Inaccuracy in automated measurement of hematocrit and corpuscular indices in the presence of severe hyperglycemia

Because we recently observed two patients with severe diabetic hyperglycemia and spuriously elevated electronically determined hematocrit and mean corpuscular volume (MCV), we investigated the effect of hyperglycemia on two popular automated hematology systems, the Coulter S and Ortho ELT-8. Marked hyperglycemia (blood glucose 800--2000 mg/dl) caused consistent overestimation of the electronically determined MCV compared to that derived from a simultaneous spun microhematocrit. The resultant overestimation and underestimation, respectively, of the derived values for hematocrit and mean corpuscular hemoglobin concentration may be clinically misleading. The mechanism of MCV elevation in hyperglycemia appears to be swelling of hyperosmolar glucose "loaded" erythrocytes when diluted into "isotonic" counting medium. This effect is readily circumvented by determination of a spun microhematocrit.