The active conformation of von Willebrand factor in patients with thrombotic thrombocytopenic purpura in remission

Summary.  Background:  Functional deficiency of ADAMTS13 in thrombotic thrombocytopenic purpura (TTP) patients is associated with circulating ultralarge von Willebrand factor (VWF) molecules that display spontaneous platelet-binding capacities. Upon remission, however, ADAMTS13 activity does not always return to baseline. Objective:  To study ADAMTS13 and VWF-related features in TTP patients in remission. Methods:  ADAMTS13 activity, anti-ADAMTS13 antibodies, VWF antigen, ultralarge VWF and levels of VWF that circulate in a glycoprotein Ibα-binding conformation were determined in plasma samples of 22 acquired TTP patients in remission between 1 month and 6 years after achieving remission. The composition of active multimers was investigated with a novel immunoprecipitation assay based on monoclonal antibody AU/VWF-a12, which specifically recognizes the active conformation of VWF. Results:  ADAMTS13 activity was undetectable in 23% of the patients, even years after they had achieved remission, and lack of ADAMTS13 activity was associated with increased active VWF levels and the presence of ultralarge VWF multimers. Active VWF levels and ultralarge VWF were also associated with blood groups. Results from immunoprecipitation experiments revealed the full range of multimers to be present. Conclusion : ADAMTS13 deficiency and the concurrent presence of ultralarge VWF and increased active VWF levels can be detected in TTP patients for years after they have achieved remission. Immunoprecipitation results suggest that the active conformation of VWF may be present in the lower molecular weight multimers, but future studies are necessary to confirm our findings.     

ADAMTS13: origins,applications, and prospects

ADAMTS13 is an enzyme that acts by cleaving prothrombotic von Willebrand factor (VWF) multimers from the vasculature in a highly regulated manner. In pathologic states such as thrombotic thrombocytopenic purpura (TTP) and other thrombotic microangiopathies (TMAs), VWF can bind to the endothelium and form large multimers. As the anchored VWF chains grow, they provide a greater surface area to bind circulating platelets (PLTs), generating unique thrombi that characterize TTP. This results in microvasculature thrombosis, obstruction of blood flow, and ultimately end‐organ damage. Initial presentations of TTP usually occur in an acute manner, typically developing due to an autoimmune response toward, or less commonly a congenital deficiency of, ADAMTS13. Triggers for TMAs that can be associated with ADAMTS13 deficiency, including TTP, have been linked to events that place a burden on hemostatic regulation, such as major trauma and pregnancy. The treatment plan for cases of suspected TTP consists of emergent therapeutic plasma exchange that is continued on a daily basis until normalization of PLT counts. However, a subset of these patients does not respond favorably to standard therapies. These patients necessitate a better understanding of their diseases for the advancement of future therapeutic options. Given ADAMTS13’s key role in the cleavage of VWF and the prevention of PLT‐rich thrombi within the microvasculature, future treatments may include anti‐VWF therapeutics, recombinant ADAMTS13 infusions, and ADAMTS13 expression via gene therapy.     

Cloning, expression and functional characterization of the full-length murine ADAMTS13.

Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts13 gene from BALB/c mice liver poly A+ mRNA. Murine ADAMTS13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS13 was inhibited by EDTA and the plasma from a TTP patient.     

ADAMTS13 activity and the presence of acquired inhibitors in human immunodeficiency virus-related thrombotic thrombocytopenic purpura

BACKGROUND: Little is known about the pathophysiology of human immunodeficiency virus (HIV)-related thrombotic thrombocytopenic purpura (TTP). It is generally assumed that acquired ADAMTS13 deficiency is due to the presence of autoantibody inhibitors, but limited data are available regarding ADAMTS13 activity and inhibitors in such patients. STUDY DESIGN AND METHODS: By use of a collagen-binding assay, ADAMTS13 activity was analyzed at presentation in 20 patients with HIV-related TTP. The presence of inhibitors in patients with reduced ADAMTS13 activity was assessed with mixing studies. The correlation between ADAMTS13 activity and inhibitors and other laboratory and clinical parameters was assessed. RESULTS: The patients fell clearly into two groups with regard to ADAMTS13 activity. Six patients (30%) had activity within the normal range, whereas the remaining 14 patients had severely reduced levels. Of the patients with reduced activity, only 5 patients had a detectable inhibitor whereas 8 showed no evidence of an inhibitor. There was significant correlation between normal ADAMTS13 activity and lower CD4 counts (p = 0.049). von Willebrand factor (VWF) antigen levels were significantly higher in patients with reduced ADAMTS13 activity (p = 0.03). Low activity in the absence of a detectable inhibitor was associated with significantly higher D-dimer levels (p = 0.01) and worse clinical outcome. CONCLUSION: The heterogeneity with regard to ADAMTS13 activity and the absence of inhibitors in the majority of patients indicate that other factors are important in the pathogenesis of HIV-related TTP. VWF release and localized coagulation activation due to direct viral or cytokine-mediated endothelial cell injury is likely to be playing a major role.     

