Maximum Allowable Blood Loss Calculation

Introduction to MABL

The maximum allowable blood loss (MABL) should be calculated based on the patient's estimated blood volume (EBV), initial hematocrit (Ho), and minimum acceptable hematocrit (Hf), using the formula: MABL = EBV × (Ho - Hf) / Hav, where Hav is the average of initial and final hematocrit, as recommended by the British Journal of Anaesthesia 1

Estimated blood volume (EBV) is typically calculated as 70 mL/kg body weight in adults, according to the British Journal of Anaesthesia and Critical Care 1, 2, 3
The American College of Surgeons classifies hemorrhage severity based on percentage of blood volume lost, with Class I being up to 15% blood volume, Class II being 15-30% blood volume, Class III being 30-40% blood volume, and Class IV being greater than 40% blood volume 2, 3, 4

For patients with severe cardiac disease, higher hemoglobin thresholds may be necessary, particularly those with unstable angina, severe aortic stenosis, or critical left main stem disease, as suggested by the British Journal of Anaesthesia 1
Patients over 45 years require careful assessment for silent myocardial ischemia before accepting lower hemoglobin thresholds, according to the British Journal of Anaesthesia 1

Maintain fibrinogen >1.0 g/L during active bleeding, with treatment recommended when fibrinogen falls below 1.5-2.0 g/L, as recommended by Critical Care 5
Patients with impaired erythropoiesis may become severely anemic within 9-14 days with blood loss of only 53 mL/day, according to Critical Care 6
Liver disease patients develop clinically significant dilutional coagulopathy with blood loss less than one blood volume, as reported by Anaesthesia 7, 8

EBV (liters) = Body weight (kg) × 0.070, as calculated by the British Journal of Anaesthesia and Critical Care 1, 2, 3
Reduce calculated MABL by 20-30% if patient has liver disease, coagulopathy, or impaired erythropoiesis, as suggested by Anaesthesia and Critical Care 6, 7, 8
Blood loss is typically underestimated clinically, and hemoglobin/hematocrit values do not fall for several hours after acute hemorrhage, according to the British Journal of Anaesthesia 4