The significance of platelet counts in coagulation studies

Traditionally, the platelet count recommended for coagulation studies has been less than 10 x 10(9)/L, but the documentation for this is obscure. In the present study, platelet rich plasma (PRPs) and platelet poor plasmas (PPPs) were prepared from the same blood specimen to determine prothrombin times (PTs), International Normalized Ratios (INRs), partial thromboplastin times (PTTs), and their results compared. The measurements of all three of these parameters are not statistically or clinically significant in 100 paired comparisons. Incremented platelet count studies, selected by the number of platelets in the PRPs, showed that platelet counts of at least 199 x 10(9)/L, or perhaps even higher, did not compromise the results of PTs, INRs or PTTs. Such increased platelet counts, however, cannot be tolerated in the various studies for antiphospholipid antibodies, the Lupus Anticoagulant (LAC), or when monitoring heparin therapy with PTTs. Here, the < 10 x 10(9)/L platelet levels must be respected; otherwise the tests would be compromised by platelet-liberated phospholipid (Triplett, Brand et al., 1983) or by Platelet Factor 4, respectively.