Winter's Formula

Winter's formula predicts the expected PCO₂ in a patient with metabolic acidosis if respiratory compensation is appropriate. It helps identify whether a patient has a simple metabolic acidosis (appropriate compensation) or a mixed acid-base disorder (concurrent respiratory disturbance).

• The ±2 represents the normal range of variation
• A quick rule: Expected PCO₂ ≈ last two digits of pH (e.g., pH 7.25 → PCO₂ ~25)
• Only applies to metabolic acidosis - different formulas for alkalosis
• In severe acidosis (HCO₃ <8), maximum respiratory compensation may be reached (~10-12 mmHg)
• Remember: Compensation never overcorrects - if pH is normal with abnormal HCO₃ and PCO₂, it's a mixed disorder

• Evaluate respiratory compensation in metabolic acidosis
• Detect mixed acid-base disorders: If actual PCO₂ differs from expected
• Critical care: Essential for ABG interpretation
• DKA, sepsis, toxicology: Common scenarios requiring this calculation