Platelet testing plays a role in the diagnosis of VWD. Although platelet testing is of much lower volume than coagulation assays like the PT and aPTT, it is relatively simple to perform and small analyzers can be purchased to automate the testing. The old Ivy and Duke bleeding time assays in which a patient is cut and a stopwatch is used to measure the time until the patient stops bleeding, are now considered obsolete. More precise platelet activity assays such as the method pictured, are routinely used to assess inherited, acquired, or drug-induced platelet dysfunctions.
Platelet function tests use various activators to induce platelet clotting (see figure). Collagen, ADP, ristocetin, and epinephrine should all induce a platelet plug to form. For the method pictured, a sample should have a 'closure time' in the presence of collagen and Epinephrine of <180 seconds. If the collagen/epinephrine closure time is prolonged, an ADP platelet test is performed. If the ADP result is normal (<120 seconds) then aspirin-induced platelet dysfunction is most likely. If both tests are prolonged then one or more of the following may be present:
- Anemia (hematocrit <28%)
- Thrombocytopenia (platelet count <100 x 109/L)
- A significant platelet function defect, most commonly, VWD
