Allowable blood loss calculator: the Gross 1983 dilution-corrected formula

Allowable blood loss is the volume of blood that can be lost before the hematocrit falls from its starting value to a chosen lower value. This allowable blood loss calculator uses the Gross 1983 formula, which divides by the average of the two hematocrits to allow for dilution as blood is lost and replaced. You supply the estimated blood volume and both hematocrits; it returns millilitres to one decimal place.

Not a medical device. Not a diagnosis or dosing recommendation.

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Hematology calculators
Gross JB · 1983
The allowable blood loss formula
Worked examples
Frequently asked questions

What is allowable blood loss?

Gross's paper, "Estimating allowable blood loss: corrected for dilution", gave a version of the calculation that uses the average of the starting and lower hematocrits. A simpler form divides by the starting hematocrit; this calculator doesn't use that form.

The calculator doesn't choose the lower hematocrit and doesn't estimate blood volume. Both come from the caller.

The allowable blood loss formula

ABL (mL) = EBV × (Hct initial − Hct target)
           ÷ ((Hct initial + Hct target) ÷ 2)

EBV = estimated blood volume in mL
Hct = hematocrit in percent

Exact expression the calculator uses:

abl_ml = estimated_blood_volume_ml * (hct_initial_percent - hct_target_percent) / ((hct_initial_percent + hct_target_percent) / 2)

Inputs

Input	Meaning	Range
`estimated_blood_volume_ml`	Estimated blood volume, mL	50 to 15000
`hct_initial_percent`	Starting hematocrit, %	10 to 70
`hct_target_percent`	Lower hematocrit, %	10 to 70, below the starting value

How the calculation works

Subtract the lower hematocrit from the starting hematocrit.
Average the two hematocrits.
Multiply the blood volume by the difference and divide by the average.
Round to one decimal place.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: 5000 mL, hematocrit 40 to 30

Difference 40 − 30 = 10; average (40 + 30) ÷ 2 = 35.
5000 × 10 ÷ 35 = 1428.57.
Allowable blood loss 1428.6 mL.

Example 2: 5000 mL, hematocrit 40 to 20

Difference 40 − 20 = 20; average (40 + 20) ÷ 2 = 30.
5000 × 20 ÷ 30 = 3333.33.
Allowable blood loss 3333.3 mL.

Example 3: 500 mL, hematocrit 40 to 30

Difference 10; average 35.
500 × 10 ÷ 35 = 142.86.
Allowable blood loss 142.9 mL.

History of the formula

Gross published "Estimating allowable blood loss: corrected for dilution" in Anesthesiology in 1983. The dilution-corrected formula the calculator uses comes from that paper.

When it's used

When the blood volume and two hematocrit values are already known and the volume of blood loss between them is wanted, for example in surgical planning or teaching.

Where it's used

Operating rooms, anaesthesia teaching and perioperative planning.

Who uses it

Anaesthetists, surgeons and students. The calculator does the arithmetic.

Limits of the calculation

It's arithmetic on the numbers supplied. A wrong blood volume or hematocrit gives a wrong result.
It doesn't estimate blood volume from height, weight or sex.
It doesn't choose the lower hematocrit.
It isn't a transfusion trigger or order.
The lower hematocrit must be below the starting value.

Clinical decisions belong to a qualified clinician who knows the patient.

The uncorrected form divides by the starting hematocrit instead of the average, so it gives a smaller number for the same inputs. This calculator uses the dilution-corrected form.

Glossary

Hematocrit (Hct): the percentage of blood volume made up of red cells.
Estimated blood volume (EBV): the estimated total blood volume, in mL.
Dilution: the fall in hematocrit as lost blood is replaced by fluid without red cells.

Common mistakes

Entering hematocrit as a fraction. Enter percent, such as 40.
Putting the lower value first. The starting hematocrit must be the higher one.
Expecting the calculator to estimate blood volume. You supply it.

Source

Gross JB. Estimating allowable blood loss: corrected for dilution. Anesthesiology. 1983;58(3):277-280. DOI: 10.1097/00000542-198303000-00016. PMID: 6829965

Frequently asked questions

What is the formula for allowable blood loss?

Gross 1983: blood volume × (starting hematocrit − lower hematocrit) ÷ the average of the two hematocrits.

Does the calculator work out blood volume?

No. You supply the estimated blood volume in mL.

Why divide by the average hematocrit?

That's the dilution correction in Gross 1983. It allows for the hematocrit falling as blood is lost and replaced.

What units does it use?

Blood volume in mL, hematocrit in percent, and the result in mL.