Platelet Volume Measurements: The Importance Of Anticoagulation

Recent advances in electrical cell sizing have made mean platelet volume (MPV) routinely available in most clinical laboratories. To study the importance of anticoagulation on platelet size stability, blood was collected in 7 different anticoagulants and stored at room temperature for up to 8 hours. Platelet counts and platelet sizing were performed using whole blood on a Coulter S⁺ and using platelet-rich plasma on a Coulter H4 Channelyzer. The results suggest that both calcium chelation and acidification were required to inhibit platelet shape change and aggregation. Electrolyte composition, pH, and tonicity of the anticoagulant all influenced the stability of the MPV. As a result of these studies, an anticoagulant combining ACD and Na₂EDTA at a pH of 5.0 and an osmotic strength of 308 m0sm/1 was used to study platelet volume and counts in whole blood on a Coulter S⁺ used in the hematology laboratory of our hospital. Platelet counts with ACD-Na₂EDTA anticoagulant were no different from routine Na₂EDTA platelet counts and exhibited 2.9% error in reproducibility and a 4.4% variability over the 8 hours of storage. Mean platelet volumes were reproducible to within 3% and had less than 1% variability over the 8 hours of storage. Platelets anticoagulated with ACD-Na₂EDTA remained discoid in shape and could be shown to undergo a shape change on stimulation with ADP up to at least 8 hours after collection, when the pH was adjusted to 7.4 and the Ca⁺⁺ concentration restored.

These data demonstrate that platelet count and platelet volumes remained stable in blood collected in ACD-Na2EDTA anticoagulant for up to 8 hours at room temperature. In 52 volunteers studied, an inverse correlation (r=.72, p<0.001) was observed between platelet count and MPV, suggesting that the circulating platelet mass may be a more important indicator of platelet homeostasis than either the platelet count or the mean platelet volume alone.