Anion Gap Calculator

Background.

Plasma carries exactly as much negative charge as positive, but a routine chemistry panel measures only part of it. Sodium accounts for most of the positive side; chloride and bicarbonate for most of the negative. Subtract the two measured anions from sodium and what remains — the anion gap — stands for everything negatively charged that the panel did not measure: albumin above all, plus phosphate, sulphate and small amounts of organic acids.

That leftover is useful because it changes when acids the panel cannot see pile up. Lactate in shock, ketoacids in diabetic ketoacidosis, the acids a failing kidney cannot excrete, and the metabolites of poisons such as methanol or ethylene glycol all consume bicarbonate and replace it with an anion outside the panel, so the gap widens. When bicarbonate is lost instead, as in severe diarrhoea or some kinds of renal tubular acidosis, chloride rises to take its place and the gap stays where it was. Splitting a metabolic acidosis into these two groups is the first fork in working out its cause.

Albumin is the complication. It is the largest single unmeasured anion, so a patient with low albumin — common in critical illness, liver disease, nephrotic syndrome and malnutrition — starts from a lower baseline, and a real accumulation of acid can raise the gap only back to an ordinary-looking number. The correction used here, from Figge and colleagues, adds 2.5 mEq/L for every 1 g/dL of albumin below 4.0 g/dL. With an albumin of 2.8 g/dL, a measured gap of 21 becomes 24.

The calculator gives the gap with and without potassium, the albumin-corrected versions and a delta ratio. It does the arithmetic and stops there: it names no diagnosis, and the numbers are meant to support clinical judgement alongside a blood gas, lactate, ketones and kidney function, not to stand in for it.

What is anion gap calculator?

The serum anion gap is the difference, in mEq/L, between the main measured cation and the main measured anions: AG = Na − (Cl + HCO₃). For singly charged ions, mEq/L and mmol/L are numerically the same, so results reported in either unit can be entered directly. A potassium-inclusive version adds potassium to the cation side, which raises the gap by the potassium value. Because the gap is a construct of what the analyser measures rather than a physical quantity, its normal value depends on the laboratory: normal gaps have drifted lower as analysers have changed how chloride is measured, which is why the reference here is a field for your lab's figure rather than a fixed number. The delta ratio compares how far the gap has risen above that reference with how far bicarbonate has fallen below its own reference, and is used to look for a second acid-base process hiding behind the first.

How to use this calculator.

1. 01Enter sodium, chloride and bicarbonate from the same sample, in mEq/L or mmol/L. Most chemistry panels report total CO₂ rather than bicarbonate; it is the usual stand-in.
2. 02Add potassium only if your laboratory or protocol uses the potassium-inclusive gap, and compare that result with a reference range built the same way. Left blank, the potassium lines read 0, meaning not calculated.
3. 03Enter serum albumin in g/dL to get the corrected gap. A result in g/L divides by 10: 28 g/L is 2.8 g/dL.
4. 04Replace the normal anion gap of 12 with your laboratory's reference value, and keep the reference bicarbonate at 24 mEq/L unless your unit uses another, before reading the delta ratio.
5. 05Read the corrected gap and delta ratio together with the blood gas, lactate, ketones, creatinine and the clinical picture.

The formula.

The basic gap is AG = Na − (Cl + HCO₃). With potassium it is AGₖ = Na + K − (Cl + HCO₃), always larger by exactly the potassium value. The albumin correction is 2.5 × (4.0 − albumin), with albumin in g/dL, and it is added to both versions of the gap. The coefficient comes from Figge and colleagues' 1998 analysis of how the gap falls with albumin: each gram per decilitre lost removes about 2.5 mEq/L of negative charge from the unmeasured pool. An albumin above 4.0 g/dL makes the correction negative, lowering the gap slightly.

The delta ratio is (corrected AG − normal AG) ÷ (reference HCO₃ − measured HCO₃). The top line is the extra unmeasured anion; the bottom line is the bicarbonate that has been used up. If a single gap-raising acid were responsible and every unit of it consumed one unit of bicarbonate, the two would match and the ratio would be 1. A ratio well below that means bicarbonate has fallen further than the new anions explain, pointing towards an additional normal-gap acidosis; a ratio well above it means bicarbonate is higher than it should be, as with a coexisting metabolic alkalosis or a raised starting bicarbonate. Buffering inside cells and differences in how quickly each anion is cleared blur the one-for-one picture, which is why the ratio is a prompt to look further rather than a classification.

The arithmetic has hard limits. The ratio is not calculated (it shows 0) when measured bicarbonate equals or exceeds the reference, because there is no bicarbonate fall to explain. The ratio always uses the albumin-corrected gap without potassium, so enter the normal gap on that same basis.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Figge, J., Jabor, A., Kazda, A., and Fencl, V. (1998). Anion gap and hypoalbuminemia. Critical Care Medicine 26(11):1807-1810.
2. [2]Hatherill, M., Waggie, Z., Purves, L., Reynolds, L., and Argent, A. (2002). Correction of the anion gap for albumin in order to detect occult tissue anions in shock. Archives of Disease in Childhood 87(6):526-529.
3. [3]Chawla, L.S., Shih, S., Davison, D., Junker, C., and Seneff, M.G. (2008). Anion gap, anion gap corrected for albumin, base deficit and unmeasured anions in critically ill patients. BMC Emergency Medicine 8:18.
4. [4]Kraut, J.A. and Madias, N.E. (2007). Serum Anion Gap: Its Uses and Limitations in Clinical Medicine. Clinical Journal of the American Society of Nephrology 2(1):162-174.
5. [5]National Kidney Foundation (n.d.). Kidney Failure Risk Factor: Serum Bicarbonate. NKF.
6. [6]Kidney Disease: Improving Global Outcomes (2024). KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. KDIGO.