The Reliability of Activated Partial Thromboplastin Time Methods and the Relationship to Lipid Composition and Ultrastructure

The coagulation system in humans

Complex, interrelated systems exist to maintain the fluidity of the blood in the vascular system while allowing for the rapid formation of a solid blood clot to prevent hemorrhaging subsequent to blood vessel injury. These interrelated systems are collectively referred to as haemostasis. The components involved in the haemostatic mechanism consist of vessel walls, platelets, coagulation factors, inhibitors, and the fibrinolytic system. In the broadest sense, a series of cascades involving coagulation proteins and enzymes, as well as cell surfaces (platelets and endothelial cells), work together to generate thrombin, the key enzyme in coagulation, subsequently leading to the formation of a fibrin clot. However, there also exist direct and indirect inhibitors of thrombin to ensure that clot formation does not go uncontrolled. Once the fibrin clot is formed, the fibrinolytic system ensures that the clot is lysed so that it does not become a pathological complication. Taken together, the systems exist to balance each other and maintain order. The balance of coagulation and fibrinolysis keeps the haemostatic system functioning efficiently.

The responsiveness of different APTT reagents to mild factor VIII, IX and XI deficiencies

INTRODUCTION

The sensitivity of APTT reagents to deficiencies of factors VIII, IX, XI and XII varies because of their composition. The APTT is used as a screening test for these factors, and a deficiency should manifest with a prolongation to the APTT, which may trigger the need for specific factor assays to be performed.

METHODS

The suitability of APTT reagents to detect mild deficiencies can be assessed by the analysis of the APTT of plasma, which has an increasing concentration of the factor in question. The APTT responsiveness can be determined from the intersection of the curve and the upper limit of the APTT normal reference range for that APTT reagent. We assessed the APTT responsiveness (in U/dl) to factors VIII, IX and XI of four APTT reagents; Actin FS (Siemens), Synthasil (IL), STA-PTTA (Stago) and Dapttin (Technoclone).

RESULTS

Actin FS was the most sensitive reagent to mild reductions of factors VIII, IX and XI [Correction added on 26 October 2010, after first online publication: Synthasil was corrected to Actin FS]. STA-PTTA showed less sensitivity than Synthasil and Actin FS; Dapttin was insensitive to mild deficiencies of factors IX and XI and should not be used as a screening test.

CONCLUSION

Both Synthasil and Actin FS are acceptable reagents to screen for reduced factors VIII, IX and XI, and the number of mildly reduced factors not diagnosed will be limited.

Usefulness of synthetic phospholipid in measurement of activated partial thromboplastin time: a new preparation procedure to reduce batch difference

Commercial activated partial thromboplastin time (APTT) reagents prepared with phospholipid extracted from animal or plant sources often differ in their response to heparin and coagulation factors and in their reference values. It is also known that there are variations in phospholipid composition and preparation procedure. The present study attempted to demonstrate that an APTT reagent based on synthetic phospholipids (phosphatidylethanolamine, phosphatidylcholine and phosphatidylserine), which are substances of high purity and homogeneity, reduces batch-to-batch difference when compared with two traditional APTT reagents derived from rabbit brain and soybean. Three types of APTT reagent (SYN-APTT, RBT-APTT, SOY-APTT), prepared respectively from synthetic phospholipid, rabbit brain, and soybean, were tested. The total batch-to-batch difference was coefficient of variation (CV) 0.7-2.4% in the five reagents prepared from synthetic phospholipid (SYN-APTT), but CV 1.5-10.3% in the two traditional reagents (RBT-APTT and SOY-APTT). Additionally, high-performance liquid chromatography (HPLC) analysis showed clear variation in the phospholipid composition of the RBT-APTT and SOY-APTT reagents. In conclusion, the SYN-APTT reagent derived from synthetic phospholipid was shown to reduce batch-to-batch difference, and we therefore suggest that synthetic phospholipid is a substance useful in the preparation of APTT reagent and could contribute to stability of supply and uniform diagnosis.

Heparin dosing and outcome in acute coronary syndromes: the GUSTO-IIb experience. Global Use of Strategies to Open Occluded Coronary Arteries

BACKGROUND

This study analyzed relationships among heparin dosage, patient characteristics, and 30-day outcome because optimal unfractionated-heparin dosing in acute coronary syndromes remains uncertain.

