Unexpected Hemoglobin A Results after an Erythrocyte Exchange: Importance of Specimen Mixing

Erythrocyte exchange, a procedure in which blood is removed and replaced with donor cells, is often used to prevent or treat severe vasocclusion in patients with sickling hemoglobinopathies. This procedure increases the percentage of hemoglobin A without dramatically increasing the hematocrit or viscosity. The efficacy of erythrocyte exchange is often measured by hemoglobin electrophoresis and densitometry to determine the posttransfusion percentages of hemoglobin A and S.We performed a manual erythrocyte exchange on a 28-year-old pregnant woman with hemoglobin SD-Punjab. Five units of whole blood were withdrawn and replaced with packed erythrocytes and saline. The expected percentage of hemoglobin A in each unit withdrawn was calculated in an iterative fashion. We calculated the total volume of hemoglobin SD-Punjab erythrocytes using the patient’s total blood volume estimated by nomogram, measured hematocrit, and assuming 100% hemoglobin SD-Punjab. The volume of erythrocytes removed was subtracted from the hemoglobin SD-Punjab erythrocyte volume tally, and the volume of erythrocytes administered (assumed to be an average of 160 mL) …