Estimating allowable blood loss with correction for variations in blood volume.

All the theoretical relationships between blood loss and change in haematocrit used for calculating the allowable pre-transfusion blood loss assume a strictly normovolaemic situation. In this study a formula was derived in which account was taken of the variation in blood volume. The formula was based on clinical data. Measurements of the blood loss, the blood haemoglobin concentration and the haematocrit (HCT) were performed on 230 occasions in the course of 35 transurethral prostatic resections. The change in blood volume was estimated by the haemoglobin dilution method and the value so obtained was compared to both the measured haematocrit and the theoretical value that would presumably have been recorded if there had been no change in blood volume. The relationship established from these comparisons was: blood loss = preoperative blood volume x [In preop HCT - In postop HCT (1 + 0.15 x blood volume change)].