimmune thrombocytopenia.

A Canadian hospital-based HIV/hepatitis C look-back notification program

OBJECTIVE

To describe the process used to notify pediatric patients who received transfusions of blood or blood products at our institution before donor blood was routinely screened for antibodies to HIV (1985) and hepatitis C virus (1990), and to evaluate the effectiveness of the notification program.

DESIGN

Patients who had received transfusions were identified through the hospital's medical records and the records from the Transfusion Medicine Laboratory. Patients were contacted by registered mail to provide notification of transfusion. A questionnaire was included with the notification to obtain information about the patient's awareness of the transfusion and whether he or she had undergone or planned to undergo testing for HIV and hepatitis C virus.

SETTING

Tertiary care university-affiliated teaching hospital in Hamilton, Ont.

PATIENTS

Patients 16 years of age or younger who had received blood products between February 1978 and November 1985. Patients who had received only albumin or immune serum globulin were not included as these products were not associated with viral transmission in Canada.

RESULTS

Notification letters were sent to 1546 patients. Of these letters 522 (33.8%) were returned undelivered. Of the 1024 patients contacted 493 (48.1%) responded to the questionnaire, of whom 157 (31.8%) were not aware of their transfusion. A total of 130 (26.4%) of the respondents had already undergone testing for HIV, and 342 (69.4%) indicated that they would undergo such testing as a result of the notification. In contrast, only 30 (6.3%) of 474 respondents had undergone testing for hepatitis C virus, but 425 (89.7%) indicated that they would undergo such testing. Overall, the patients' response to the notification was neutral or positive; however, a number of patients expressed dissatisfaction and anxiety.

CONCLUSIONS

The high proportion of patients who were unaware that they had undergone transfusion and who decided to undergo testing for HIV and hepatitis C virus as a result of notification supports the use of notification programs such as this one.

Proteolytic degradation of high molecular weight kininogen in acute thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disorder characterized by thrombocytopenia and schistocytic haemolytic anaemia. The majority of patients have normal coagulation parameters including the activated partial thromboplastin time (APTT). An intracellular cysteine protease, calpain, has been found in the plasma of many patients with acute TTP, and we hypothesize that it participates in the pathogenesis of the disorder. However, certain aspects of the disorder remain unresolved. For example, high molecular weight kininogen (HK) is one of the primary plasma inhibitors of calpain, and it also can act as a substrate for calpain. Consequently, one might anticipate that the HK could be defective or altered in TTP. In this report we describe the analysis of HK in plasma from live patients with acute TTP and following recovery. The HK was studied immunogenically and functionally. We observed that the HK in plasma samples from patients with acute TTP was proteolysed. This degradation was not observed in remission samples from the same patients. However, both acute and remission TTP samples had normal HK coagulant activity (acute samples, x = 0.84 +/- 0.26 U/ml; remission samples, x = 0.89 +/- 0.21 U/ml; control samples, x = 0.87 +/- 0.05 U/ml). Although the TTP plasmas were able to inhibit calpain activity, less inhibition activity was found in the acute samples (acute: mean inhibition 71 +/- 2.4%; remission: mean 92 +/- 2.1%; control samples: mean 93 +/- 5.4%; P < 0.001). Normal HK treated with calpain also had reduced calpain inhibition capacity (mean 58%). This study suggests that although HK is proteolysed during acute TTP, the proteolysis occurs without a major loss in the coagulation function or depletion of the protease inhibitory activity of HK.

High-dose intravenous immune globulin and the response to splenectomy in patients with idiopathic thrombocytopenic purpura

BACKGROUND

High-dose intravenous immune globulin produces a temporary rise in the platelet count in patients with idiopathic thrombocytopenic purpura. Splenectomy may also be effective, but it is not possible to predict which patients will have a good response. We hypothesized that the response to intravenous immune globulin predicts the response to splenectomy.

METHODS

We studied retrospectively 30 patients with idiopathic thrombocytopenic purpura who had first been treated with immune globulin and then undergone splenectomy. The responses to the two treatments were classified on the basis of the platelet count as poor (<50,000 per cubic millimeter), good (50,000 to 150,000 per cubic millimeter), or excellent (>150,000 per cubic millimeter).