Blood group O and black race are independent risk factors for thrombotic thrombocytopenic purpura associated with severe ADAMTS13 deficiency

BACKGROUND: It has been postulated that blood group O subjects may be partially protected against thrombotic thrombocytopenic purpura (TTP) because they have lower plasma levels of von Willebrand factor. STUDY DESIGN AND METHODS: The Oklahoma TTP Registry enrolled 301 consecutive patients from November 13, 1995 (when systematic ADAMTS13 measurements began), through 2009; 281 (93%) patients had ADAMTS13 measurements. Patients were designated as having severe ADAMTS13 deficiency when the activity measurement by either method was less than 10%. ABO blood group was determined in all 281 patients. The observed frequency of blood group O was compared to the expected frequency. The association between severe ADAMTS13 deficiency and blood group, race, sex, and age were analyzed by logistic regression. RESULTS: The frequency of blood group O was unexpectedly and significantly greater than the race‐ethnicity–adjusted expected frequency in 65 patients with severe ADAMTS13 deficiency (60.0% vs. 47.4%, p = 0.042) but not in 216 patients without severe ADAMTS13 deficiency (44.9% vs. 46.5%, p = 0.639). Blood group O and race‐ethnicity were independently associated with severe ADAMTS13 deficiency among patients with TTP. The probability for severe ADAMTS13 deficiency was 45.8% with O and 32.1% with non‐O blood groups for black patients and 24.1% with O and 15.1% with non‐O blood groups for white patients. CONCLUSION: Among patients with TTP and severe ADAMTS13 deficiency the relative frequency of patients with blood group O was greater than expected, suggesting that blood group O may be a risk factor for TTP associated with severe ADAMTS13 deficiency.     

Von Willebrand factor and ADAMTS13 balancing primary haemostasis

Von Willebrand factor (VWF) is an adhesive, multi-functional huge multimerized protein with multiple domains harboring binding sites for collagen, platelet glycoprotein receptors and coagulation factor VIII (FVIII). The functional domains enable VWF to bind to the injured vessel wall, to recruit platelets to the site of injury by adhesion and aggregation and to bind and protect FVIII, an important cofactor of the coagulation cascade. VWF function in primary haemostasis is located in particular in the arterial and micro-circulation. This environment is exposed to high shear forces with hydrodynamic shear rates ranging over several orders of magnitude from 10?1 to 10? s-1 and requires particular mechanisms to enable platelet adhesion and aggregation under these variable conditions. The respective VWF function is strictly correlating with its multimer size. Lack or reduction of large VWF multimers is seen in patients with von Willebrand disease (VWD) type 2A which correlates with reduction of both VWF:platelet GPIb-binding and VWF:collagen binding and a bleeding phenotype. To prevent unlimited platelet adhesion and aggregation which is the cause of the microangiopathic disorder thrombotic thrombocytopenic purpura (TTP), VWF function is regulated by its specific protease ADAMTS13. Whereas a particular susceptibility of VWF to ADAMTS13 proteolysis is the cause of a frequent VWD type 2A phenotype, lack or dysfunction of ADAMTS13, either acquired by ADAMTS13 antibodies or by inherited ADAMTS13 deficiency (Upshaw-Schulman Syndrome), causes TTP. Therefore VWD and TTP represent the opposite manifestations of VWF related disorders, tightly linked to each other.     

