Interpreting tests for von Willebrand disease (VWD)

Von Willebrand disease (VWD) is one of the most common genetic bleeding disorders and, yet, it tends to be underdiagnosed. By the end of this video from our Hematology and Coagulation Essentials course, you'll be able to understand and interpret the various tests available for the subtypes of VWD.

Demystify hematology and coagulation with our fascinating Hematology and Coagulation Essentials course. Hematological and coagulation disorders can be difficult to diagnose. By the end of this course, you'll be able to confidently assess the most important diagnostic aspects of hematology and coagulation, and shine during your next night on call! Never again will you wonder what a prolonged PTT really means, how to differentiate between a factor deficiency and an inhibitor, or what test to order next.

Thrombocytopenia in von Willebrand disease

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Here, we are going to discuss the various tests for von Willebrand disease. Patients with von Willebrand disease may demonstrate thrombocytopenia in cases of type 2B or pseudo-von Willebrand disease. In both of these types, there is increased binding of von Willebrand factor with the platelets. This results in increased consumption of the platelets and resultant thrombocytopenia.

Bleeding time and clotting test results

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Bleeding time, although not a test ordered often, will be prolonged. This is because in von Willebrand disease there is defective platelet adhesion. In von Willebrand disease, there are reduced levels of factor VIII. This will result in prolonged PTT [partial thromboplastin time]. A mixing study should show correction.

The von Willebrand disease panel

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There is a test called the von Willebrand disease panel, and this panel consists of three tests. The first is measuring factor VIII levels. The second is measuring von Willebrand factor antigen levels, and the third is measuring von Willebrand factor functional activity. This is also known as ristocetin cofactor activity.

Panel results by disease subtype

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Type 1 von Willebrand disease is a mild abnormality. In this type, there is mild deficiency of all three levels tested. In type 3, there is severe deficiency of all three factors. In type 2, results vary according to the subtype. In type 2N, factor VIII levels are low. This is because in type 2N, von Willebrand factor and factor VIII cannot bind properly. Factor VIII is thus destroyed. In type 2M, von Willebrand factor antigen levels will be normal; functional activity and factor VIII levels are low. This is because in type 2M the multimers are present but do not work properly. In types 2A and 2B, the results of the von Willebrand disease panel varies.

Ristocetin-induced platelet aggregation

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Now, I would like to discuss ristocetin-induced platelet aggregation. Ristocetin was originally an antibiotic, which is no longer used; however, it enhances the interaction of von Willebrand factor with GP1b. Ristocetin induces von Willebrand factor and platelet GP1b to interact and results in platelet clumping. This is normally seen in healthy people with a high dose of ristocetin but not with a low dose of ristocetin.

Ristocetin test results in von Willebrand disease

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In von Willebrand disease, there is reduced aggregation of platelets with a high dose of ristocetin. The low dose is not affected. However, in the subtype type 2B, even with a low dose of ristocetin, platelet aggregation is observed. This is because in type 2B, the von Willebrand factor has increased affinity for platelet GP1b, and even a small dose of ristocetin is enough to cause aggregation.

This is a graph of a patient with ristocetin-induced platelet aggregation, and it shows that with ristocetin, the amount of platelet aggregation is negligible. However, platelet aggregation with ADP, collagen, and epinephrine are normal. This is the characteristic pattern of ristocetin-induced platelet aggregation for von Willebrand disease.

Von Willebrand factor multimer analysis

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Another test we can order is what’s called multimer analysis. Multimer analysis is done by electrophoresis, and all the von Willebrand factor multimers line up according to their size. This allows us to analyze if there is global deficiency of the multimers or deficiency of any particular subtype of multimers.

Multimer patterns by disease subtype

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Here, N stands for normal. If we compare type 1 von Willebrand disease to a normal person, we can see that there is mild deficiency of all the subtypes of multimers. In type 3, there is severe deficiency of all the subtypes of multimers. In type 2A, there is selective loss of the large and intermediate multimers. In type 2B, there is selective loss of the large multimers. Individuals with type 2M and type 2N would have a normal multimer analysis.

Just wanted to remind you that in type 2M, the multimers are present but they do not function well. In type 2N, everything related to von Willebrand disease is fine; however, the von Willebrand factor and factor VIII cannot bind.

Factor VIII binding in type 2N disease

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How would we go about diagnosing type 2N? There is a factor VIII–von Willebrand factor binding assay for this condition. In type 2N, the binding of von Willebrand factor and factor VIII is abnormal. This results in easy degradation of factor VIII. Low levels of factor VIII in such patients make us think of hemophilia. The factor VIII–von Willebrand factor binding assay is an ELISA-based test and is usually a send out to a reference laboratory.