Winter’s formula is used to estimate expected PaCO2 compensation in metabolic acidosis:

Winter’s Formula:

Expected PaCO2 = (1.5 x HCO3-) + 8 ± 2

Where:

• Expected PaCO2 = predicted partial pressure of carbon dioxide in mmHg
• HCO3- = actual bicarbonate level in mmol/L

Interpretation:

• If actual PaCO2 > expected PaCO2, respiratory acidosis coexists.
• If actual PaCO2 < expected PaCO2, respiratory alkalosis coexists.
• If actual PaCO2 ≈ expected PaCO2, metabolic acidosis is compensated solely by respiratory mechanism.