Electronic blood cell counters. Faulty calibration due to type and amount of anticoagulant in collection tubes

Electronic blood cell counters calibrated for mean corpuscular volume against normal venous blood collected in K3 EDTA (using a microhematocrit centrifuge) consistently yielded unacceptable data when used to assay commercial control materials. After recalibration using samples collected in Na2 EDTA acceptable results were obtained. Both types of sample tubes probably contained excess anticoagulant, and plasma was hypertonic in both. However, blood collected in Na2 EDTA had a lower pH, and the consequent swelling of RBCs due to a low pH compensated for osmotically induced shrinkage when microhematocrit values were measured.