Testing for platelet dysfunction usually doesn’t change the therapy used, since the end result is the dissolution or prevention of clots. However, it is important to know why a patient experiences blood clots that are life-threatening.
- Complete Blood Count (CBC): a manual or automated platelet count from a peripheral smear of blood on a slide.
- Bleeding time is determined by making a small cut and timing how long hemostasis takes. These tests the interaction of platelets with blood vessel wall injury
- Platelet Function Analyzer: commercially available, this has largely replaced the cumbersome bleeding time test
- PTT and PT: the prothrombin time (PT) is a measure of one of the pathways (extrinsic) of the clotting cascade in which platelets take part. Activated partial thromboplastin time (aPTT) tests another pathway, the intrinsic clotting cascade
- Genetic testing of blood DNA: To identify gene mutations
Platelet Function Testing
The evaluation of platelet function inside the body has always been a challenge until recently. New technology has made it possible to test platelet aggregation, adhesion, and circulating properties. These use light to measure the amount of opaqueness there is due to clots or non-clotting in platelet-rich plasma or whole blood, or they use the time it takes for clots to form:
- Platelet Aggregometry: The VerifyNow Assay is the latest technology in aggregometry
- Whole Blood Aggregometry
- Light Scattering methods
- Clot Signature Analyzer: uses a tube of blood to measure platelet function via platelet “plugs” observed
- Thrombotic Status Analyzer: measures platelet activation via capillary tube occlusion. The Platelet Function Analyzer measures the drop in flow rate through a capillary tube due to platelet plugs
- High Shear Filterometer: measures the time it takes for a filter to stop allowing blood through
Diagnosis of inherited platelet dysfunction can be made from genetic testing and platelet aggregometry.
Other Testing for Platelet Hyperfunction
The end result of platelet hyperfunction is clotting. In this regard, confirmation is done via ultrasound of blood vessels, or when more accuracy is needed, CT or MRI of arteries or veins suspected to harbor thrombi (clots).
Testing Used After Antiplatelet Medicines are Begun
Anticoagulation using antiplatelet drugs is challenging, in that the effect must be enough to discourage clot formation but not so much to provoke bleeding. Once antiplatelet medications are begun, it is necessary to do interval testing to ensure the patient’s clotting tendencies fall into the narrow range of what is considered ideal anticoagulation. For this purpose, the following interval blood tests are used:
- PT (prothrombin time): Measures the time for plasma to clot when exposed to tissue factor.
- INR (the international normalized ratio): A PT ratio applied to a world-wide standard and measures how well the PT is affected by anticoagulants. It is expressed in the multiples of decreased speeds from normal clotting. The therapeutic goal for anticoagulation is an INR of 2-3 (ideally, 2.5). Because it is an international standard, it is especially useful for travelers.
- aPTT (activated partial thromboplastin time): measures the time for plasma to clot after exposure to contact factors.