Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1129 page)

   Lipemic or hemolyzed blood may affect the in vitro platelet response.

   Assays cannot be performed in severely thrombocytopenic patients.

   Platelet aggregation studies have not been standardized to test for aspirin or clopidogrel “resistance” or hyperaggregability.

   Platelet aggregation studies are labor intensive and require highly skilled, experienced technicians.

PLATELET ANTIBODY DETECTION
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   Definition

   Platelet antibodies can be divided into two categories: autoimmune and alloimmune. Autoimmune antibodies are part of an autoimmune condition, such as autoimmune thrombocytopenic purpura (ITP) or SLE, or they may develop following administration of certain drugs. Alloimmune antibodies develop as the result of immunization of transfused, incompatible, platelets.

   The development of platelet antibodies may result in shortened platelet survival and refractoriness to platelet transfusions (lack of adequate and sustained increment in platelet number). Therefore, from 20 to 70% of multitransfused thrombocytopenic patients become refractory to transfused platelets. Platelet antibodies in pregnant women may cause neonatal alloimmune thrombocytopenia. Platelet antibodies react with several antigenic groups on the platelet surface: ABO antibodies, HLA antibodies.

   The most common platelet antigen is known as HPA-1, also known as Pl
^A1
, present in 98% of the Caucasian population. Anti-HPA-1 are the most common clinically significant antibodies. The HPA-1b (PLA2) antigen occurs in 27% of the Caucasian population. Both reside on the platelet membrane protein GPIIIa.

   Use

   In refractory, multitransfused patients, the common approach is to determine the HLA type of the patient (ideally to be done before treatments that predictably result in the need for repeated platelet transfusions) and transfuse platelets from the best HLA-matched, ABO compatible, donor. Platelet cross-matching may also be used to select the best cross-matched compatible donors. Unfortunately, cross-matched platelets are effective in only 50% of transfused patients.

   Many hematologists used the platelet antibody assays to diagnose ITP. Because of the low specificity, this assay is presently not recommended.

   Limitations