Anion Gap Calculator
Metabolic acidosis screening from serum electrolytes
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Working out metabolic acidosis screening from serum electrolytes is easier with Anion Gap Calculator, which answers as you type. Drop Sodium Na⁺ (mEq/L), Chloride Cl⁻ (mEq/L) and Bicarbonate HCO₃⁻ (mEq/L) into Anion Gap — the figure refreshes as you type. Nothing in Anion Gap is a black box. Students and professionals reach for Anion Gap when speed matters. You can disconnect once Anion Gap has loaded; the arithmetic runs locally. Anion Gap handles anion gap calculator metabolic acidosis electrolytes sodium chloride the same way as free online anion gap calculator — one of the Health tools on CalcProMaster. Keep Anion Gap bookmarked for next time. Anion Gap answers the next Health question you meet.
What the defaults give
Na⁺ 140, Cl⁻ 104 and HCO₃⁻ 24 give an anion gap of 12.0 mEq/L, and the albumin-corrected value is the same because albumin is entered at 4.
The unmeasured anions and cations balance at the top of the usual 8–12 band.
Anion gap, albumin correction and the delta ratio
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
At the default inputs: 140 − (104 + 24) = 12 mEq/L, the top of the usual 8–12 range.
Low albumin hides a high gap, because albumin is the main unmeasured anion. Corrected gap = gap + 2.5 × (4 − albumin in g/dL): an albumin of 2.0 adds 5 mEq/L to the measured value.
The delta ratio asks whether a high gap is explained by the fall in bicarbonate:
Delta ratio = (gap − 12) ÷ (24 − HCO₃⁻)
| Anion gap | HCO₃⁻ | Delta ratio |
|---|---|---|
| 16 | 20 | 1.00 |
| 20 | 12 | 0.67 |
| 24 | 8 | 0.75 |
Rough reading: under 0.4 suggests a normal-gap acidosis, 0.4–1 a mixed picture, 1–2 a pure high-gap acidosis, and above 2 a concurrent metabolic alkalosis. The delta gap is the same arithmetic without the division: (20 − 12) − (24 − 12) = −4. This is a screening aid, not a diagnosis. Related: Winter’s formula and serum osmolality.
The formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Corrected AG = AG + 2.5 × (4 − albumin)
Serum must be electrically neutral, so the measured cations and anions should balance. The gap is the difference that unmeasured ions — proteins, phosphate, sulphate and organic acids — make up. A gap that grows means acid is accumulating with an unmeasured anion attached, which is what happens in lactic acidosis, ketoacidosis and some poisonings.
Worked examples
140 / 104 / 24 with albumin 4 → 12.0 mEq/L
Bicarbonate 12 instead of 24 → 24.0
Chloride 52 → 64.0
Albumin 2 g/dL → gap still 12.0, corrected 17.0
Albumin 8 g/dL → corrected 2.0
The chloride rows show the direction: raising chloride lowers the gap because chloride is a measured anion. That is why a normal-gap acidosis is called hyperchloraemic — the acid is accompanied by chloride rather than by an unmeasured anion.
Why the albumin correction matters
Albumin is the largest unmeasured anion, and each 1 g/dL below 4 removes about 2.5 mEq/L from the measured gap. A patient with a gap of 12 and albumin of 2 has a corrected gap of 17 — abnormal, and masked by the low protein. In sick patients with low albumin, the uncorrected value can look reassuring when the real gap is wide.
Potassium is not in this formula. Some laboratories include it, which raises the reference range by about 4 mEq/L; if your lab reports a gap that looks 4 higher than expected, that is usually why.
Limits
The gap is a screening calculation, not a diagnosis. It is unreliable when sodium or chloride is measured by a different method, when triglycerides are very high, or when the sample is haemolysed. Extreme inputs produce arithmetically valid but physiologically impossible numbers, which is why the inputs need to be plausible before the output means anything. A sodium of 70 or 280 is not a patient, it is a typo, and the formula will happily return minus fifty-eight or plus one hundred and fifty-two.
For orientation: a gap above 12 with a low bicarbonate suggests a high-anion-gap acidosis, a normal gap with a low bicarbonate suggests a hyperchloraemic cause such as diarrhoea or saline infusion, and a gap below 6 is unusual enough to question the measurement.
Frequently Asked Questions
What is a normal anion gap?
About 8–12 mEq/L without potassium. The defaults give 12.0, which is at the top of that range.
What does a high gap mean?
An unmeasured acid is accumulating — lactic acidosis, ketoacidosis, renal failure or a toxin. It is a screening finding that needs clinical interpretation.
Why correct for albumin?
Each 1 g/dL of albumin below 4 hides about 2.5 mEq/L of gap. At albumin 2 the measured 12 is really 17.
Is potassium included?
No. Some labs include it, which shifts the reference range up by roughly 4 mEq/L.
How is the gap calculated?
Sodium minus the sum of chloride and bicarbonate. 140 − (104 + 24) = 12.0 mEq/L.
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