Anion Gap
Serum anion gap from a metabolic panel, with or without potassium and corrected for albumin.
Compared with a commonly quoted range of 8–12 mEq/L without potassium — always use your own laboratory’s reference range.
Subtract chloride and bicarbonate from sodium. With sodium 140, chloride 104, and bicarbonate 24 mEq/L the anion gap is 140 − (104 + 24) = 12 mEq/L. If albumin is low, add 2.5 for every g/dL below 4 — at an albumin of 2.5 that gives 15.75.
Why there is a gap at all
Blood is electrically neutral: the positive and negative charges balance exactly. A routine metabolic panel, though, measures only some of the ions present — sodium among the cations, chloride and bicarbonate among the anions. Subtracting the measured anions from the measured cations therefore leaves an apparent gap, which represents the anions the panel does not measure: albumin above all, plus phosphate, sulfate, and organic acids.
That makes the gap a detector. When an acid accumulates in the blood — lactate, ketones, certain toxins — its anion joins the unmeasured pool while bicarbonate is consumed buffering the hydrogen ion. The gap widens, and the pattern points towards a class of causes that a bicarbonate level alone would not distinguish.
Two conventions
Some units include potassium among the cations and some do not. Both are defensible; what matters is that the reference range matches the formula. Without potassium the range is commonly quoted as 8–12 mEq/L, and with it 12–16 mEq/L. Modern ion-selective analysers often report a lower range than the classic teaching figures, so the laboratory’s own range should always take precedence.
All electrolytes in mEq/L, albumin in g/dL. The albumin correction matters because albumin is the largest single unmeasured anion.
Worked example: Na 140, Cl 104, HCO₃ 24, albumin 2.5 g/dL
Calculate the raw gap first, then adjust for the low albumin:
- 1 Take the sodium. Sodium is 140 mEq/L. This calculation excludes potassium — check which convention your unit uses.
- 2 Add the measured anions. Chloride 104 + bicarbonate 24 = 128 mEq/L.
- 3 Subtract. 140 − 128 = 12 mEq/L, which sits at the top of the commonly quoted 8 to 12 range.
- 4 Check the albumin. Albumin is 2.5 g/dL, which is 1.5 g/dL below the reference of 4.
- 5 Correct the gap. 12 + 2.5 × 1.5 = 12 + 3.75 = 15.75 mEq/L — a raised gap that the uncorrected figure concealed.
Reference ranges and what a raised gap suggests
Ranges vary between laboratories and analysers — always use your own. The causes listed are the classic teaching groups, not a diagnostic list.
| Item | Value or meaning |
|---|---|
| Anion gap without K⁺ | Commonly quoted as 8 – 12 mEq/L |
| Anion gap with K⁺ | Commonly quoted as 12 – 16 mEq/L |
| Albumin correction | Add 2.5 mEq/L per 1 g/dL of albumin below 4 g/dL |
| Raised gap acidosis | Lactate, ketoacidosis, kidney failure, and certain ingestions |
| Normal gap acidosis | Bicarbonate loss — diarrhoea, renal tubular acidosis |
| Low gap | Low albumin, or rarely a paraprotein or laboratory artefact |
Why the albumin correction matters
Albumin carries a negative charge and is the single largest contributor to the unmeasured anion pool. A patient whose albumin has fallen to 2 g/dL has lost a substantial part of that pool, so their gap starts several units lower than the reference range assumes. A metabolic acidosis that would widen the gap in a healthy person can then leave it looking entirely normal.
Correcting for albumin restores the comparison. It is particularly relevant in critical care and in chronic illness, where low albumin is common and a masked raised gap is easy to miss. The correction is an approximation of about 2.5 mEq/L per g/dL, and like the gap itself it supports clinical assessment rather than replacing it.
This tool is an informational calculation for study and practice, not medical advice. Interpretation of acid-base results requires clinical assessment by a qualified professional.