METHODS

Patients (n = 5335) randomized to heparin therapy in the Global Use of Strategies to Open Occluded Coronary Arteries (GUSTO) IIb trial were studied. The heparin dose was adjusted to a target activated partial thromboplastin time (aPTT) and normalized for the patient's weight. Mortality and cardiac (re)infarction within 30 days and their association with patient characteristics and heparin dosing were evaluated.

RESULTS

The lowest mortality rate appeared with a heparin dose of approximately 14 U/kg/h or an aPTT of approximately 70 seconds. Heparin dosing was a significant predictor of outcome after adjusting for presenting coronary syndrome; a trend remained after adjusting for other baseline differences. This association was lost when adjusted for the aPTT result. Patients who died early appeared to have lower heparin dosing than those with later mortality (P =.012). Heparin "resistance" with relatively high heparin dosages and low aPTT values did not increase the risk for adverse outcome.

CONCLUSIONS

There is a defined, dose-associated benefit of unfractionated heparin in acute coronary syndromes similar to that seen previously in thrombolytic-treated infarctions. Heparin therapy is complicated by its complex biologic interactions and relatively crude measures of its effect. Better measures of heparin effectiveness and strategies need to be developed with either better antithrombin agents or adjunctive therapies such as antiplatelet regimens to treat patients who require benefits beyond that supplied by unfractionated heparin.

Disagreement between bedside and laboratory activated partial thromboplastin time and international normalized ratio for various novel anticoagulants

During studies on warfarin, heparin and various anticoagulants with novel mechanisms of action, the activated partial thromboplastin time (aPTT) and the (apparent) international normalized ratio (INR) from a bedside monitor (Coagucheck Plus(R)) were compared with laboratory assay results. Data were compared using the Bland and Altman method of comparison where systematic differences result in significant slopes of the regression line. During heparin treatment, the bedside monitor largely underestimated the aPTT (slope = -0.80). During treatment with the direct thrombin inhibitor napsagatran (slope = 0.99), the pentasaccharides Org31540/SR90107A (slope = 0.77) and SanOrg34006 (slope = 0.35), and warfarin (slope = 0.60), the bedside monitor underestimated the aPTT at lower aPTT levels, while at higher aPTT levels it overestimated the laboratory values. The bedside monitor slightly overestimated the INR during treatment with warfarin (slope = 0.33). Apparent INR was largely overestimated during treatment with Org31540/SR90107A (slope = 1.38), SanOrg34006 (slope = 0.97), Napsagatran (slope = 1.23), and recombinant tissue factor pathway inhibitor (slope = 1.48, P < 0.001 for all regression lines). These results indicate that a substantial disagreement in aPTT or (apparent) INR exists between the bedside monitor and laboratory assay during treatment with the studied 'classic' and novel anticoagulants. The amount of disagreement depended on the anticoagulant given.

New heparin dosing recommendations for patients with acute coronary syndromes

Despite major innovations in antithrombotic and antiplatelet therapy, unfractionated intravenous heparin is widely used to treat acute coronary syndromes. Recommendations for unfractionated heparin dosing in acute myocardial infarction and unstable angina have been issued in two recent American College of Cardiology/American Heart Association guidelines. An initial heparin bolus of 60 U/kg (maximum, 4000 U) followed by a 12-U/kg/h infusion (maximum 1000 U/h) is recommended with alteplase for ST-elevation myocardial infarction. When intravenous heparin is administered for myocardial infarction with non-ST elevation and unstable angina, an initial bolus of 60 to 70 U/kg (maximum, 5000 U) followed by a 12- to 15-U/kg/h infusion is recommended. The goal is to achieve an activated partial thromboplastin time of 50 to 70 seconds. Here, we review these new dosing regimens and explain the rationale for their use. We also review the risk of bleeding with heparin, especially when administered concurrently with aspirin, thrombolytic agents, and glycoprotein IIb/IIIa antagonists, and the relationship between activated partial thromboplastin time and cardiac events.

