Why compensation matters
In a metabolic acidosis, bicarbonate (HCO₃⁻) falls and the blood becomes more acidic. The body responds by breathing faster and deeper to blow off carbon dioxide, lowering PaCO₂ and limiting the drop in pH. Winters’ formula predicts how far PaCO₂ should fall. If the measured value is outside that range, a second, respiratory disorder is present.
The formulas
Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 mmHg
The calculator also computes:
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8 to 12 mmol/L depending on the laboratory
- Delta ratio = (anion gap − normal gap) ÷ (24 − HCO₃⁻)
Worked example
A patient with diabetic ketoacidosis has HCO₃⁻ 12 mmol/L, PaCO₂ 26 mmHg, Na⁺ 140 and Cl⁻ 105.
- Expected PaCO₂ = 1.5 × 12 + 8 = 26 mmHg (24–28): the measured 26 is in range, so respiratory compensation is appropriate
- Anion gap = 140 − 105 − 12 = 23: a high anion gap acidosis
- Delta ratio = (23 − 12) ÷ (24 − 12) = 0.92: a pure high anion gap acidosis
Interpreting the measured PaCO₂
| Measured PaCO₂ | Meaning | Examples |
|---|---|---|
| Within the expected range | Appropriate compensation | Simple metabolic acidosis |
| Above the range | Added respiratory acidosis | Exhaustion, sedation, lung disease |
| Below the range | Added respiratory alkalosis | Salicylate poisoning, sepsis, liver failure |
Interpreting the delta ratio
| Delta ratio | Suggests |
|---|---|
| Below 0.4 | Normal anion gap (hyperchloremic) acidosis, such as diarrhea or renal tubular acidosis |
| 0.4–0.8 | Mixed high and normal anion gap acidosis |
| 0.8–2 | Pure high anion gap acidosis, such as ketoacidosis or lactic acidosis |
| Above 2 | Added metabolic alkalosis (vomiting, diuretics) or a pre-existing chronic respiratory acidosis |
Causes of a high anion gap acidosis
The classic memory aid GOLDMARK lists glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure and ketoacidosis.
Limits
- Compensation takes 12 to 24 hours to develop fully.
- The anion gap falls by about 2.5 mmol/L for every 1 g/dL drop in albumin below 4 g/dL; correct it in patients with low albumin.
- Winters’ formula applies only to metabolic acidosis, not alkalosis.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.