Recombinant ADAMTS13 normalizes von Willebrand factor‐cleaving activity in plasma of acquired TTP patients by overriding inhibitory antibodies

Summary. Background: Severe deficiency of the von Willebrand factor (VWF)‐cleaving protease ADAMTS13 as observed in acquired thrombotic thrombocytopenic purpura (TTP) is caused by inhibitory and non‐inhibitory autoantibodies directed against the protease. Current treatment with plasma exchange is considered to remove circulating antibodies and to concurrently replenish the deficient enzyme. Objectives: To explore the use of recombinant ADAMTS13 (rADAMTS13) as a potential therapeutic agent in acquired TTP, we investigated its efficacy in normalizing VWF‐cleaving activity in the presence of ADAMTS13 inhibitors. Methods: Thirty‐six plasma samples from TTP patients were adjusted to predefined inhibitor titers, and recovery of ADAMTS13 activity was analyzed following supplementation with rADAMTS13. Results: We showed a linear relation between the inhibitor titer measured and effective rADAMTS13 concentration necessary for reconstitution of VWF‐cleaving activity in the presence of neutralizing autoantibodies. Conclusions: Our results support the further investigation of the potential therapeutic applicability of rADAMTS13 as an adjunctive therapy in acquired TTP.     

External validation of the PLASMIC score: a clinical prediction tool for thrombotic thrombocytopenic purpura diagnosis and treatment

Essentials

• Severe ADAMTS‐13 deficiency is key to thrombotic thrombocytopenic purpura (TTP) diagnosis.
• PLASMIC score predicts ADAMTS‐13 deficiency in suspected TTP with high discrimination.
• PLASMIC score is more generalizable with fewer missing data than alternative clinical scores.
• PLASMIC score identifies a subgroup of patients lacking significant response to plasma exchange.

Summary

Background

The PLASMIC score was recently published to distinguish patients with severe ADAMTS‐13 deficiency from those without for early identification of thrombotic thrombocytopenia purpura (TTP).

Objective

We performed an independent external validation of the PLASMIC score for clinical prediction of severe ADAMTS‐13 deficiency.

Patients/Methods

We studied an independent cohort of 112 consecutive hospitalized patients with suspected thrombotic microangiopathy and appropriate ADAMTS‐13 testing (including 21 patients with TTP diagnosis).

Results

The PLASMIC score model predicted severe ADAMTS‐13 deficiency with a c statistic of 0.94 (0.88–0.98). When dichotomized at high (score 6–7) vs. low‐intermediate risk (score 0–5), the model predicted severe ADAMTS‐13 deficiency with positive predictive value of 72%, negative predictive value of 98%, sensitivity of 90% and specificity of 92%. In the low‐intermediate risk group (score 0–5) there was no significant improvement in overall survival associated with plasma exchange.

Conclusions

The PLASMIC score model had excellent applicability, discrimination and calibration for predicting severe ADAMTS‐13 deficiency. The clinical algorithm allowed identification of a subgroup of patients who lacked a significant response to empiric treatment.     

ADAMTS‐13 assays in thrombotic thrombocytopenic purpura

Summary. ADAMTS‐13, the thirteenth member of the ADAMTS (A Disintegrin And Metalloprotease with Thrombo‐Spondin 1 repeats) family, is the plasma metalloprotease responsible for regulating the multimeric structure of VWF. In congenital or acquired deficiency it is actively involved in the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a rare but life threatening disease characterized by microangiopathic haemolytic anaemia and consumptive thrombocytopenia leading to disseminated microvascular thrombosis and variable signs and symptoms of organ ischemia and damage. In the last few years, a number of in house and commercial laboratory assays for ADAMTS‐13 and its autoantibodies have been developed. The features and clinical utility of ADAMTS‐13 assays are summarized in this review.     

Thrombotic thrombocytopenic purpura

This overview summarizes the history of thrombotic thrombocytopenic purpura (TTP) from its initial recognition in 1924 as a most often fatal disease to the discovery in 1997 of ADAMTS-13 deficiency as a major risk factor for acute disease manifestation. The cloning of the metalloprotease, ADAMTS-13, an essential regulator of the extremely adhesive unusually large von Willebrand factor (VWF) multimers secreted by endothelial cells, as well as ADAMTS-13 structure and function are reviewed. The complex, initially devised assays for ADAMTS-13 activity and the possible limitations of static in vitro assays are described. A new, simple assay using a recombinant 73-amino acid VWF peptide as substrate will hopefully be useful. Hereditary TTP caused by homozygous or double heterozygous ADAMTS-13 mutations and the nature of the mutations so far identified are discussed. Recognition of this condition by clinicians is of utmost importance, because it can be easily treated and--if untreated--frequently results in death. Acquired TTP is often but not always associated with severe, autoantibody-mediated ADAMTS-13 deficiency. The pathogenesis of cases without severe deficiency of the VWF-cleaving protease remains unknown, affected patients cannot be distinguished clinically from those with severely decreased ADAMTS-13 activity. Survivors of acute TTP, especially those with autoantibody-induced ADAMTS-13 deficiency, are at a high risk for relapse, as are patients with hereditary TTP. Patients with thrombotic microangiopathies (TMA) associated with hematopoietic stem cell transplantation, neo-plasia and several drugs, usually have normal or only moderately reduced ADAMTS-13 activity, with the exception of ticlopidine-induced TMA. Diarrhea-positive-hemolytic uremic syndrome (D+ HUS), mainly occurring in children is due to enterohemorrhagic Escherichia coli infection, and cases with atypical, D- HUS may be associated with factor H abnormalities. Treatment of acquired idiopathic TTP involves plasma exchange with fresh frozen plasma (FFP), and probably immunosuppression with corticosteroids is indicated. We believe that, at present, patients without severe acquired ADAMTS-13 deficiency should be treated with plasma exchange as well, until better strategies become available. Constitutional TTP can be treated by simple FFP infusion that rapidly reverses acute disease and--given prophylactically every 2-3 weeks--prevents relapses. There remains a large research agenda to improve diagnosis of TMA, gain further insight into the pathophysiology of the various TMA and to improve and possibly tailor the management of affected patients.     

