Estimated Blood Loss
Estimate surgical or trauma blood loss from the drop in hematocrit using the Gross formula, with an approximate red-cell unit equivalent.
Formula
EBL = EBV × (Hct_initial − Hct_final) ÷ Hct_average
- The Gross formula estimates blood loss from the fall in hematocrit during surgery or trauma.
- EBV is the estimated blood volume (often about 70 mL/kg in adults); Hct_average is the mean of the starting and ending hematocrit.
- The result assumes losses are replaced with crystalloid, so it can under- or over-estimate when blood products are given.
- The approximate red-cell unit figure divides estimated loss by about 300 mL per unit — a rough planning guide only.
- This is a clinical estimate; actual transfusion decisions depend on the full picture, not a single formula.
5000 mL volume, Hct 42% to 32%
Inputs
- Estimated Blood Volume: 5000 mL
- Initial Hematocrit: 42 %
- Final Hematocrit: 32 %
With an average hematocrit of 37%, loss = 5000 × (42 − 32) ÷ 37 ≈ 1351 mL, roughly 4.5 red-cell units.
Frequently asked questions
What is the Gross formula?
It estimates blood loss as the estimated blood volume times the change in hematocrit divided by the average hematocrit, assuming losses are replaced with fluid.
How do I estimate blood volume?
A common adult approximation is about 70 mL per kg of body weight, though it varies with age, sex, and body composition.
Why use the average hematocrit?
Hematocrit falls progressively during bleeding and dilution, so the average of the start and end values better reflects the period of loss.
How accurate is the estimate?
It is an approximation. Timing of measurements, fluid resuscitation, and transfusions all affect hematocrit and can shift the result.
Should this guide transfusion?
Only as one input. Transfusion decisions rest on the patient's overall condition and clinical judgement, not a formula alone.