How is estimated blood volume calculated here?

EBV is weight multiplied by an average blood volume coefficient that depends on patient type, around 70 mL/kg for adult males and 65 mL/kg for adult females, with higher values for children and neonates.

Can I use haematocrit instead of haemoglobin?

Yes. The same formula works with haematocrit because the ratio cancels units, so MABL = EBV x (Hcti - Hctf) / Hcti gives an equivalent result. Use whichever value your unit reports, but be consistent.

Does reaching MABL mean transfusion is mandatory?

Not automatically. MABL is a planning estimate of when the chosen haemoglobin floor is reached. The decision to transfuse also considers ongoing bleeding, haemodynamics, comorbidity, and the patient's tolerance of anaemia.

Maximum Allowable Blood Loss (MABL) Calculator

Q: What formula does this use?

It uses MABL = EBV x (Hi - Hf) / Hi, where EBV is estimated blood volume, Hi is the initial haemoglobin, and Hf is the lowest acceptable haemoglobin. This is the standard average-method formula taught in anaesthesia.

Maximum allowable blood loss

Maximum allowable blood loss (MABL) is the volume of blood a patient can lose intraoperatively before their haemoglobin falls to a predefined floor that triggers transfusion. Calculating it in advance lets the anaesthetic and surgical team plan blood availability and cell salvage rather than react to unexpected bleeding.

How it works

The standard average-method formula is:

EBV  = weight (kg) x coefficient (mL/kg)
MABL = EBV x (Hi - Hf) / Hi

where Hi is the initial (starting) haemoglobin and Hf is the lowest acceptable haemoglobin. The same equation works with haematocrit in place of haemoglobin. The coefficient depends on patient type, for example about 70 mL/kg for an adult male and 65 mL/kg for an adult female.

Tips and notes

Choose the lowest acceptable haemoglobin based on the patient’s comorbidity and the operation, not a single universal trigger. MABL assumes blood is replaced isovolaemically and the haemoglobin falls linearly, which is a simplification; ongoing rapid bleeding can overshoot it. Treat MABL as a planning threshold and always integrate haemodynamics, point-of-care testing, and clinical judgement.