FLOW CYTOMETRIC ANALYSIS OF PLATELET ACTIVATION THROUGHOUT NORMAL GESTATION

Objective: To measure platelet activation in normal pregnancy, before and after stimulation with agonists, with a whole blood flow cytometri c technique. Methods: In a cross-sectional study, 5 mt of whole blood was collected from healthy volunteers (nine in the first trimester, te n in the second trimester, 35 in the third trimester, and 32 nonpregna nt controls). Platelets were treated with an agonist (thrombin or U-46 619, a thromboxane A2 analogue) or buffer and were exposed to saturati ng concentrations of monoclonal antibodies directed against platelet m embrane glycoproteins (GPs): 7E3 (fibrinogen receptor GPIIb/IIIa), S12 (alpha granule marker P-selectin), and 6D1 (van Willebrand factor rec eptor GPIb). Mean fluorescence intensity was determined for 5000 plate lets per sample by using a flow cytometer. Results: In the absence of agonist, no significant difference between groups was found in antibod y binding. At no stage of pregnancy were circulating activated platele ts detected. platelets from third-trimester subjects bound significant ly less 7E3 than platelets of controls or of first-or second-trimester subjects after stimulation with high-dose thrombin (P < .05 for all c omparisons). Down-regulation of 6D1 on platelets after stimulation wit h high-dose U-46619 was significantly greater in third-trimester gravi das than in controls or first-trimester subjects (P < .05). Conclusion : Pregnancy does not increase the percentage of activated platelets in the circulation. Platelet reactivity is altered in the third trimeste r, as evidenced by decreased antibody binding to a fibrinogen receptor epitope and enhanced down-regulation of a von Willebrand factor recep tor epitope. (C) 1997 by The American College of Obstetricians and Gyn ecologists.