Measurement of factor VIII activity of B-domain deleted recombinant factor VIII

The factor VIII activity of B-domain deleted recombinant factor VIII (BDDrFVIII) measured by activated partial thromboplastin time (APTT)-based one-stage assays is approximately 50% of the activity obtained by the chromogenic assay. Similar results have been reported for the two licensed full-length recombinant factor VIII products. In view of these findings, comprehensive studies have been undertaken to find the cause of the assay differences. Only the phospholipid reagent, used as a platelet substitute in the one-stage assay, proved to be crucial for explaining the assay difference. When platelet-rich plasma was used as the source of phospholipid in the one-stage assay, the factor VIII activity assay results correlated well with those measured by the chromogenic assay. Similar results were obtained when the platelets were replaced by liposomes prepared using platelet factor 3 (PF3) as a model that has a low content (5% to 10%) of phosphatidylserine (PS). In contrast, the use of liposomes with 20% to 30% PS, as in the crude lipid extracts used in ordinary APTT reagents, resulted in underestimation of the factor VIII activity. Antigen measurements using an enzyme-linked immunosorbent assay (ELISA) method demonstrated a good correlation between the antigen and chromogenic activity, but not always between antigen and one-stage activity results. Based on these findings and the clinical data, it can be concluded that the chromogenic assay most accurately measures the functional activity of BDDrFVIII. However, modifications of the one-stage assay, such as the use of a product-specific standard or development of a PF3-like phospholipid reagent, could address the observed assay discrepancies.

Activated partial thromboplastin time as a screening test of minor or moderate coagulation factor deficiencies for canine plasma: sensitivity of different commercial reagents

To determine the sensitivity for detection of coagulation factor deficiencies by commercial reagents for canine plasma, 5 commercial activated partial thromboplastin time (APTT) reagents with different types of contact activator and phospholipid of various origin were examined. Thirty canine plasma samples with minor or moderate deficiencies of coagualition factors that influence the APTT were examined. Significant differences were found for the sensitivity of various reagents, but no correlation was found with the type of contact activator. Following the test instructions provided by the manufacturers, the number of APTT results that were prolonged beyond the reference range varied between 20 and 30 (sensitivity = 0.67-1.00); the number of corresponding results using a standardized test protocol varied between 19 and 28 (sensitivity: 0.63-0.93). The most sensitive reagent contained kaolin as a contact activator and a human placental thromboplastin. The results of this study indicate that the APTT test optimized for human plasma is also a sensitive screening test of the intrinsic system of canine plasma, provided that a suitable reagent is used.

Lipid composition of seven APTT reagents in relation to heparin sensitivity

The phospholipid content of different activated partial thromboplastin time (APTT) reagents was determined and compared to heparin sensitivity. The seven reagents included were those most widely used amongst participants of the U.K. National External Quality Assessment Scheme (NEQAS) at the time of study. Heparin sensitivity was assessed using the APTT ratios obtained by more than 300 NEQAS participants on five plasmas prepared from patients receiving unfractionated heparin. The concentrations of three neutral lipids and six phospholipids present in the seven APTT reagents were determined by high-performance thin-layer chromatography (HPTLC) and densitometry. Both the concentrations and the relative percentages of individual phospholipid components varied markedly between reagents. The total phospholipid concentration included a 12-fold range from 16 to 205 microgram/ml. Phosphatidylserine (PS) was completely lacking from one reagent prepared from vegetable material and ranged from 3 to 22 microgram/ml in the other six reagents containing extracts from animal tissue. The concentration of phosphatidylcholine ranged from 3 to 109 microgram/ml. There was no demonstrable relationship between the concentration of any individual lipid components and heparin sensitivity. However, the relative percentage phospholipid composition was important since a lower % of PS or phosphatidylinositol (PI) correlated with increasing heparin sensitivity.

Improving measurement stability and reproducibility of potentiometric sensors for polyions such as heparin

The growing importance of polymer membrane-based potentiometric polyion sensors in biomedical research and clinical measurements has brought up the question of how accurate and reproducible these sensors are. Indeed, recent research has revealed that these sensors behave quite differently than classical ion-selective electrodes. This paper explores ways to improve measurement reproducibility and long term potential stability by considering the unique pseudo steady-state response mechanism of the polyion sensors developed so far. Heparin may be stripped out of the phase boundary membrane surface with a high sample NaCl concentration and this characteristic is used to modify the calibration procedure in order to avoid memory effects. It is also attempted to reduce long term potential drifts by continuously stripping heparin out of the membrane at the membrane-inner filling solution side.

Beta2-glycoprotein I is necessary to inhibit protein C activity by monoclonal anticardiolipin antibodies

OBJECTIVE

To clarify mechanisms of the thrombosis associated with anticardiolipin antibodies (aCL), we examined the effects on activated protein C (APC) of monoclonal aCL and beta2-glycoprotein I (beta2GPI), which is required for formation of the epitopes of aCL.

