How it's calculated
Delta ratio = (anion gap − 12) / (24 − bicarbonate) — comparing how much the anion gap has risen above normal against how much the bicarbonate has fallen below normal.
Interpretation
| Delta ratio | Interpretation |
|---|---|
| < 0.4 | Concurrent normal-anion-gap (hyperchloraemic) metabolic acidosis |
| 0.4-0.8 | Combined high-anion-gap and normal-anion-gap metabolic acidosis |
| 1-2 | Pure high-anion-gap metabolic acidosis |
| > 2 | Concurrent metabolic alkalosis, or pre-existing compensated chronic respiratory acidosis |
Clinical use
- Used after identifying an elevated anion gap, to check whether the change in bicarbonate matches what would be expected from the anion gap rise alone, or whether a second acid-base process is also present.
- Particularly useful in complex, critically ill patients where more than one acid-base disturbance is common (e.g. diabetic ketoacidosis with vomiting-related metabolic alkalosis).
- Only meaningful when there is a genuine elevated anion gap to begin with — not applicable to a normal-anion-gap acidosis.
Frequently asked questions
What does a delta ratio outside the 1-2 range mean?
It suggests a second, independent acid-base process is present alongside the high-anion-gap metabolic acidosis — below 1 suggests an additional normal-anion-gap acidosis, above 2 suggests a concurrent alkalosis or a pre-existing compensated chronic respiratory acidosis.
Why is 12 used as the baseline anion gap and 24 as the baseline bicarbonate?
These are the widely used population-average normal values for anion gap and bicarbonate — the delta ratio compares each value's deviation from its respective normal baseline.
Is the delta ratio the same as the anion gap itself?
No — the anion gap identifies the type of acidosis (high vs normal gap), while the delta ratio is a further calculation applied once a high anion gap is found, to check for a second, coexisting acid-base disorder.
References
- Berend K, et al. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371:1434-1445.
- Rastegar A. Clinical utility of Stewart's method in diagnosis and management of acid-base disorders. Clin J Am Soc Nephrol. 2009.