Delta Ratio Calculator (ΔAG / ΔHCO₃⁻)
Calculate the delta-delta ratio to detect mixed acid–base disorders. Enter the anion gap and bicarbonate for instant interpretation across pure HAGMA, mixed, and concurrent alkalosis bands.
How the delta ratio works
The delta-delta ratio (also written ΔΔ, or ΔAG/ΔHCO₃⁻) compares the rise in the anion gap against the fall in serum bicarbonate. In a pure high-anion-gap metabolic acidosis (HAGMA), every 1 mEq/L of unmeasured anion added to the blood consumes 1 mEq/L of bicarbonate as a buffer — so the rise in the gap should approximately equal the fall in bicarbonate, and the ratio should sit near 1. When the ratio deviates from that 1:1 relationship, a mixed acid–base disorder is present. The formal definition is the delta ratio formula: ΔAG / ΔHCO₃⁻ = (observed AG − 12) / (24 − observed HCO₃⁻), using the conventional normals of AG 12 and HCO₃⁻ 24 mEq/L.
To use the calculator, enter the observed anion gap (or toggle the switch to compute it from Na⁺, Cl⁻, HCO₃⁻ via the standard anion gap equation), then enter the patient's bicarbonate. The ratio updates live, the marker slides along the colored band, and a plain-language interpretation appears with a color-coded badge.
Delta ratio interpretation bands
The ratio is read against four classic bands, each of which implies a different combination of underlying disorders:
| Delta ratio | Interpretation | What it means |
|---|---|---|
| < 0.4 | Pure NAGMA | Bicarbonate falls more than the gap rises — a pure hyperchloremic (normal-anion-gap) metabolic acidosis. Think diarrhea, RTA, or saline dilution. |
| 0.4 – 0.8 | Mixed HAGMA + NAGMA | The gap rises less than expected for the bicarbonate fall — a concurrent normal-anion-gap acidosis is dragging the ratio down. |
| 1 – 2 | Pure HAGMA | The textbook 1:1 buffering relationship — a uncomplicated high-anion-gap metabolic acidosis (lactate, ketones, toxins, uremia). |
| > 2 | HAGMA + metabolic alkalosis | The gap rises more than the bicarbonate falls — a concurrent metabolic alkalosis (vomiting, diuretics) is "refilling" bicarbonate. |
Values in the 0.8–1 region are transitional and are usually read as a pure HAGMA with a small concurrent NAGMA component; values above 2 but below 4 should prompt a search for an additional alkalotic process, while very high ratios (> 4) in the right setting can flag ethylene glycol or methanol toxicity, where the unmeasured anion load is extreme.
Worked example
A septic patient has an anion gap of 25 and a bicarbonate of 14 mEq/L. Applying the equation: ΔAG = 25 − 12 = 13; ΔHCO₃⁻ = 24 − 14 = 10; ratio = 13 / 10 = 1.3. That sits squarely in the 1–2 band — a pure high anion gap metabolic acidosis with no evidence of a mixed disturbance. If the same patient instead had a bicarbonate of 18, the ratio would fall to 13 / 6 = 2.2, hinting at a concurrent metabolic alkalosis on top of the HAGMA.
Delta-ratio cut-offs assume a normal baseline anion gap of 12 and bicarbonate of 24. In chronic respiratory acidosis (compensatory bicarbonate retention) or hypoalbuminemia, the baseline shifts and the ratio must be interpreted with the corrected anion gap. Always read it alongside the clinical picture, not in isolation.
When the ratio cannot be computed
If the observed bicarbonate equals exactly 24 mEq/L, the denominator (24 − HCO₃⁻) becomes zero and the ratio is undefined — the calculator returns an explicit "cannot compute" message rather than showing a misleading number. When bicarbonate is above 24, the denominator turns negative and the ratio goes negative; the calculator still computes it but the result signals that the bicarbonate is not depressed, so a metabolic alkalosis or compensation (rather than an acidosis) is at play. This graceful handling is important because a divide-by-zero would otherwise silently produce Infinity or NaN.
This calculator runs entirely in your browser — no values are transmitted or stored. Reference logic is reviewed against StatPearls and LITFL. Results are informational only and should always be interpreted in full clinical context by a qualified clinician.