FRETS-VWF73: a rapid and predictive tool for thrombotic thrombocytopenic purpura

Laboratory investigation and diagnosis of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare and potentially fatal disease for which rapid diagnosis is crucial for patient outcomes. Deficient activity (< 10%) of the liver enzyme, ADAMTS13, is the pathophysiological hallmark of TTP, and measurement of the enzyme activity can establish the diagnosis of TTP with high accuracy. Thus, along with the clinical history, appropriate laboratory assessment of a suspected case of TTP is essential for diagnosis and treatment. Here, we present a review of the available laboratory tests that can assist clinicians in establishing the diagnosis of TTP, with special focus on ADAMTS13 assays, including the measurement of the antigen and activity, and detection of autoantibodies to ADAMTS13.

ISTH guidelines for the diagnosis of thrombotic thrombocytopenic purpura

BACKGROUND

Despite an increase in our understandings of pathogenesis of thrombotic thrombocytopenic purpura (TTP), the approaches for initial diagnosis and management of TTP vary significantly.

OBJECTIVE

The evidence-based guidelines of the International Society on Thrombosis and Haemostasis (ISTH) are intended to support patients, clinicians, and other health care professionals in their decisions about the initial diagnosis and management of acute TTP.

METHODS

In June 2018, ISTH formed a multidisciplinary panel that included hematologists, an intensive care physician, nephrologist, clinical pathologist, biostatistician, and patient representatives, as well as a methodology team from McMaster University. The panel composition was designed to minimize the potential conflicts of interests. The panel used the Grading of Recommendations Assessment, Development, and Evaluation approach and the Population, Intervention, Comparison, Outcome framework to develop and grade their recommendations. Public comments were sought and incorporated in the final document.

RESULTS

The panel agreed on three recommendations covering the initial diagnosis with emphasis on the importance of ADAMTS13 testing (eg, activity, anti-ADAMTS13 IgG or inhibitor) and assessment of the pretest probability of TTP by clinical assessment and/or the risk assessment models like the PLASMIC or French score. The panel noted how availability and turnaround time of ADAMTS13 test results might affect early diagnosis and management, in particular the use of caplacizumab.

CONCLUSIONS

There is a lack of high-quality evidence to support strong recommendations for the initial diagnosis and management of a suspected TTP. The panel emphasized the importance of obtaining ADAMTS13 testing in a proper clinical context. Future research should focus on how to monitor and act on ADAMTS13 levels during remission.

Potential impact of a delayed ADAMTS13 result in the treatment of thrombotic microangiopathy: an economic analysis

BACKGROUND

A pre-plasma exchange ADAMTS13 measurement differentiates thrombotic thrombocytopenic purpura (TTP) from other forms of thrombotic microangiopathy (TMA). Given that many hospitals do not perform the ADAMTS13 assay in-house and that the turnaround time (TAT) differs among reference laboratories, we performed an analysis investigating the potential impact of a delay in obtaining the results on the healthcare system.

METHODS

An economic model was developed to estimate the impact of a delay in obtaining the pretreatment ADAMTS13 results on patients admitted with TMA with cost (US dollars) as the primary outcome. Incremental cost-effectiveness ratio (ICER) as a composite outcome was calculated from both cost and life days [LDs], an effectiveness surrogate marker. Model parameters were gathered from the medical literature, except for the institutional cost of the ADAMTS13 test.

RESULTS

In patients with TMA, during the 6-day study period, the incremental cost to the healthcare system ranged from approximately $4155 to $5123 for every 1-day delay in obtaining the pre-exchange ADAMTS13 results with virtually no change in the effectiveness marker. The ICER composite outcome established the cost-effectiveness of having a fast TAT for pre-exchange ADAMTS13 results. Probabilistic sensitivity analyses also confirmed the robustness of the model.

CONCLUSIONS

In patients with clinical presentations of TMAs, having a rapid TAT for pre-exchange ADAMTS13 measurement appeared to be cost-effective. If testing cannot be performed in-house, then our findings support the necessity of contracting with a reference laboratory that can reliably provide the result, preferably within 1 day of admission.