RESULTS

All nine patients who had poor responses to intravenous immune globulin also had poor responses to splenectomy at one year. Of the 21 patients with good or excellent responses to intravenous immune globulin, 19 had good or excellent responses to splenectomy.

CONCLUSIONS

Patients with idiopathic thrombocytopenic purpura who have good or excellent responses to intravenous immune globulin are likely to have good or excellent responses to splenectomy, whereas patients who have poor responses to intravenous immune globulin are unlikely to have good or excellent responses to splenectomy.

Failure of pulse high-dose dexamethasone in chronic idiopathic immune thrombocytopenia

Idiopathic thrombocytopenic purpura (ITP) is a disorder characterized by increased platelet destruction in the setting of normal megakaryopoiesis. Approximately 20% of patients with ITP are refractory to corticosteroids and splenectomy. Recently, pulse high-dose dexamethasone was reported to be effective in the treatment of chronic ITP in adult patients. We treated 9 patients with severe chronic ITP with monthly high-dose dexamethasone. None of the 9 patients responded with a sustained increase in platelet count. Five of these patients were unable to tolerate the regimen. The failure of high-dose dexamethasone in our hands contrasts with the good results of an earlier publication and suggests that there could be a subset of responders who will require better identification.

A diagnostic test for heparin-induced thrombocytopenia: detection of platelet microparticles using flow cytometry

Based on our previous observation that heparin-induced thrombocytopenia (HIT) sera can generate platelet microparticles from washed platelets in a heparin-dependent fashion, we developed a test for HIT using flow-cytometry to measure heparin-dependent platelet microparticle formation. During the developmental phase of the assay the optimal physical conditions for microparticle generation were defined. 133 sera were then evaluated using the microparticle assay and the serotonin release assay to determine the threshold for defining a positive result that gave optimal sensitivity and specificity. The microparticle assay was then prospectively evaluated against the serotonin release assay in 202 sera referred to our laboratory for HIT testing. Overall agreement between the two assays was 96% (Cohen's kappa = 0.91). When the clinical data were reviewed on patients whose sera gave discrepant results between the two assays, no case of HIT was detected by one assay and missed by the other. The platelet microparticle assay is as accurate as the serotonin release assay and may be a useful non-radioactive test for HIT.

A 14-year study of heparin-induced thrombocytopenia

PURPOSE

To determine the sites of thromboses (venous versus arterial circulation) that complicate the clinical course of immunemediated heparin-induced thrombocytopenia, and to determine the 30-day risk for thrombosis in patients who are initially recognized with isolated heparin-induced thrombocytopenia.

PATIENTS AND METHODS

We analyzed objectively documented thrombotic events that complicated the clinical course of 127 patients with serologically confirmed heparin-induced thrombocytopenia identified in one medical community over a 14-year period. We classified heparin-induced thrombocytopenia patients into two groups: patients recognized with heparin-induced thrombocytopenia only after a new thrombosis had occurred, and patients initially recognized with isolated heparin-induced thrombocytopenia. We determined the subsequent 30-day risk for thrombosis for the cohort of patients initially recognized with isolated thrombocytopenia.

RESULTS

Heparin-induced thrombocytopenia was associated with the development of new venous thrombotic events and arterial thrombotic events in 78 and 18 patients, respectively (ratio venous/arterial thrombosis = 4:1). Pulmonary embolism was the most common life-threatening thrombotic event, occurring in 25% of all patients. Approximately half of all heparin-induced thrombocytopenia patients were recognized only after they had a complicating thrombotic event. Of the remaining 62-patient cohort initially recognized with isolated thrombocytopenia, the subsequent 30-day risk of thrombosis was 52.8%. The risk of thrombosis did not differ whether the heparin had been discontinued alone or whether warfarin had been substituted for the heparin.

CONCLUSIONS

Venous thrombosis complicates heparin-induced thrombocytopenia more frequently than does arterial thrombosis. The high risk of thrombosis in patients initially recognized with isolated thrombocytopenia suggests that conventional management approaches require reappraisal.