Thrombotic thrombocytopenic purpura in childhood

Thrombotic thrombocytopenic purpura (TTP) is a micro-angiopathic disease due to deficiency of the specific VWF cleaving protease (VWF-CP) ADAMTS13. The acquired form is caused by autoantibodies against VWF-CP, whereas mutations of the ADAMTS13 gene are responsible for inherited TTP. In childhood both forms exist with predominance of inherited TTP. The phenotype of TTP in childhood can be rather variable. Besides the classical clinical picture, abortive forms may occur that can delay the identification of patients at risk. The patients are frequently assumed to suffer from idiopathic thrombocytopenia (ITP) or Evans syndrome. Further efforts are necessary to accelerate correct diagnosis and to establish a risk-adapted prophylactic therapy.     

A shear-based assay for assessing plasma ADAMTS13 activity and inhibitors in patients with thrombotic thrombocytopenic purpura

BACKGROUND: Severe deficiency of plasma ADAMTS13 activity is a frequent finding in patients with hereditary and acquired thrombotic thrombocytopenic purpura (TTP). To date, plasma ADAMTS13 activity is determined by cleavage of either predenatured von Willebrand factor (VWF) or small peptides derived from the VWF‐A2 domain. The physiologic relevance of the assay results is uncertain. STUDY DESIGN AND METHODS: We sought to develop a novel shear‐based assay to assess plasma ADAMTS13 activity and inhibitors. We also compared this assay with a fluorogenic peptide assay. RESULTS: We found that an incubation of purified plasma VWF with 0.5 to 1.0 µL of citrated plasma under constant vortexing at 2500 rpm for 60 minutes in the presence of 5 mmol/L CaCl₂ and 1.7 µmol/L ZnCl₂ and low concentration of NaCl resulted in the maximal cleavage of VWF. The cleavage product could be separated by a 2.5% agarose gel and detected by Western blotting. The assay revealed that plasma and recombinant ADAMTS13 are highly sensitive to inhibition by zinc and chloride ions. Under the optimal conditions, the shear‐based assay appeared to be more sensitive than the guanidine‐denaturization assay for determining plasma ADAMTS13 activity. CONCLUSIONS: Our fluid shear‐based assay may be useful for investigating basic biologic function and regulation of ADAMTS13 metalloprotease. It may also be applicable for assessing plasma ADAMTS13 activity and inhibitors in TTP patients.     

Novel monoclonal antibody-based enzyme immunoassay for determining plasma levels of ADAMTS13 activity

BACKGROUND: ADAMTS13 specifically cleaves unusually large von Willebrand factor (VWF) multimers, which induce platelet thrombi formation under high shear stress. ADAMTS13 activity is deficient in patients with thrombotic thrombocytopenic purpura (TTP). The determination of plasma levels of ADAMTS13 activity is a prerequisite for a differential diagnosis of thrombotic microangiopathies. Here, a unique and highly sensitive enzyme immunoassay (EIA) of ADAMTS13 activity is described. STUDY DESIGN AND METHODS: ADAMTS13 hydrolyzes the peptide bond between Y1605 and M1606 of VWF. In this assay, a recombinant fusion protein (GST-VWF73-His) is used as a substrate. A panel of mouse monoclonal antibodies (MoAbs) that specifically recognizes Y1605, which is the C-terminal edge residue of the VWF-A2 domain and is generated by the enzymatic cleavage, has been produced. These antibodies were prepared with a synthetic decapeptide, termed N-10 (1596-DREQAPNLVY-1605), as the immunogen. Twenty-six clones specific to N10 were obtained, and one anti-N10 MoAb was used in this study. RESULTS: With horseradish peroxidase-conjugated anti-N10 MoAb, a standard enzyme assay was established. This assay was highly sensitive, and the detection limit was 0.5 percent of the normal. Further, an inhibitor of ADAMTS13 was measured to a level of 0.1 Bethesda units per mL. ADAMTS13 activity was measured in 20 patients with Upshaw-Schulman syndrome, a congenital TTP, and 61 acquired TTP patients. The activity measured by this assay and by the classic VWF multimer assay showed high correlation. CONCLUSION: A convenient and highly sensitive EIA for ADAMTS13 activity has been established. This assay can be introduced for routine laboratory work in transfusion medicine.     

