What is the maximum allowable blood loss (MABL) for a patient?

Maximum Allowable Blood Loss (MABL)

The maximum allowable blood loss 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. 1

Calculation Framework

Basic Formula Components

• Estimated blood volume (EBV) is typically calculated as 70 mL/kg body weight in adults 1
• Initial hematocrit (Ho) represents the patient's preoperative hematocrit value 1
• Final hematocrit (Hf) is the minimum acceptable hematocrit threshold for that patient 1
• Average hematocrit (Hav) is calculated as the mean of Ho and Hf 1

Minimum Acceptable Hemoglobin Thresholds

• For most hemodynamically stable hospitalized adults: Consider transfusion when hemoglobin falls below 7 g/dL (hematocrit ~21%) 2
• For cardiac surgery patients: A threshold of 7.5 g/dL (hematocrit ~22.5%) is appropriate 2
• For orthopedic surgery or preexisting cardiovascular disease: Use 8 g/dL (hematocrit ~24%) as the threshold 2
• For patients with severe cardiac disease: Higher thresholds may be necessary, particularly those with unstable angina, severe aortic stenosis, or critical left main stem disease 1

Clinical Context for Blood Loss Classification

ATLS Blood Loss Categories

The American College of Surgeons classifies hemorrhage severity based on percentage of blood volume lost 1:

• Class I (up to 15% blood volume): Up to 750 mL in a 70 kg adult; minimal physiologic impact 1
• Class II (15-30% blood volume): 750-1500 mL; tachycardia develops, blood pressure remains normal 1
• Class III (30-40% blood volume): 1500-2000 mL; hypotension and tachycardia present, crystalloid and blood required 1
• Class IV (>40% blood volume): >2000 mL; immediately life-threatening, requires immediate blood transfusion 1

Surgical Planning Thresholds

• Acute normovolemic hemodilution (ANH) should only be considered when potential blood loss is likely to exceed 20% of blood volume 1
• ANH should not be performed unless preoperative hemoglobin is ≥110 g/L (hematocrit ~33%) 1

Critical Modifying Factors

Hemostatic Limitations

Plasma fibrinogen concentration can limit allowable blood loss more frequently than platelet count, particularly when initial fibrinogen levels are in the lower normal range (<300 mg/dL) 3:

• Critical fibrinogen levels (<100 mg/dL) can limit hemodilution in up to 20% of patients 3
• Platelet concentrations rarely (<2% of patients) limit hemodilution 3
• Maintain fibrinogen >1.0 g/L during active bleeding, with treatment recommended when fibrinogen falls below 1.5-2.0 g/L 1

Patient-Specific Risk Factors

• Patients with impaired erythropoiesis (typical of ICU patients with low body weight and initial hemoglobin at lower normal limits) may become severely anemic (hemoglobin ≤70 g/L) within 9-14 days with blood loss of only 53 mL/day 1
• Patients over 45 years require careful assessment for silent myocardial ischemia before accepting lower hemoglobin thresholds 1
• Liver disease patients develop clinically significant dilutional coagulopathy with blood loss less than one blood volume 1

Practical Application Algorithm

Step 1: Calculate Patient's EBV

• EBV (liters) = Body weight (kg) × 0.070 1

Step 2: Determine Minimum Acceptable Hematocrit

• Use 21% (Hb 7 g/dL) for most stable patients 2
• Use 22.5% (Hb 7.5 g/dL) for cardiac surgery 2
• Use 24% (Hb 8 g/dL) for orthopedic surgery or cardiovascular disease 2

Step 3: Apply MABL Formula

• MABL (liters) = EBV × (Ho - Hf) / [(Ho + Hf) / 2] 1

Step 4: Adjust for Clinical Context

• Reduce calculated MABL by 20-30% if patient has liver disease, coagulopathy, or impaired erythropoiesis 1
• Consider fibrinogen levels: If preoperative fibrinogen <300 mg/dL, hemostatic failure may occur before reaching calculated MABL 3

Common Pitfalls

• The linear formula underestimates allowable blood loss because it assumes all lost blood contains initial hemoglobin concentration, when in reality progressive hemodilution occurs 4
• Failing to account for transfused blood volume in calculations leads to significant errors in estimating actual blood loss 5
• Relying solely on hemoglobin/hematocrit without considering coagulation factors, particularly fibrinogen, can result in unexpected hemostatic failure 3
• Blood loss is typically underestimated clinically, and hemoglobin/hematocrit values do not fall for several hours after acute hemorrhage 1