Winter's Formula
Determine the expected arterial PCO₂ in metabolic acidosis and identify concurrent respiratory disorders by comparing with the measured PCO₂.
Compare measured PCO₂ with Winter's predicted range to detect mixed disorders.
Expected PCO₂
35.0 mm Hg
Lower bound (−2)
33.0 mm Hg
Upper bound (+2)
37.0 mm Hg
How to Use This Calculator
Confirm metabolic acidosis
Use serum chemistry or arterial/venous blood gas to verify low bicarbonate and low pH before applying Winter's formula.
Input bicarbonate and PCO₂
Enter serum bicarbonate (mEq/L). Add measured PCO₂ to evaluate compensation; leave blank if not available.
Compare measured vs expected
Measured PCO₂ within ±2 mm Hg of predicted indicates appropriate compensation. Deviations suggest mixed respiratory disorders.
Formula
Winter's formula: Expected PCO₂ (mm Hg) = 1.5 × [HCO₃⁻] + 8 ± 2
Applies to metabolic acidosis with HCO₃⁻ ≤ 22 mEq/L. For other primary disorders, use appropriate compensation formulas (e.g., metabolic alkalosis, respiratory acidosis).
Full Description
Winter's formula predicts respiratory compensation to metabolic acidosis by estimating alveolar ventilation response. Comparing measured PCO₂ with the predicted range helps detect mixed acid-base disorders (e.g., concurrent respiratory acidosis from hypoventilation). Use alongside anion gap analysis and clinical evaluation for comprehensive acid-base assessment.
Frequently Asked Questions
When is Winter's formula valid?
Only in primary metabolic acidosis with reduced bicarbonate. Do not apply in metabolic alkalosis or primary respiratory disorders.
What if bicarbonate is >24 mEq/L?
Winter's formula is not applicable; consider metabolic alkalosis compensation formulas instead.
How precise is the ±2 range?
The ±2 mm Hg accounts for physiologic variability. Values slightly outside range may still be clinically acceptable; trends matter.
What other compensation rules should I know?
Use specific formulas for metabolic alkalosis, acute/chronic respiratory acidosis, and respiratory alkalosis to evaluate mixed disorders comprehensively.