The role of ADAMTS‐13 in the coagulopathy of sepsis

Summary

The interaction between platelets and the vessel wall is mediated by various receptors and adhesive proteins, of which von Willebrand factor (VWF) is the most prominent. The multimeric size of VWF is an important determinant of a more intense platelet–vessel wall interaction, and is regulated by the VWF‐cleaving protease ADAMTS‐13. A deficiency in ADAMTS‐13 leads to higher concentrations of ultralarge VWF multimers and pathological platelet–vessel wall interactions, in its most typical and extreme form leading to thrombocytopenic thrombotic purpura, a thrombotic microangiopathy characterized by thrombocytopenia, non‐immune hemolysis, and organ dysfunction. Thrombotic microangiopathy associated with low levels of ADAMTS‐13 may be a component of the coagulopathy observed in patients with sepsis. Here, we review the potential role of ADAMTS‐13 deficiency and ultralarge VWF multimers in sepsis, and their relationship with sepsis severity and prognosis. In addition, we discuss the possible benefit of restoring ADAMTS‐13 levels or reducing the effect of ultralarge VWF as an adjunctive treatment in patients with sepsis.

     

Ten candidate ADAMTS13 mutations in six French families with congenital thrombotic thrombocytopenic purpura (Upshaw–Schulman syndrome)

Summary.  ADAMTS13, the specific von Willebrand factor (VWF)-cleaving metalloprotease, prevents the spontaneous formation of platelet thrombi in the microcirculation by degrading the highly adhesive ultralarge VWF multimers into smaller forms. ADAMTS13 severe enzymatic deficiency and mutations have been described in the congenital thrombotic thrombocytopenic purpura (TTP or Upshaw–Schulman syndrome), a rare and severe disease related to multivisceral microvascular thrombosis. We investigated six French families with congenital TTP for ADAMTS13 enzymatic activity and gene mutations. Six probands with congenital TTP and their family were tested for ADAMTS13 activity in plasma using a two-site immunoradiometric assay and for ADAMTS13 gene mutations using polymerase chain reaction and sequencing. ADAMTS13 activity was severely deficient (< 5%) in the six probands and one mildly symptomatic sibling but normal (> 50%) in all the parents and the asymptomatic siblings. Ten novel candidate ADAMTS13 mutations were identified in all families, showing either a compound heterozygous or a homozygous status in all probands plus the previous sibling and a heterozygous status in the parents. The mutations were spread all over the gene, involving the metalloprotease domain (I79M, S203P, R268P), the disintegrin domain (29 bp deletion in intron/exon 8), the cystein-rich domain (acceptor splice exon 12, R507Q), the spacer domain (A596V), the 3rd TSP1 repeat (C758R), the 5th TSP1 repeat (C908S) and the 8th TSP1 repeat (R1096stop). This study emphasizes the role of ADAMTS13 mutations in the pathogenesis of congenital TTP and suggests that several structural domains of this metalloprotease are involved in both its biogenesis and its substrate recognition process.     

Systemic antithrombotic effects of ADAMTS13

The metalloprotease ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type I repeats 13) cleaves highly adhesive large von Willebrand factor (VWF) multimers after their release from the endothelium. ADAMTS13 deficiency is linked to a life-threatening disorder, thrombotic thrombocytopenic purpura (TTP), characterized by platelet-rich thrombi in the microvasculature. Here, we show spontaneous thrombus formation in activated microvenules of Adamts13-/- mice by intravital microscopy. Strikingly, we found that ADAMTS13 down-regulates both platelet adhesion to exposed subendothelium and thrombus formation in injured arterioles. An inhibitory antibody to ADAMTS13 infused in wild-type mice prolonged adhesion of platelets to endothelium and induced thrombi formation with embolization in the activated microvenules. Absence of ADAMTS13 did not promote thrombi formation in alphaIIbbeta3 integrin-inhibited blood. Recombinant ADAMTS13 reduced platelet adhesion and aggregation in histamine-activated venules and promoted thrombus dissolution in injured arterioles. Our findings reveal that ADAMTS13 has a powerful natural antithrombotic activity and recombinant ADAMTS13 could be used as an antithrombotic agent.     