Desialylation of platelets induced by Von Willebrand Factor is a novel mechanism of platelet clearance in dengue

Thrombocytopenia and platelet dysfunction are commonly observed in patients with dengue virus (DENV) infection and may contribute to complications such as bleeding and plasma leakage. The etiology of dengue-associated thrombocytopenia is multifactorial and includes increased platelet clearance. The binding of the coagulation protein von Willebrand factor (VWF) to the platelet membrane and removal of sialic acid (desialylation) are two well-known mechanisms of platelet clearance, but whether these conditions also contribute to thrombocytopenia in dengue infection is unknown. In two observational cohort studies in Bandung and Jepara, Indonesia, we show that adult patients with dengue not only had higher plasma concentrations of plasma VWF antigen and active VWF, but that circulating platelets had also bound more VWF to their membrane. The amount of platelet-VWF binding correlated well with platelet count. Furthermore, sialic acid levels in dengue patients were significantly reduced as assessed by the binding of Sambucus nigra lectin (SNA) and Maackia amurensis lectin II (MAL-II) to platelets. Sialic acid on the platelet membrane is neuraminidase-labile, but dengue virus has no known neuraminidase activity. Indeed, no detectable activity of neuraminidase was present in plasma of dengue patients and no desialylation was found of plasma transferrin. Platelet sialylation was also not altered by in vitro exposure of platelets to DENV nonstructural protein 1 or cultured DENV. In contrast, induction of binding of VWF to glycoprotein 1b on platelets using the VWF-activating protein ristocetin resulted in the removal of platelet sialic acid by translocation of platelet neuraminidase to the platelet surface. The neuraminidase inhibitor oseltamivir reduced VWF-induced platelet desialylation. Our data demonstrate that excessive binding of VWF to platelets in dengue results in neuraminidase-mediated platelet desialylation and platelet clearance. Oseltamivir might be a novel treatment option for severe thrombocytopenia in dengue infection.

Dengue is the most common arbovirus infection in the world. A decrease in the number of blood platelets is an almost universal finding in severe dengue. Binding of the coagulation protein von Willebrand factor (VWF) and loss of sialic acid residues from the platelet membrane are two main mechanisms of clearance of senescent platelets under non-pathological conditions. Here, we show that platelets from patients with acute dengue have bound more VWF and have lost sialic acid from their membrane. Sialic acid can be cleaved by the enzyme neuraminidase. We show that neuraminidase activity in the plasma is not increased and that neither dengue virus itself nor nonstructural protein 1, a protein secreted by dengue virus, cleave sialic acid from the platelet membrane. In contrast, binding of VWF to platelets results in translocation of neuraminidase to the platelet membrane and subsequent cleavage of sialic acid. This process could be inhibited by the neuraminidase inhibitor oseltamivir, a commonly used anti-influenza drug. Altogether, our results indicate that VWF binding to platelets is increased in dengue infection, leading to the removal of sialic acid and platelet clearance. Oseltamivir may prevent this process and thus represent a novel treatment option for low platelet numbers in dengue infection.

Pharmacokinetics of plasma infusion in congenital thrombotic thrombocytopenic purpura

Essentials Congenital thrombotic thrombocytopenic purpura (TTP) is primarily treated with plasma infusion. We present a pharmacokinetic analysis of ADAMTS-13 in six patients following plasma infusion. A median half-life of 130 h was demonstrated, ranging between 82.6 and 189.5 h. Investigation of interindividual clearance of ADAMTS-13 is necessary to optimize treatment. SUMMARY: Background Congenital thrombotic thrombocytopenic purpura (TTP) is defined by persistent severe deficiency of ADAMTS-13 in the absence of anti-ADAMTS-13 inhibitory antibodies, confirmed by mutational analysis. Replacement of the missing protease prevents disease relapse, primarily using plasma infusion (PI). Objectives, patients and methods There is scant evidence regarding optimal dose and frequency of treatment, which tends to be empirically guided. We present a pharmacokinetic analysis of ADAMTS-13 in six patients with congenital TTP on established regimes following PI. Results We found a median clearance of 25.41 mL h-1 and half-life of 130 h, ranging between 82.6 and 189.5 h (3.4-7.9 days, respectively). All patients reached baseline ADAMTS-13 level within 7-10 days post-plasma. Median ADAMTS-13 activity peak post-PI was 24.05 IU dL-1 . Variation was related to elimination rate, which, in turn, was affected by weight and metabolism, but not to von Willebrand factor antigen or activity levels. Using the pharmacokinetic parameters, we simulated individualized protocols based on PI dose or frequency to target hypothetical optimal plasma levels of ADAMTS-13 of 10 and 50 IU dL-1 , respectively. Results suggest a target trough ADAMTS-13 of 10 IU dL-1 is feasible but 50 IU dL-1 would not be achievable taking into account volume required. Conclusions Further work is needed to compare treatment of congenital TTP with PI vs. recombinant ADAMTS-13. PI may provide longer duration of ADAMTS-13 effect, but is limited by plasma volume required, whereas recombinant therapy can provide a higher ADAMTS-13 peak. We propose that investigation of interindividual clearance of ADAMTS-13 is necessary to optimize treatment and provide the rationale for dose and frequency of prophylaxis.

