Delta ratio (Δ anion gap / Δ HCO₃⁻)
Look for a second metabolic disorder alongside a high anion gap acidosis
Sources last checked: 2026-10-13
Delta ratio (Δ anion gap / Δ HCO₃⁻)
Look for a second metabolic disorder alongside a high anion gap acidosis
Only meaningful when the anion gap is raised. Uses normal values of 12 (anion gap) and 24 mmol/L (HCO₃⁻) without albumin correction or potassium; cut-offs differ between textbooks, and Rastegar showed a wide spread even in pure acidoses.
Interpretation
| < 0.8 — additional normal-gap acidosis | Bicarbonate has fallen more than the anion gap has risen: mixed high-anion-gap and normal-anion-gap (hyperchloraemic) metabolic acidosis. |
|---|---|
| 0.8–1.2 — pure high-gap acidosis | Rise in anion gap matches the fall in bicarbonate: uncomplicated high-anion-gap metabolic acidosis. |
| > 1.2 — additional metabolic alkalosis | Anion gap has risen more than bicarbonate has fallen: high-anion-gap acidosis with a coexisting metabolic alkalosis. |
What is the Delta ratio (Δ anion gap / Δ HCO₃⁻)?
The delta ratio compares how much the anion gap has risen with how much bicarbonate has fallen in a high-anion-gap metabolic acidosis. If one unmeasured acid accounts for the whole picture, the two changes should be similar; a clear mismatch suggests a second metabolic process. The cut-offs here follow the interpretation table in Tucker et al. (2025); Rastegar (2007) reviewed the physiology and its limits.
When to use it
- High-anion-gap metabolic acidosis (ketoacidosis, lactic acidosis, toxic alcohols, uraemia)
- Suspicion of an additional normal-gap acidosis, e.g. diarrhoea or saline loading in DKA
- Suspicion of an additional metabolic alkalosis, e.g. vomiting or diuretics
When not to use it
- Normal anion gap — the ratio has no meaning without a raised gap
- Bicarbonate at or above 24 mmol/L, where the denominator is zero or negative
How it is calculated
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) Delta ratio = (anion gap − 12) / (24 − HCO₃⁻) Delta gap = (anion gap − 12) − (24 − HCO₃⁻)
Clinical pearls and limitations
- Only meaningful when the anion gap is raised. Uses normal values of 12 (anion gap) and 24 mmol/L (HCO₃⁻) without albumin correction or potassium; cut-offs differ between textbooks, and Rastegar showed a wide spread even in pure acidoses.
- The delta gap (ΔAG − ΔHCO₃⁻) carries the same information as a difference instead of a ratio: values near 0 match a pure high-gap acidosis.
- Correct the anion gap for low albumin before interpreting it; hypoalbuminaemia hides unmeasured anions.
- Lactic acidosis tends to give higher ratios than ketoacidosis because lactate is retained while ketones are lost in urine.
Frequently asked questions
How is the delta ratio calculated?
(Anion gap − 12) / (24 − HCO₃⁻). It uses 12 mmol/L as the normal anion gap and 24 mmol/L as the normal bicarbonate.
What does a delta ratio above 1.2 mean?
The anion gap has risen more than bicarbonate has fallen, which suggests a coexisting metabolic alkalosis (or a pre-existing high bicarbonate).
What is the difference between delta gap and delta ratio?
Both compare the rise in anion gap with the fall in bicarbonate; the delta gap subtracts them, the delta ratio divides them.
Evidence and guidelines
- Primary source: Tucker AM et al. Breathing and balance: clinical insights and management strategies of respiratory acid-base disorders. Nutr Clin Pract 2025 — Henderson–Hasselbalch equation (text) and Table 5 (delta ratio and interpretation).
- Rastegar A. Use of the ΔAG/ΔHCO₃⁻ ratio in the diagnosis of mixed acid-base disorders. J Am Soc Nephrol 2007;18(9):2429-2431.
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