American Board of Internal Medicine (ABIM) Certification Practice Exam

Which test may not detect mild von Willebrand disease (vWD)?

1. PT test

2. PFA-100® assay

3. Factor VIII assay

4. Complete blood count

The correct answer is: PFA-100® assay

The PFA-100® assay is specifically designed to assess platelet function and is often influenced by von Willebrand factor (vWF) levels. However, in cases of mild von Willebrand disease (vWD), the vWF levels may be only slightly decreased or within the normal range, which may lead to a false negative result in the PFA-100® assay. This means that even if a patient has mild vWD, the test may indicate that their platelet function is normal, thus failing to identify the underlying issue. In contrast, the other tests listed are more reliable in detecting aspects of vWD. For example, the PT test primarily measures the extrinsic and common pathway of coagulation, which is generally unaffected by mild vWD. The Factor VIII assay can help measure the levels of Factor VIII, which is often decreased in vWD, and a complete blood count can be used to assess platelet count and morphology, although it is not a direct measure of vWF. Understanding the specific limitations of the PFA-100® assay is crucial for effectively diagnosing and managing von Willebrand disease, especially in its milder forms, where clinical suspicion is necessary for appropriate testing.