Discrepancies between ADAMTS13 activity assays in patients with thrombotic microangiopathies

ADAMTS13 activity assays are sometimes useful in confirming the clinical diagnosis or to distinguish different thrombotic microangiopathies (TMA). We investigated the commonly used clinical assays for ADAMTS13 activity. 159 samples from normal subjects or acquired TMA patients were studied in collagen binding (CBA), Fret and chromogenic peptide substrate assays. Frozen aliquots of pooled normal plasma gave similar values by CBA, Fret-VWF73 peptide, Fret-VWF86 and chromogenic VWF73 ELISA (chr-VWF73). Two lyophilised commercial calibrants gave lower ADAMTS13 activity by CBA than peptide substrate assays. The addition of solid HEPES to normal plasma caused a significant fall in CBA, but not Fret-VWF73 activity and might partly explain the differences, since lyophilised plasmas are often HEPES buffered. Normal plasmas showed good agreement between CBA and Fret assays, although chr-VWF73 gave slightly higher values. In acquired TMA, there was reasonable agreement between assays for samples with <11% ADAMTS13 activity (83% of samples showed agreement between CBA, Fret-VWF73 and chr-VWF73), but samples with moderate deficiency frequently showed lower CBA levels (only 41–52% agreement). However, there were also some discrepancies among the peptide substrate assays, with Fret-VWF86 sometimes giving slightly higher values than the VWF73 substrate assays. An International reference plasma might improve standardisation, but is not the only problem. It is unclear which assay has greatest clinical utility, this may depend on the nature of the sample. If the activity does not match the clinical picture, an alternative method should be performed. Where therapeutic monitoring is required, the same activity assay should be used throughout.

ADAMTS13 test and/or PLASMIC clinical score in management of acquired thrombotic thrombocytopenic purpura: a cost-effective analysis

BACKGROUND

The ADAMTS13 test distinguishes thrombotic thrombocytopenic purpura (TTP) from other thrombotic microangiopathies (TMAs). The PLASMIC score helps determine the pretest probability of ADAMTS13 deficiency. Due to inherent limitations of both tests, and potential adverse effects and cost of unnecessary treatments, we performed a cost-effectiveness analysis (CEA) investigating the benefits of incorporating an in-hospital ADAMTS13 test and/or PLASMIC score into our clinical practice.

STUDY DESIGN AND METHODS

A CEA model was created to compare four scenarios for patients with TMAs, utilizing either an in-house or a send-out ADAMTS13 assay with or without prior risk stratification using PLASMIC scoring. Model variables, including probabilities and costs, were gathered from the medical literature, except for the ADAMTS13 send-out and in-house tests, which were obtained from our institutional data.

RESULTS

If only the cost is considered, in-house ADAMTS13 test for patients with intermediate- to high-risk PLASMIC score is the least expensive option ($4,732/patient). If effectiveness is assessed as measured by the number of averted deaths, send-out ADAMTS13 test is the most effective. Considering the cost/effectiveness ratio, the in-house ADAMTS13 test in patients with intermediate- to high-risk PLASMIC score is the best option, followed by the in-house ADAMTS13 test without the PLASMIC score.

CONCLUSIONS

In patients with clinical presentations of TMAs, having an in-hospital ADAMTS13 test to promptly establish the diagnosis of TTP appears to be cost-effective. Utilizing the PLASMIC score further increases the cost-effectiveness of the in-house ADAMTS13 test. Our findings indicate the benefit of having a rapid and reliable in-house ADAMTS13 test, especially in the tertiary medical center.

Thrombotic thrombocytopenic purpura and other thrombotic microangiopathic hemolytic anemias: diagnosis and classification

