Quantitative detection of platelet aggregates in whole blood without fixation

Platelet counting detects lesser degrees of platelet aggregation than conventional aggregometry. In order to prevent progressive platelet aggregation or disaggregation after sampling it is customary to fix blood samples. However fixation may introduce other artefacts. We first compared stability of platelet counts in EDTA-, citrate- and r-hirudin-anticoagulated blood from healthy volunteers. Second, the stability of platelet counts in unfixed EDTA- and hirudin-anticoagulated blood was compared with glutaraldehyde-fixed blood in the same anticoagulants. Third, the effect of in vivo heparin administration on platelet counts in EDTA- and hirudin-anticoagulated blood was studied. Platelet counts within 2 h of collection were significantly higher in EDTA- than in hirudin- or citrate-anticoagulated blood (P = 0.002 vs. hirudin and P = 0.001 vs. citrate). Twenty-four hour counts in hirudin and EDTA were unchanged (P = 0.3 and P = 0.2, respectively, vs. earlier counts). Counts in citrate increased significantly (P = 0.007; n = 10). Platelet counts in fixed blood did not differ significantly from those in unfixed blood. Heparin administered for cardiopulmonary bypass reduced platelet counts in hirudin-anticoagulated blood from (mean +/- 1 standard deviation) 180 +/- 45 to 162 +/- 30 x 10(9) l-1 (P = 0.01; n = 14), without significantly lowering counts with EDTA-anticoagulation, consistent with increased platelet aggregation. Hirudin and EDTA provided stable platelet counts, suggesting that fixation is unnecessary.