METHODS

We developed the chromogenic assay, in which the degradation of coagulation factor Va by APC is reflected in the reduced generation of thrombin from prothrombin, using soybean trypsin inhibitor to inhibit APC. APC activities were measured in the presence and absence of 3.4 microM beta2GPI and/or 2.5 microg/ml of IgM monoclonal aCL (EY2C9 and EY1C8) established from peripheral blood lymphocytes obtained from a patient with aCL.

RESULTS

Without APC, the formed thrombin activity decreased by the addition of 3.4 microM beta2GPI. When 12.8 nM APC was added, beta2GPI partially reversed the APC-induced inhibition of thrombin generation in a concentration-dependent manner. With 3.4 microM beta2GPI, the thrombin generation in monoclonal aCL (2.5 microg/ml) decreased to 77.1-80.2% by the addition of 12.8 nM APC, but the values were above that in the control IgM (72.7%). Without beta2GPI, the APC activity was unaffected by the addition of monoclonal aCL.

CONCLUSION

Beta2-glycoprotein I exhibits procoagulant activity by inhibiting APC activity and anticoagulant activity by inhibiting thrombin generation. Any further inhibition of APC activity was caused by monoclonal aCL and only in the presence of beta2GPI.

Influence of phospholipids on the assessment of factor VIII activity

In view of reported discrepancies between different factor VIII assays, the influence of phospholipids on the performance of one-stage clotting (OS) and chromogenic substrate (CS) assays was evaluated. The B domain deleted recombinant factor VIII, rVIII SQ, two full-length recombinant products and a plasma derived factor VIII concentrate were each diluted into severe haemophilia A plasma and assayed against a plasma standard. The one-stage activity was 50, 80, 75 and 106%, respectively, of the chromogenic result. Variations in the phospholipid concentration did not affect the chromogenic assay, except at very low levels where the apparent activity increased. In contrast, dilution of the phospholipid reagent had a substantial influence on the activity measured by OS assays, especially in the case of rVIII SQ. At low levels of phospholipid, the one-stage activity of rVIII SQ exceeded the chromogenic result. When mixtures of phosphatidylserine (PS) and phosphatidyl-choline (PC) were used as a source of phospholipid, the OS results for rVIII SQ agreed well with the CS activity as long as the content of PS was below 10%, i.e., closer to the physiological level. At higher levels of PS, as in most commercial APTT reagents, the OS activity decreased. When the APTT reagent was replaced by platelets in the OS assay, the results compared well with those obtained by the CS assay for both t-VIII SQ and full-length factor VIII products.

Reevaluation of a weight-based heparin dosing nomogram: is institution-specific modification necessary?

OBJECTIVE

To compare a heparin dosing nomogram using an initial infusion rate of 18 units/kg/h with physician-directed heparin prescribing and with a modified version of the nomogram adjusted for institution-specific data.

METHODS

During consecutive phases of this cohort study, patients' intravenous heparin therapies were initiated and adjusted by using one of the following three methods: (1) physician-directed dosing, (2) a body weight-based dosing nomogram with an initial infusion rate of 18 units/kg/h, and (3) a body weight-based dosing nomogram with an initial infusion rate determined by the median dose of heparin (in units/kg/h) required to achieve therapeutic activated partial thromboplastin times (aPTTs) during the first two phases. The time required to achieve therapeutic aPTTs as well as the percentage of initial aPTTs in the therapeutic range were compared for the three phases.

RESULTS

The heparin dosing nomogram in which the initial infusion rate was adjusted for our individual institution resulted in a statistically shorter median time until aPTTs were in the therapeutic range than did either the physician-directed dosing or unmodified nomogram groups (6.1 h in the modified nomogram group, 10.5 h in the physician-directed group, 21.5 h in the unmodified nomogram group; p < 0.05 for all differences). Use of the institution-specific nomogram resulted in the greatest percentage of initial aPTTs in the therapeutic range (84% in the 13 units/kg/h nomogram group vs. 47% in the physician-directed group and 18% in the 18 units/kg/h nomogram group; p < 0.05 for all differences).

CONCLUSIONS

Use of a heparin dosing nomogram with an initial infusion rate of 18 units/kg/h resulted in prolongation of the time to reach therapeutic aPTTs. By modifying the nomogram for use at an individual institution, we reduced the time to achieve therapeutic range of aPTTs while still reducing the likelihood of excessive anticoagulation of patients.

