To find out the maximum allowable blood loss, or ABL, subtract the initial hemoglobin count, or Hi, by the final hemoglobin count, or Hf, then multiply that number by the estimated blood volume, or EBV. Divide that number by the initial hemoglobin count. To find the estimated blood volume, multiply the body weight, in kilograms, by the average blood volume, in milliliters over kilograms. OpenAnesthesia lists the written calculation as: ABL=[EBV x (Hi-Hf)]/Hi.
The average blood volume for a premature neonate is 95 milliliters per kilogram; a full term neonate is 85 milliliters per kilogram; an infant is 80 milliliters per kilogram; an adult man is 75 milliliters per kilogram; and an adult woman is 65 milliliters per kilogram. The average hematocrit, or Hct, for a man is 42 to 52 percent, and for a woman is 37 to 47 percent. An 80-kilogram man has an estimated blood volume of 6,000, which is 80 kilograms muliplied by 75 milliliters per kilogram. If his initial hemoglobin count is 50 percent and the lowest allowable final hemoglobin count is 40 percent, his maximum allowable blood loss is 1,200 milliliters. If the man lost more than 1,200 milliliters of blood, he would then require a transfusion. OpenAnesthesia states that pediatric cases should have sponges and gauze weighed for blood loss.