Base Excess Formula
Base excess (BE) is calculated using the Van Slyke equation, with the Zander formula providing the highest accuracy (<1 mmol/L) for clinical use: BE = (1 - 0.0143 × cHb) × [[0.0304 × PCO₂ × 10^(pH-6.1) - 24.26] + (9.5 + 1.63 × cHb) × (pH - 7.4)] - 0.2 × cHb × (1 - sO₂) 1
Required Variables for Calculation
The formula requires four measured parameters from blood gas analysis 1:
- pH: Arterial or venous blood pH
- PCO₂: Partial pressure of carbon dioxide (mmHg)
- cHb: Total hemoglobin concentration (g/dL)
- sO₂: Oxygen saturation (expressed as decimal, e.g., 0.95 for 95%)
Formula Components Explained
The equation consists of three main components 1:
- First term (1 - 0.0143 × cHb): Correction factor Z that depends on hemoglobin concentration
- Second term [0.0304 × PCO₂ × 10^(pH-6.1) - 24.26]: Calculates the difference between actual and normal plasma bicarbonate concentration at reference pH 7.4
- Third term (9.5 + 1.63 × cHb) × (pH - 7.4): Represents the slope of the CO₂-buffer line for whole blood (beta value)
- Fourth term 0.2 × cHb × (1 - sO₂): Correction for oxygen desaturation effects on hemoglobin buffering
Clinical Interpretation
Base excess indicates the metabolic (non-respiratory) component of acid-base disturbances 2:
- Normal range: -2 to +2 mEq/L
- Negative BE: Metabolic acidosis (deficit of base or excess of acid)
- Positive BE: Metabolic alkalosis (excess of base or deficit of acid)
The BE value represents the amount of strong acid or strong base (in mEq/L) needed to restore pH to 7.4 at a PCO₂ of 40 mmHg 1, 3.
Accuracy Considerations
The Zander formula demonstrates superior accuracy compared to alternative equations 1:
- Mean accuracy: ±0.86 mmol/L across the entire BE range (-30 to +30 mmol/L)
- Valid over PCO₂ range of 12 to 96 mmHg
- Superior to Siggaard-Andersen (±1.48 mmol/L) and NCCLS (±1.40 mmol/L) formulas
Blood Sample Type
Any blood sample type can be used for BE calculation 1, 4:
- Arterial blood
- Mixed venous blood
- Peripheral venous blood
- Only a few microliters of blood are required for optimal diagnostics 4
Common Pitfalls
Avoid using simplified BE calculations that omit oxygen saturation correction, as desaturation renders hemoglobin a stronger weak acid buffer, artificially increasing calculated BE without actual change in strong acid/base 5.
Do not confuse base excess of blood (BE) with standard base excess (SBE) - SBE assumes hemoglobin is diluted in extracellular volume and may be more appropriate for assessing whole-body metabolic status, while BE reflects the actual blood sample 6.
The oxygen desaturation correction factor (0.2 × cHb × (1 - sO₂)) is critical for accuracy in venous blood, where oxygen saturation is lower than arterial blood 1, 5.
Clinical Applications
BE is a diagnostic tool for multiple clinical scenarios 4:
- Mortality prediction after multiple trauma or shock
- Assessment of metabolic acidosis or alkalosis
- Monitoring during artificial ventilation
- Evaluation of bleeding and clotting disorders
- Fluid resuscitation guidance (e.g., lactate ≥5 mmol/L or base excess ≤-6 as criteria for fibrinogen administration in trauma) 2