The role of lipids in the detection of lupus anticoagulant by the dilute Russell Viper venom test: are platelets or reagents containing hexagonal HII phases necessary?

Liposomes prepared from rabbit brain extracts (RBE) and individual pure lipids (high phosphatidyl serine content, HIPS) were compared with frozen-thawed platelets (PLTS) in the dilute Russell Viper venom time (dRVVt). While all three preparations demonstrated sensitivity to lupus anticoagulant (LA) the highest detection rate was seen with RBE. For confirmation of LA, high concentration RBE achieved the most efficient correction of the defect. Electron microscopy and particle sizing showed RBE to be small, discrete liposomes, whereas HIPS and PLTS were aggregates of larger diameter particles. Low-angle X-ray diffraction showed no evidence of hexagonal HII phase. There appears to be no specific requirement either for platelets or for reagents containing hexagonal HII phases in the dRVVt. The dRVVt can be optimized by incorporating a simple dilute rabbit brain lipid mixture for detection of LA and a concentrated mixture as a correcting reagent.

Methods for detecting lupus anticoagulants and their relation to thrombosis and miscarriage in patients with systemic lupus erythematosus

AIMS

To examine the sensitivity and specificity to past thrombotic events of four different coagulation tests, which screen for lupus anticoagulant (LA), and of anticardiolipin antibodies in patients with systemic lupus erythematosus.

METHODS

Fifty three consecutive patients with systemic lupus erythematosus were studied of whom three males and 21 females, aged 21-60 years, had a history of venous and arterial thrombosis, or miscarriage, or both. Activated partial thromboplastin time (aPTT), dilute Russell's viper venom time (dRVVT), kaolin clotting time (KCT), dilute aPTT and the circulating titre of anticardiolipin antibodies were investigated in the two groups of patients and in 20 healthy control subjects.

RESULTS

The prolonged dilute aPTT was more sensitive to thromboses or miscarriages, or both than dRVVT (p less than 0.05), KCT (p less than 0.01), and aPTT (p less than 0.001). No significant differences in specificity were found among aPTT (100%), dRVVT (93%), KCT (93%) and dilute aPTT (86.2%); but aPTT and dRVVT were significantly more specific (p less than 0.01, p less than 0.05, respectively) than anticardiolipin antibodies.

CONCLUSIONS

The study shows a strong association between lupus anticoagulant and thrombosis when a very sensitive test such as the dilute aPTT is used. The combination of this assay with a very specific test such as dRVVT might enable patients with SLE at high risk of thrombosis to be identified.

Laboratory diagnosis of von Willebrand's disease

The laboratory diagnosis of von Willebrand's disease (vWD) has become much more difficult because of the identification of numerous variant forms of vWD. The biologic and pathologic variability in individual patients necessitates a comprehensive assessment. Patients with classic type I vWD may be easily identified by using the bleeding time, activated partial thromboplastin time, platelet count, von Willebrand antigen, and ristocetin cofactor tests. In patients with variant forms of vWD, however, multimeric analysis of both platelet and plasma von Willebrand factor may be necessary. Furthermore, more than one assessment may be needed to detect an abnormal result in many of the aforementioned tests.

Use of the activated partial thromboplastin time for monitoring heparin therapy: problems and possible solutions

There is considerable variation in available methods for the activated partial thromboplastin time (APTT), giving widely differing results with patients on heparin treatment. The study is primarily concerned with the assessment of five of the widest used APTT reagents. The heparin response of these reagents has been related to their lipid composition and physical properties. Of the various correlations between lipid composition of the reagents and clotting performance only electrophoretic mobility was associated with the APTT response to heparin. There was a highly significant negative correlation between the APTT prolongation with heparin and electrophoretic mobility. When plasma is heparinized in vitro a differing order of ranking for APTT reagents is obtained than when heparinized patients are tested. The APTT response in patients with recent thrombosis must therefore be the best guide to the clinical dose of heparin. The therapeutic range of conventional heparin therapy is generally regarded as 1.5-2.5 times the control. External quality assessment programmes in the UK and USA have shown considerable differences between heparin dosage according to the APTT test systems. The definition of the therapeutic range must be derived from randomized clinical studies. The need for progress in standardization of the APTT monitoring of heparin is demonstrated.