Thrombotic microangiopathies (TMAs) include several diseases, most prominently are thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS). TMAs are characterized by profound thrombocytopenia, microangiopathic hemolytic anemia and organ ischemia. In most cases TTP results from deficiency of ADAMTS13, the von Willebrand factor-cleaving protease leading to increase of ultra-large von Willebrand factor (ULVWF) multimers. Congenital TTP is due to mutations in the gene of ADAMTS13 whereas acquired TTP is due to production of autoantibodies against ADAMTS13. In both cases severe deficiency of ADAMTS13 exists. However, the presence of ADAMTS13 activity does not rule out TTP. Diagnostic criteria of TTP are based on clinical features of neurologic and renal disfunction along with anemia and thrombocytopenia, low ADAMTS13 activity, and the presence of ULVWF. The standard treatment of TTP includes plasma exchange, protein A immunoabsobtion, immunosuppressive drugs, CD20 antibodies against B cells, and splenectomy. HUS is commonly caused by infection with Shiga-toxin produced by Escherichia coli. HUS is characterized by thrombocytopenia, anemia, renal impairment and diarrhea. Rarely, atypical HUS appears as a consequence of mutations related to the alternative pathway for the compliment system. Plasmapheresis in HUS is not efficient. Alternatively, plasma therapy and in some cases dialysis are used. TMA diseases may be associated with other infections, bone marrow transplantation, pregnancy, systemic vasculitis, and certain drugs.

Advantages and limits of ADAMTS13 testing in the prognostic assessment of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare but severe disease characterized by mechanical hemolytic anemia and consumptive thrombocytopenia leading to disseminated microvascular thrombosis that causes signs and symptoms of organ ischemia and functional damage. After the elucidation of the pathophysiology of TTP, thanks to the demonstration of the congenital or acquired (autoimmune) plasma deficiency of the von Willebrand factor cleaving metalloprotease A Disintegrin And Metalloprotease with ThromboSpondin 1 repeats (ADAMTS13), a number of laboratory assays for measuring ADAMTS13 and related autoantibodies have been developed. Current knowledge on the diagnostic and prognostic value of ADAMTS13 and anti-ADAMTS13 assays is summarized in this review.

ADAMTS13 activity and antigen during therapy and follow-up of patients with idiopathic thrombotic thrombocytopenic purpura: correlation with clinical outcome

BACKGROUND

The assay for ADAMTS13 activity helps clinicians to confirm the clinical diagnosis of idiopathic thrombotic thrombocytopenic purpura. The clinical value of testing for the antigen level of ADAMTS13 protein is, however, less clear.

DESIGN AND METHODS

In this study, both ADAMTS13 antigen and activity levels were measured in 835 sequential samples from 40 consecutive patients who were followed for an average of 29 months throughout the course of acute episode plasma exchange treatment and clinical remission.

RESULTS

During acute episodes, ADAMTS13 activity was severely deficient while ADAMTS13 antigen levels were more variable, ranging from severely deficient to as high as within the reference range. A severe depletion of ADAMTS13 antigen level during acute disease was, however, statistically associated with disease mortality (P=0.0322). For patients who achieved initial clinical responses, ADAMTS13 antigen levels appeared to be restored faster than ADAMTS13 activity to the normal range. Further analysis demonstrated that the ADAMTS13 antigen level at the time of initial clinical recovery was significantly higher in the patients who subsequently achieved a sustained clinical remission than in the group who soon after had an exacerbation (P=0.0187).

CONCLUSIONS

Our data suggest that evaluation of ADAMTS13 antigen levels during the course of therapy and follow-up may offer additional useful information for the management of patients with thrombotic thrombocytopenic purpura.

Decreased active von Willebrand factor level owing to shear stress in aortic stenosis patients

BACKGROUND

Aortic stenosis patients often show bleeding complications. Previously, a prolonged platelet function analyzer (PFA-100) closure time was observed with plasma of severe aortic stenosis patients. To elucidate a possible role of circulating preactivated von Willebrand factor (VWF), we determined the level of VWF in its active, platelet-binding conformation in plasma of patients with aortic stenosis.

PATIENTS/METHODS

Sixty-two aortic stenosis patients were included in this study. VWF and related parameters were measured, and the results were related to severity of aortic stenosis.

RESULTS

VWF activation factor, indicating the proportion of circulating VWF able to bind to platelets, correlated negatively with peak transvalvular gradient and PFA-100 closure time. No correlation was observed between ADAMTS13 activity and peak transvalvular gradient or PFA-100 closure time. Both VWF antigen and VWF propeptide levels were significantly higher in patients with mild and moderate aortic stenosis, but not in those with severe stenosis.

CONCLUSIONS

Our data demonstrate that the aortic pressure gradient is inversely associated with VWF activation factor, but not with VWF antigen or VWF multimerization in patients with aortic stenosis. These findings might have implications for the bleeding observed in patients with aortic stenosis.

