Calculate Allowable Blood Loss

Allowable Blood Loss (ABL) Formula*

E × ( H i H f ) H i
  • E = Estimated Blood Volume (EBV)
  • Hi = Initial hematocrit (Hct)
  • Hf = Final lowest acceptable Hct

Estimated Blood Volume (EBV) Formula

W × B

  • W = Weight in kilograms (kg)
  • B = Average blood volume
Average blood volumes**
Age Blood Volume
Premature neonates 95 mL/kg
Full term neonates 85 mL/kg
Infants 80 mL/kg
Adult men 75 mL/kg
Adult women 65 mL/kg
Normal hematocrit values
Men 42–52%
Women 37–47%
If the patient is obese, using the IBW or ABW will give more accuracy than an actual weight.


Before surgery is to take place, what is the EBV of a female patient weighing 50 kg? Also, what is the ABL of this patient if her Hct is 45?

  • EBV = 65 mL/kg
  • Initial hematocrit Hct = 45%
  • Final lowest acceptable Hct = 30%
3250 × ( 45 30 ) 45

The answer is 1083. Using this rough estimate, the patient could loose 1083 mL of blood without needing a transfusion.

The final lowest acceptable Hct is determined by the clinician or facility. Thirty percent is used in this calculator but in reality this will vary from case to case.

Replacing Blood Loss

Ideally, blood loss should be replaced with crystalloid or colloid solutions to maintain intravascular volume (normovolemia) until the danger of anemia outweighs the risks of transfusion. At that point, further blood loss is replaced with transfusions of red blood cells to maintain hemoglobin concentration (or hematocrit) at that level. For most patients, that point corresponds to a hemoglobin between 7–10 g/dL (or a hematocrit of 21–30%). Below a hemoglobin concentration of 7 g/dL, the resting cardiac output has to increase greatly to maintain normal oxygen delivery.
Morgan & Mikhail, 1996

Estimating Blood Loss

Dry sponges

  • 4x4 hold ˜ 10 mL blood
  • Ray-techs ˜ 10–20 mL blood
  • Lap sponges ˜ 100 mL blood
  • **Pediatric cases should have sponges & gauze weighed for blood loss

Blood Loss Replacement

Replace 1 mL of blood with:

  • 3 mL crystalloid (e.g., NS, Dextrose, LR)
  • 1 mL colloid (e.g., **albumin, Hespan®, Dextran®)
  • 1 mL whole blood
  • 1 mL PRBC


  • Miller, R. D. (Ed.). (2000). Anesthesia. (5th ed.). Philadelphia: Churchill Livingstone.
  • Morgan, G. E., Jr., & Mikhail, M., S. (1996). Clinical anesthesiology. (2nd ed.). New York: Lange Medical Books/McGraw-Hill
  • Nagelhout, J. J., & Zaglaniczy, K., L. (2001). Nurse anesthesia (2nd ed.). Philadelphia: W.B. Saunders Co.