Screening for the lupus anticoagulant

The lupus anticoagulant may be defined as an immunoglobulin (IgG, IgM or both) which interferes with one or more of the in vitro phospholipid-dependent tests of coagulation. For many years, lupus anticoagulants were regarded as a laboratory nuisance; consequently, reagents were often selected on the basis of insensitivity to lupus anticoagulants. Recently, lupus anticoagulants have been associated with a variety of clinical conditions including recurrent thromboembolic events (both arterial and venous), obstetrical complications including fetal death and spontaneous abortion, and a variety of hematologic and neurologic complications. As a result, many laboratories are now being asked to identify the presence of lupus anticoagulants in selected patient populations. In addition to assays for lupus anticoagulants, there are immunologic assays designed to detect phospholipid antibodies using solid phase systems (RIA or ELISA). A variety of screening tests have been designed to enhance sensitivity to lupus anticoagulants. Test systems with decreased amounts of phospholipid (phosphatidylserine) appear to be most sensitive to lupus anticoagulants. Of the various tests used, the activated partial thromboplastin time (APTT) appears to be most sensitive. The sensitivity of any screening test system is inversely proportional to the residual platelets in the patient sample. APTT reagents differ widely in their sensitivity to lupus anticoagulants. The dilute Russell viper venom time is also highly dependent on the choice and concentration of phospholipid with respect to its sensitivity. Once an abnormality of a screening test has been identified, it is necessary to prove the abnormal result is due to the presence of an inhibitor. This step in the diagnosis may utilize either mixing studies or plasma agarose gels. The final step in the diagnosis of lupus anticoagulants is the demonstration of phospholipid specificity of the inhibitor. Two approaches have been utilized: 1. test systems designed to enhance anticoagulant effect (phospholipid-depleted), and 2. test systems with increased or altered phospholipids which will bypass or neutralize the anticoagulant.

Use of the activated partial thromboplastin time for the diagnosis of congenital coagulation disorders: problems and possible solutions

The activated partial thromboplastin time (APTT) is a commonly performed laboratory procedure which is used for multiple purposes including monitoring of heparin therapy, detection of coagulation factor deficiency, and detection of lupus anticoagulants. Among the hereditary coagulation deficiencies, factor VIII and factor IX are the most common. APTT reagents differ widely in both their sensitivity to factor VIII and factor IX deficiencies as well as their responsiveness. Sensitivity may be defined as the ability to identify a deficiency state while responsiveness is indicated by the degree of prolongation of the APTT result as compared to the upper limit of normal. Reagents may be both sensitive and responsive or alternatively sensitive and relatively nonresponsive. Consequently, it is extremely important for each laboratory to carefully identify the upper limit of the normal range. A variety of preanalytical variables will also effect the sensitivity of the APTT to factor deficiency states. These variables include specimen handling and the preparation of platelet poor plasma. The instrument effect is also of importance. Selection of the reagent tends to have the most impact on sensitivity and responsiveness while instrumentation affects the precision of a given APTT. The composition and concentration of phospholipid in APTT reagents does have an effect on reagent responsiveness and sensitivity. Sensitivity to factor deficiencies does not necessarily parallel sensitivity to lupus anticoagulants.

A comparison of two APTT reagents which use silica activators

The Ortho activated partial thromboplastin time (APTT) reagent, Thrombosil 1 (TS), was compared to the General Diagnostics automated APTT reagent (GD). TS produced more precise results over a 38-day period of testing a normal control plasma, indicating that the upper limit of the normal range could be more precisely set with TS. This normal range was better represented if the normal values with both reagents were logarithmically transformed before calculating the mean +/- 2 SD. TS was more sensitive to plasma which had been heparinized in vitro. This was also demonstrated in vivo by the testing of 100 plasmas from heparinized patients. On testing of in-vitro dilutions of normal plasma with factor-deficient plasmas, TS was more sensitive to decreasing levels of factors VIII, IX and XI but less sensitive to decreasing factor XII. This was demonstrable in vivo in 71% of cases with plasmas from factor-deficient patients. GD was more sensitive to the lupus anticoagulant in most cases.

