From the Research
The formula for estimating allowable blood loss (ABL) is ABL = [(starting hematocrit - lowest acceptable hematocrit) ÷ starting hematocrit] × estimated blood volume, as this calculation is crucial for determining when a patient can safely lose blood during surgery before requiring a transfusion, and is supported by the most recent and highest quality study 1.
Importance of Accurate Estimation
To use this formula, one needs to know the patient's starting hematocrit, the lowest acceptable hematocrit, and the estimated blood volume.
- Starting hematocrit is the percentage of red blood cells in the total blood volume.
- Lowest acceptable hematocrit typically ranges from 21-30% depending on the patient's condition.
- Estimated blood volume is approximately 70 mL/kg in adults.
Calculation Example
For example, if a 70 kg patient has a starting hematocrit of 45% and the lowest acceptable hematocrit is 30%, their estimated blood volume would be 4,900 mL, and their allowable blood loss would be 1,633 mL, as calculated using the formula provided above.
Clinical Judgment
This calculation is important for surgical planning and anesthetic management, as it helps determine when blood transfusion might be necessary. However, it is an estimate, and clinical judgment should always be used alongside this calculation, particularly in patients with comorbidities or altered physiological states, as noted in studies such as 2, 3, 4, 5.
Limitations and Considerations
The accuracy of blood loss estimation formulas has been questioned in various studies, including 1 which suggests that estimating haemoglobin mass loss might be a more accurate approach than estimating blood volume loss. Despite these considerations, the formula ABL = [(starting hematocrit - lowest acceptable hematocrit) ÷ starting hematocrit] × estimated blood volume remains a widely used and accepted method for estimating allowable blood loss, with its application supported by the need for a standardized approach to patient blood management, as discussed in 1.