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.
Looking for a hidden second disorder
In a high anion gap metabolic acidosis, such as lactic acidosis, diabetic ketoacidosis or kidney failure, an unmeasured acid is added to the blood. Each hydrogen ion is buffered by bicarbonate, so in a simple case the anion gap rises by about as much as bicarbonate falls. Comparing the two changes, with the delta gap or delta ratio, reveals whether a second acid-base disorder is hiding behind the first.
The formulas
- Anion gap: AG = Na − (Cl + HCO₃), normally about 12 mmol/L (range varies by lab)
- Albumin-corrected AG: AG + 2.5 × (4 − albumin in g/dL)
- ΔAG = corrected AG − normal AG
- ΔHCO₃ = normal HCO₃ (24) − measured HCO₃
- Delta ratio = ΔAG ÷ ΔHCO₃
- Delta gap = ΔAG − ΔHCO₃
Interpreting the delta ratio
| Delta ratio | Interpretation |
|---|---|
| < 0.4 | Normal anion gap (hyperchloremic) acidosis |
| 0.4 – 1 | High AG acidosis plus a normal AG acidosis |
| 1 – 2 | Pure high AG acidosis |
| > 2 | High AG acidosis plus a metabolic alkalosis, or a chronic respiratory acidosis |
In lactic acidosis, the ratio is often around 1.6, because some of the acid is buffered inside cells and bone. In ketoacidosis it is closer to 1, because ketones are lost in the urine.
Example
Sodium 140, chloride 100 and bicarbonate 12 mmol/L, with a normal albumin of 4 g/dL:
- Anion gap: 140 − 112 = 28
- ΔAG: 28 − 12 = 16; ΔHCO₃: 24 − 12 = 12
- Delta ratio: 16 ÷ 12 = 1.33: a pure high anion gap acidosis
- Delta gap: +4, within the usual range of about ±6
If bicarbonate had been 20 with the same anion gap, the ratio would be 4: bicarbonate is much higher than expected, pointing to an associated metabolic alkalosis, for example from vomiting.
Pitfalls
- Use your own lab’s normal anion gap, which depends on the analyzer and can be as low as 6 to 8 mmol/L with modern ion-selective electrodes.
- Correct for low albumin, common in sick patients, or a high gap can be missed.
- The delta ratio is a rough guide with wide normal limits. Always read it alongside the clinical picture, the pH, PaCO₂ and Winter’s formula for respiratory compensation.