Binding specificity of lupus anticoagulants and anticardiolipin antibodies

Antiphospholipid antibodies have been found to be strongly associated with syndromes characterised by spontaneous arterial and venous thromboses, recurrent miscarriage, immune thrombocytopenia, and occasionally neurological manifestations. These antibodies can be detected using solid phase immunoassays, and by their effect on prolonging phospholipid dependent clotting tests. This latter phenomenon is termed the lupus anticoagulant (LA). The relationship between anticardiolipin antibodies (ACA) and the LA activity of plasma was investigated in 14 patients. Plasma of these patients exhibited both LA activity and high levels of ACA. The patients included 7 with systemic lupus erythematosus, 6 without and 1 chlorpromazine induced lupus anticoagulant. 7 patients had a history of thrombosis and 7 did not, despite high antibody levels. Plasma was incubated in a serial fashion with solid phase cardiolipin and the residual ACA level and LA activity were monitored using a solid phase enzyme linked immunoassay, and the kaolin clotting time (KCT) and activated partial thromboplastin time (APTT) respectively. There was no correlation between baseline ACA levels and parameters of LA activity (dKCT or dAPTT) in contrast to previous reports. However, there was a concurrent reduction in both LA and ACA levels over 24 hours during incubation with cardiolipin in all patients. The rate of reduction of both parameters was highly correlated (r = 0.99. p less than 0.001). The relative reduction of LA activity versus ACA level varied between patients, and may represent different affinities for phospholipid in thromboplastin versus phospholipid in solid phase. Thus, despite the lack of concordance between LA and ACA in many patients, the two activities can be removed concurrently in vitro, suggesting similar binding specificities of the antibodies. The incomplete concordance could be explained by varying affinities for different structural presentations of the lipid antigen.

Protein C response to induction and withdrawal of oral anticoagulant treatment

Protein C activity and antigen levels have been related to clotting activities of factors VII and X during the induction and withdrawal periods of oral anticoagulant treatment. Both factor VII and protein C activities fell rapidly during induction but factor VII showed a more rapid and much more marked depression than protein C. In contrast, reductions in factor X were much slower. Protein C antigen, although depressed rapidly at the initiation of treatment, did not subsequently fall to the same degree as protein C activity. The ratio of activity to antigen became progressively smaller. On discontinuation there was a reversal of the pattern but with two important differences. Firstly, there was evidence of an excessive rise ('rebound') of factor VII compared with the steady state levels in these patients; and secondly there was a surprisingly slow return of protein C to normal levels after the oral anticoagulant was withdrawn (levels were still below normal on day 4). These observations lend support to gradual withdrawal of oral anticoagulants after a period of long-term administration. The results suggest that after discontinuation of long-term anticoagulants patients may have increased coagulability up to four days.

Monoclonal IgM with lupus anticoagulant activity in a case of Waldenström's macroglobulinaemia

Strong lupus anticoagulant activity was detected in the plasma of a 73-yr-old woman with Waldenström's macroglobulinaemia. Purified patient IgM caused significant prolongation of the APTT in concentrations as low as 30 micrograms/ml, and its activity was inhibited by intact or (more effectively) by frozen-thawed blood platelets. Employing varying concentrations of cephalin in the APTT assay, it was demonstrated that the macroglobulin had a direct cephalin-neutralizing effect. When tested in an ELISA system using phospholipid-coated microtiter wells, the IgM was shown to possess antibody activity against cephalin. With purified phospholipids as antigens, the protein was found to display antibody activity against phosphatidyl serine and phosphatidyl ethanolamine, which are main constituents of cephalin. These observations were confirmed in absorption experiments.

Measurement of heparin in plasma: influence of inter-subject and circadian variability in heparin sensitivity according to method

Heparin was measured, with respect to standard curves prepared with normal pooled plasma, by five methods (APTT, thrombin time, one and two stage coagulation, anti-factor Xa and chromogenic anti-factor Xa) after addition at three concentrations to plasmaprepared from normal young volunteers, hospitalized patients with malignancy and geriatric patients. By the APTT and TT, differences in sensitivity were observed at 0.4iu heparin/ml corresponding to an apparent difference in heparin level of 10 and 14 fold between high and low responding individuals. Such large differences were not apparent by anti-factor Xa assay. A circadian difference in sensitivity was also observed in the patient group such that in samples taken at night, heparin levels were 30-50% higher on average when measured in the APTT and TT. Again, such large differences were not apparent by anti-factor Xa methods. In light of recent findings about the usefulness of anti-factor Xa methods for efficient monitoring of heparin, it is suggested that this conclusion may arise from the tendency for anti-factor Xa methods to determine actual concentrations of heparin.