Von Willebrand factor activity assay errors

Diagnosis and laboratory monitoring of hemophilia A

Acquired hemophilia A (AHA) is a rare disorder in which autoantibodies against factor VIII (FVIII) lead to a bleeding phenotype that varies from life-threatening to no bleeding at all. Prolonged activated partial thromboplastin times (APTT) in patients with a bleeding phenotype should be investigated to rule out AHA and should never be ignored in a preprocedure patient. Most inhibitors in AHA are heat and time dependent, so mixing studies performed only on an immediate mix are not useful: both lupus anticoagulants and treatment with direct oral anticoagulants can coexist with AHA and confound the diagnosis. Assays for intrinsic coagulation factors and von Willebrand factor should always be performed, regardless of the results of mixing studies. A Bethesda or modified Bethesda assay should be performed to quantify any inhibitor, and if susoctocog alfa (rpFVIII) is available, then an assay for cross-reacting antibodies should also be performed. At diagnosis and until complete remission, if the FVIII in the patient sample is >5 IU/dL, heat inactivation should be performed before the inhibitor assays are performed. While there are no conventional tests available to measure the effects of FVIII bypassing therapies, newer therapies may require monitoring, or their effects may need to be considered when choosing appropriate assays. Measurement of rpFVIII requires a 1-stage clotting assay, and measurement of patient FVIII while on emicizumab requires a chromogenic assay that does not contain human FX. Close communication is required between the treating clinicians and the laboratory to ensure that the correct tests are performed while patients are receiving treatments.

Sources and solutions for spurious test results in coagulation

In the coagulation laboratory, much emphasis has been placed on rapid and accurate testing; however, spurious results that are inaccurate and do not reflect the actual status of the patient can potentially lead to an incorrect diagnosis and altered intervention. Errors in coagulation results and interpretation can occur at any point of the process from obtaining the specimen to interpretation and use of the result by the clinician. The main sources of error include the patient's biological and preanalytical variation, analytical testing, and postanalytical use of the reported result(s). This article reviews various sources of error leading to spurious results, providing methods to recognize these aberrant results and presenting solutions for minimizing their occurrence.

Clinically relevant differences between assays for von Willebrand factor activity

Essentials It is unclear whether there are differences between von Willebrand factor (VWF) activity assays. We compared the four most used VWF activity assays in 661 von Willebrand disease (VWD) patients. All assays correlated excellently, but a discrepant classification was seen in 20% of patients. Differences between VWF activity assays have a large impact on the classification of VWD. SUMMARY: Background Measuring the ability of von Willebrand factor (VWF) to bind to platelets is crucial for the diagnosis and classification of von Willebrand disease (VWD). Several assays that measure this VWF activity using different principles are available, but the clinical relevance of different assay principles is unclear. Objective To compare the four most widely used VWF activity assays in a large VWD patient population. Methods We measured VWF:RCo (ristocetin to activate VWF + whole platelets), VWF:GPIbR (ristocetin + platelet glycoprotein Ib receptor [GPIb] fragments), VWF:GPIbM (gain-of-function GPIb fragments that bind VWF spontaneously without ristocetin) and VWF:Ab (monoclonal antibody directed against the GPIb binding epitope of VWF to mimic platelets) in 661 VWD patients from the nationwide 'Willebrand in the Netherlands' (WiN) Study. Results All assays correlated excellently (Pearson r > 0.9), but discrepant results led to a different classification for up to one-fifth of VWD patients. VWF:RCo was not sensitive enough to classify 18% of patients and misclassified half of genotypic 2B VWD patients, especially those with p.Arg1306Trp. VWF:GPIbR was more sensitive, accurately classified the vast majority of patients, and was unaffected by the p.Asp1472His variant that causes artificially low VWF:RCo. VWF:GPIbM was the most precise assay but misclassified over a quarter of genotypic 2A, 2B and 3 patients. VWF:Ab, often not considered an actual VWF activity assay, performed at least equally to the other assays with regard to accurate VWD classification. Conclusion Although the different VWF activity assays are often considered similar, differences between assays have a large impact on the classification of VWD.

Comparison of von Willebrand factor (VWF) activity levels determined by HemosIL AcuStar assay and HemosIL LIA assay with ristocetin cofactor assay by aggregometry

INTRODUCTION

Diagnosis of von Willebrand disease (VWD) requires quantitative as well as qualitative determination of von Willebrand factor (VWF) levels. For functional assessment of VWF, ristocetin cofactor assay by aggregometry is considered to be the gold standard. However, need for technical expertise, labour intensiveness, difficult standardization and high intra- and inter- assay variabilities are some of the limitations of this methodology. Various assays for determination of VWF adhesive function using different methodologies have been developed in recent years.

AIM

To evaluate the HemosIL AcuStar chemiluminescence assay (VWF:RCo[Acu]) and the HemosIL latex immunoassay (VWF:act) as diagnostic tests for VWD and identification of type 2 VWD in comparison with the ristocetin cofactor assay performed by aggregometry (VWF:RCo[Agg]).

METHODS

Results from 96 samples analysed by VWF:RCo[Acu] and 128 samples by VWF:act were compared with VWF:RCo[Agg]. Sixty of these samples (25 normal, 17 type 1 and 18 type 2) were analysed by all three assays.

RESULTS

VWF:RCo[Acu] showed excellent agreement with VWF:RCo[Agg], and readily identified all type 2 VWD samples tested. VWF:act showed reasonable agreement with VWF:RCo[Agg] for most patients, but had a slightly lower sensitivity for detection of type 2 VWD.

CONCLUSION

VWF:RCo[Acu] assay has the potential to replace VWF:RCo[Agg] for the diagnosis of VWD.