What the anion gap measures
Blood is electrically neutral: positive and negative charges balance. A basic metabolic panel measures the main positive ion, sodium, and two of the main negative ions, chloride and bicarbonate. The difference between them is the anion gap. It stands for the negative charges that the panel does not measure, mostly albumin, phosphate, sulfate and organic acids. When an acid such as lactate or ketones builds up, it consumes bicarbonate and adds unmeasured anions, so the gap widens.
The formulas
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
- With potassium = Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻)
- Albumin-corrected anion gap = anion gap + 2.5 × (4 − albumin in g/dL)
- Delta ratio = (corrected anion gap − 12) ÷ (24 − HCO₃⁻)
All electrolytes are in mEq/L, which for these single-charge ions is the same as mmol/L.
Worked example
A panel shows sodium 140, chloride 105 and bicarbonate 24 mEq/L, with a normal albumin of 4 g/dL.
- Anion gap = 140 − (105 + 24) = 11 mEq/L, within the classic range.
- Albumin is normal, so the corrected gap is also 11.
- Bicarbonate is not below 24, so the delta ratio is not meaningful and the calculator shows 0.
Now suppose the same patient has an albumin of 2.5 g/dL, which is common in hospital. The corrected gap becomes 11 + 2.5 × 1.5 = 14.75 mEq/L: an elevated gap that the raw value hid.
Reading the result
| Anion gap (without K⁺) | Usual meaning |
|---|---|
| Below 3 | Unusual; check for lab error, very low albumin or paraproteins |
| 3 to 12 | Usual range (up to 16 when potassium is included) |
| Above 12 | High anion gap; look for an acid that has accumulated |
Reference ranges differ between laboratories, so use the one printed on the report. The causes of a high anion gap metabolic acidosis are often remembered with the mnemonic GOLDMARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure and ketoacidosis.
The delta ratio
When a high anion gap acidosis is present, the delta ratio compares the rise in the gap with the fall in bicarbonate. A ratio of about 1 to 2 suggests a pure high anion gap acidosis. Below 1 suggests a second, normal anion gap acidosis at the same time, such as from diarrhea. Above 2 suggests a metabolic alkalosis on top, for example from vomiting.
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.