Relationship between ADAMTS13 activity in clinical remission and the risk of TTP relapse

Idiopathic thrombotic thrombocytopenic purpura (TTP) is characterized by frequent recurrences. Effective screening for relapses will enable intervention prior to overt episodes of TTP. The present study used a modified assay to detect ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, 13) activity as low as 0.5%. This analytical improvement permits adequate measurement of ADAMTS13 activity levels in 97% of remission samples used for statistical modelling. ADAMTS13 activity and ADAMTS13 antibody (IgG) were measured in 157 serial samples prospectively collected from 24 TTP patients during periods of clinical remission. These patients were followed-up quarterly for an average of 23 months, during which time nine episodes of TTP relapse occurred among six patients. Finally, logistic regression modelling was used to define the relationship between ADAMTS13 activity levels (0.5-100%) and the probability of TTP relapses. Our data demonstrated that lower ADAMTS13 activity and younger age were significantly associated with higher risk of relapse in the 3 months after specimens were taken. In contrast, ADAMTS13 antibody IgG levels were not predictive of TTP relapses. Identification of low ADAMTS13 activity during clinical remission as a key risk factor for TTP relapses provides a new screening strategy to identify patients who may benefit from prophylactic therapy prior to disease relapses.

ADAMTS13 assays and ADAMTS13-deficient mice

PURPOSE OF REVIEW

Thrombotic thrombocytopenic purpura can be induced by acquired or congenital deficiency of the plasma von Willebrand factor-cleaving protease, ADAMTS13. Measurement of ADAMTS13 activity is important for the diagnosis and treatment of microangiopathies including thrombotic thrombocytopenic purpura. Phenotypic analysis of mice lacking the Adamts13 gene is valuable for understanding the pathogenesis of microangiopathies.

RECENT FINDINGS

The minimum substrate for ADAMTS13 activity was identified as 73 amino acid residues in the A2 domain of von Willebrand factor, called VWF73. Several new assays have been developed using this sequence. The VWF73-based assays are rapid, quantitative, and easy to handle, and are well correlated with the measures from previous assays. Mice lacking the Adamts13 gene were produced. The mice were viable and fertile. They showed a prothrombotic state but no symptoms of spontaneous thrombocytopenia, hemolytic anemia, or microvascular thrombosis were observed.

SUMMARY

VWF73-based ADAMTS13 assays will significantly facilitate the accurate diagnosis of microangiopathies and contribute to the improved clinical treatment of these diseases. Accumulated clinical information on patients with ADAMTS13 deficiency and mice lacking the Adamts13 gene indicates that additional environmental or genetic susceptibility factors are required to trigger thrombotic thrombocytopenic purpura.

Inhibitory autoantibodies against ADAMTS-13 in patients with thrombotic thrombocytopenic purpura bind ADAMTS-13 protease and may accelerate its clearance in vivo

BACKGROUND

Many patients with acquired thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies that may bind and/or inhibit ADAMTS-13 proteolytic activity and accelerate its clearance in vivo.

METHODS

To test this hypothesis, we determined ADAMTS-13 activity and antigen levels in parallel plasma samples from patients clinically diagnosed with TTP. Collagen binding, GST-VWF73 and FRETS-VWF73 assays were used to determine ADAMTS-13 activity and to detect inhibitory autoantibodies. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation plus Western blotting (IP/WB) were used to detect total anti-ADAMTS-13 IgG (inhibitory and non-inhibitory).

RESULTS

Among 40 patients with TTP (21 idiopathic and 19 non-idiopathic), inhibitory autoantibodies were detected (by FRETS-VWF73) in 52% of idiopathic and 0% of non-idiopathic TTP patients. In contrast, non-inhibitory IgG autoantibodies were detected in 29% of idiopathic and 50% of non-idiopathic TTP patients. The concentration of inhibitory IgG autoantibody in idiopathic TTP patients was significantly higher than that of non-inhibitory IgG in either idiopathic or non-idiopathic TTP patients. Idiopathic TTP patients demonstrated significantly reduced ADAMTS-13 activity compared with non-idiopathic patients, but only slightly lower ADAMTS-13 antigen levels. Interestingly, patients with inhibitory autoantibodies exhibited significantly lower ADAMTS-13 antigen levels than those with only non-inhibitory IgG autoantibodies or no autoantibody. Serial plasma exchanges increased levels of ADAMTS-13 activity and antigen concurrently in patients with inhibitory autoantibodies.

CONCLUSION

The identification of severe ADAMTS-13 deficiency and autoantibodies or inhibitors appears to be assay-dependent; the inhibitory IgG autoantibodies, in addition to binding and inhibiting ADAMTS-13 proteolytic activity, may accelerate ADAMTS-13 clearance in vivo.