What it is
Maximum allowable blood loss (MABL) estimates the volume of blood a patient can lose intraoperatively before the haematocrit falls to a predefined minimum acceptable threshold — the point at which red-cell transfusion would be considered. It turns a vague “watch the bleeding” into a concrete planning number, which matters most in small children where the absolute tolerable loss is tiny.
The formula
MABL (mL) = EBV × (Hct_initial − Hct_min) / Hct_initial
where EBV is the estimated blood volume (age-band mL/kg × weight). The age-band constants are:
| Age band | mL/kg |
|---|---|
| Preterm neonate | 95 |
| Term neonate | 85 |
| Infant (1–12 mo) | 80 |
| Child (1–12 yr) | 75 |
| Adolescent / adult | 70 |
The minimum acceptable haematocrit is a clinical decision — often 21–25%, set higher for patients with limited cardiopulmonary reserve.
When to use it
- Pre-operative planning for procedures with anticipated blood loss.
- Deciding when to have crossmatched blood available in theatre.
- Setting an intraoperative threshold the team can monitor against measured/estimated loss.
Worked example
A 1-year-old child weighs 10 kg (child band, 75 mL/kg). Initial haematocrit 36%, minimum acceptable 25%.
- EBV = 75 × 10 = 750 mL
- MABL = 750 × (36 − 25) / 36 = 750 × 11 / 36 ≈ 229 mL
Once loss approaches ~229 mL, transfusion enters the conversation — and you can see how a few hundred millilitres is the whole margin in a toddler.
Pitfalls
- MABL is a guide, not a hard stop. It assumes loss is replaced isovolaemically with crystalloid/colloid and that haematocrit dilutes predictably. Real bleeding is messier — reassess clinically.
- The “gross” formula above uses the initial haematocrit in the denominator. Some clinicians use the average of initial and minimum haematocrit for a more conservative (smaller) MABL. Know which your team uses.
- Minimum acceptable haematocrit is patient-specific. Cardiac disease, sepsis, or ongoing loss push it higher.
- Garbage in, garbage out: an inaccurate weight or wrong age band corrupts EBV and therefore MABL.
Run it: Maximum Allowable Blood Loss (MABL): The Pediatric Transfusion Trigger
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.