Formula
ABL = EBV × (Hi − Hf) ÷ Hav, where Hav = (Hi + Hf)/2. EBV = weight × blood volume per kg: premature neonate 95, term neonate 85, infant 80, adult man 75, adult woman 65 mL/kg. Hemoglobin can be substituted for hematocrit.
Clinical pearls
- Assumes blood loss is replaced with crystalloid/colloid to keep normovolemia; it is a planning estimate, not a transfusion trigger.
- Blood-volume values are conventional anesthesia reference values (premature 95, term neonate 85, infant 80, man 75, woman 65 mL/kg); use the override for children or obese patients if your reference differs (obese adults have less blood volume per kg).
- Choose the target hematocrit by clinical context (e.g. coronary disease, ongoing bleeding) rather than a fixed number.
Examples
allowable blood loss 80 kg man hct 42 target 27maximum allowable blood loss for a 3.5 kg term neonate, starting hematocrit 50, target 35abl 65 kg woman hct 38 to 24
FAQ
How do you calculate maximum allowable blood loss?
Multiply estimated blood volume (weight × mL/kg) by the drop in hematocrit you can accept, divided by the average of the starting and target hematocrit.
What is the estimated blood volume of an adult?
About 75 mL/kg for men and 65 mL/kg for women; neonates and infants have more per kilogram (80–95 mL/kg).