The platelet count in EDTA-anticoagulated blood from patients with thrombocytopenia may be underestimated when measured in routine laboratories

Spuriously low platelet counts (PCs) can be observed in normal blood samples anticoagulated with ethylenediamine tetra-acetic acid (EDTA) and, much less frequently, with citrate-tris-pyridossalphosphate (CPT), due to time-dependent in vitro platelet agglutination. Accuracy in PC determination is essential as PC is one of the parameters that usually guides treatment for thrombocytopenic patients. PCs of 93 thrombocytopenic patients were measured in EDTA- or CPT-anticoagulated blood samples immediately after sampling (t0) and 90 min (t90) after storage at room temperature. The presence of platelet agglutinates in blood samples was determined by examining blood smears using optical microscopy. PCs decreased at t90 with both anticoagulants. Platelet agglutinates were present at t90 in 27% of EDTA-samples vs. 2% of CPT-samples with decreased PCs (P < 0.001). Based on PCs in EDTA-samples, 15 patients (16%) shifted from a lower bleeding risk at t0 to a higher bleeding risk category at t90 (P 5 0.019), compared to 5 (5%) patients, based on PCs in CPT-samples. Therefore, time-dependent in vitro platelet agglutination in EDTA-blood samples may cause underestimation of PCs in thrombocytopenic patients, possibly leading to improper management.