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) / Hiwhere 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.
Estimated blood volume coefficients by patient type
The tool uses standard reference coefficients to derive EBV from body weight. These are population averages used for planning purposes:
| Patient type | Typical coefficient (mL/kg) |
|---|---|
| Adult male | ~70 |
| Adult female | ~65 |
| Child (1–12 years) | ~75–80 |
| Neonate (full-term) | ~85–90 |
| Premature neonate | ~95–100 |
These coefficients reflect the fact that blood volume as a proportion of body weight is highest in neonates and decreases toward adult values through childhood.
Worked example
For illustration, consider a 70 kg adult male patient with a haemoglobin of 130 g/L (Hi = 130) going for an elective colectomy. The surgical team has set the lowest acceptable haemoglobin (Hf) at 80 g/L.
EBV = 70 kg × 70 mL/kg = 4,900 mL
MABL = 4,900 × (130 − 80) / 130
= 4,900 × 50 / 130
≈ 1,885 mLThe team knows this patient can lose approximately 1,885 mL before transfusion is indicated at the chosen trigger. With four units of cross-matched red cells on hold and cell salvage running, they have a specific target for when to act rather than an intuition-based decision.
Choosing the lowest acceptable haemoglobin
The most consequential input is Hf. There is no single correct answer — it depends on the patient’s individual tolerance of anaemia:
- Cardiac or pulmonary comorbidity: Oxygen delivery is already compromised, so a higher Hf floor (often 80–100 g/L) is used.
- Young, fit patient: Can tolerate a lower floor (70–80 g/L) if haemodynamics are stable and bleeding is controlled.
- Elective vs. emergency: Elective cases allow more conservative planning; emergency cases may require accepting a lower floor while resuscitation continues.
- Anticipated ongoing loss: If the surgeon expects continued bleeding (e.g., liver surgery), the team may set Hf conservatively higher so the buffer between MABL and actual transfusion trigger is not exhausted early.
Limitations of the MABL calculation
MABL is a planning tool, not a real-time monitor. Key limitations:
- Linear haemoglobin drop. The formula assumes haemoglobin falls proportionally with volume lost, which holds only when losses are replaced isovolaemically (volume-for-volume). Rapid colloid or crystalloid replacement dilutes haemoglobin faster than the formula predicts.
- Average EBV coefficients. Actual blood volume varies with fitness, adiposity, and disease. Lean athletes have proportionally more blood volume than the coefficient implies; severely obese patients proportionally less.
- Does not account for coagulopathy. Massive transfusion triggers are guided by coagulation as much as haemoglobin. MABL deals only with red cell loss.
- Point-in-time calculation. Blood loss is dynamic. MABL gives a pre-operative estimate; intraoperative cell saver outputs, swab weights, and suction measurements refine it in real time.
Always integrate MABL with haemodynamics, point-of-care testing, and clinical judgement throughout the case.