Delta Ratio Calculator (Delta Gap)
Delta Ratio Calculator (Delta Gap)
Classify a high-anion-gap acidosis as pure or mixed by comparing the rise in the anion gap against the fall in bicarbonate.
Delta Ratio (Delta Gap)
Gap classificationAnion gap 24 mmol/L, HCO₃⁻ 14 mmol/L
Formula
- Anion gap − 12
- the rise in the gap above a normal value of 12 mmol/L — the delta gap
- 24 − HCO₃⁻
- the fall in bicarbonate below a normal value of 24 mmol/L
- input gap
- use the albumin-corrected anion gap where albumin is low — an uncorrected gap understates the delta ratio
Worked example
Anion gap 24 mmol/L, HCO₃⁻ 14 mmol/L
Δ anion gap = 24 − 12 = 12
Δ HCO₃⁻ = 24 − 14 = 10
12 ÷ 10 = 1.20
Reading the delta ratio
| Delta ratio | Pattern |
|---|---|
| < 0.4 | Pure normal-anion-gap (hyperchloraemic) acidosis |
| 0.4 – 1.0 | Mixed high- and normal-anion-gap acidosis — classically DKA after saline |
| 1.0 – 2.0 | Pure high-anion-gap acidosis |
| > 2.0 | Concurrent metabolic alkalosis, or a chronic respiratory acidosis |
What the delta ratio adds to the anion gap
The anion gap alone identifies that unmeasured anions are present, but it says nothing about whether they explain the whole acid-base picture. The delta ratio compares two changes: how far the gap has risen above normal, against how far the bicarbonate has fallen below normal. If unmeasured acid alone is responsible, those two changes should be roughly equal — a ratio near 1.0 — because each mole of acid buffered consumes one mole of bicarbonate and adds one mole to the gap.
A ratio below 1.0 means bicarbonate has fallen further than the gap rose, which happens when a normal-anion-gap process — chloride retention, diarrhoeal bicarbonate loss, or saline-driven hyperchloraemia — is present alongside the high-gap process. This pattern is classic in diabetic ketoacidosis after aggressive saline resuscitation, where chloride from the fluid replaces some of the lost bicarbonate. A ratio above 2.0 means bicarbonate has fallen less than the gap rose, which happens when something is propping bicarbonate up — a coexisting metabolic alkalosis, or a chronically elevated baseline bicarbonate from a compensated respiratory acidosis.
The ratio is only as reliable as the anion gap feeding it. In hypoalbuminaemia the raw gap understates the true excess of unmeasured anions, so the albumin-corrected gap should be used as the input whenever albumin is low — which, in the patients most likely to need this calculation, is often.
Frequently asked questions
What does a delta ratio of 1.0 mean?
It means the fall in bicarbonate matches the rise in the anion gap, consistent with a pure high-anion-gap metabolic acidosis with no second process.
What does a low delta ratio suggest?
Below 0.4 suggests a pure normal-anion-gap (hyperchloraemic) acidosis. Between 0.4 and 1.0 suggests a mixed picture — classically diabetic ketoacidosis after saline resuscitation has lowered the gap’s apparent contribution.
What does a delta ratio above 2.0 mean?
It suggests a coexisting metabolic alkalosis, or a chronic respiratory acidosis with an already-elevated baseline bicarbonate, either of which keeps bicarbonate higher than the gap alone would predict.
Should I use the corrected or uncorrected anion gap?
Use the albumin-corrected gap whenever albumin is low. An uncorrected gap in a hypoalbuminaemic patient understates the excess anions and skews the delta ratio downward.
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References
- Wrenn K. The delta (delta) gap: an approach to mixed acid-base disorders. Ann Emerg Med. 1990;19(11):1310–3.
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434–45.