Complement activation in thrombotic thrombocytopenic purpura

Summary. Background: Ultra‐large von Willebrand factor and deficiency of its cleaving protease are important factors in the events leading to thrombotic microangiopathy; however, the mechanisms involved are only partly understood. Whereas pathological activation of the alternative complement pathway is linked to atypical hemolytic uremic syndrome, the role of complement activation in thrombotic thrombocytopenic purpura (TTP) is unknown. The aim of this study was to investigate whether signs of complement activation are characteristic of TTP. Patients and methods: Twenty‐three patients with TTP (18 women, median age 38 years) and 17 healthy controls (13 women, median age 38 years) were included. Complement parameters (C3, Factors H, I, B and total alternative pathway activity) together with complement activation fragments (C3a) or complexes (C1rs‐INH, C3bBbP, sC5b9) were measured by ELISA or RID. ADAMTS13 activity and anti‐ADAMTS13 inhibitory antibodies were measured by the VWF‐FRET73 assay. Results: Increased levels of C3a, and SC5b9 were observed in TTP during acute episodes, as compared with healthy controls. Decreased complement C3 levels indicative of complement consumption occurred in 15% of acute TTP patients. Significant decrease of complement activation products C3a and SC5b9 was observed during plasma exchange (PEX). The sustained presence of anti‐ADAMTS13 inhibitory antibodies in complete remission was associated with increased complement activation. Conclusion: These data document in an observational study the presence of complement activation in TTP. Further investigation is needed to determine its potential pathogenetic significance.     

Is it quinine TTP/HUS or quinine TMA? ADAMTS13 levels and implications for therapy

Thrombocytopenia with or without microangiopathy following quinine is often referred to as quinine “hypersensitivity.” When schistocytes are present it is frequently termed “quinine‐associated TTP/HUS.” A severe deficiency of the vWF‐cleaving protease, ADAMTS13, is associated with idiopathic TTP. A previous study of patients with “quinine‐associated TTP/HUS” found that ADAMTS13 activities were not abnormal in 12/12 patients. A retrospective review of TTP patients with quinine‐associated thrombotic microangiopathy (TMA) for whom ADAMTS13 was measured before plasma exchange was performed. Six patients were identified. All were females (age range: 43 to 73, mean = 61.7 years) and had taken quinine for leg cramps. Four of the six experienced renal failure requiring dialysis. Five of the patients had D‐Dimers levels measured, all were elevated. In four patients the levels were ≥18 times the upper limit of normal. ADAMTS13 was normal in four patients and mildly decreased in two patients. We conclude that while thrombocytopenia and schistocytosis can be seen in quinine‐associated TTP/HUS, the pathophysiology seems to be distinct from that seen in most cases of idiopathic TTP (i.e., severely decreased ADAMTS13 with an inhibitor). We recommend that a TMA in association with quinine be consistently referred to as quinine‐associated thrombotic microangiopathy (quinine‐TMA) to better distinguish this entity from idiopathic TTP. The use of plasma exchange in quinine‐TMA is called into question. J. Clin. Apheresis, 2009. © 2009 Wiley‐Liss, Inc.     

Update on thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathy, which is classically associated with signs and symptoms of fever, thrombocytopenia, neurologic deficits, hemolytic anemia, and renal failure. It is caused by a deficiency of A Disintegrin-like And Metalloprotease with a ThromboSpondin type1 motif 13 (ADAMTS13), which may be an inherited disorder, but more commonly is an acquired disease due to autoantibodies directed against ADAMTS13. Low ADAMTS13 levels result in increased ultra-large von Willebrand factor multimers, which induce platelet adhesion and thrombosis. Plasma exchange therapy is the standard of care, and has greatly reduced morbidity and mortality. A recent TTP case is reviewed, and treatments for recurrent or refractory TTP are summarized. A scoring system using clinical and laboratory parameters to evaluate which suspected TTP patients will benefit from plasma exchange therapy is also discussed.