Chemistry
Albumin-Corrected Anion Gap
Anion gap adjusted for low albumin.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The albumin-corrected anion gap adjusts the measured anion gap for the patient's serum albumin, because albumin is the main unmeasured anion and a low albumin lowers the apparent gap. Hypoalbuminaemia is common in hospitalised and critically ill patients and can mask a genuine high-anion-gap metabolic acidosis. The correction adds approximately 2.5 mmol/L to the measured gap for every 1 g/dL the albumin falls below the reference value of about 4 g/dL. The result is reported in mmol/L.
Interpreting the result
A corrected gap below about 12 mmol/L is normal, 12–16 mmol/L is borderline or mildly raised, and above 16 mmol/L indicates a true high anion gap that warrants a search for causes such as lactate, ketones, renal failure or toxins. The key clinical value is in patients with low albumin, where the uncorrected gap may appear normal while the corrected gap reveals a significant acidosis. It is interpreted alongside the bicarbonate, lactate and the broader acid–base picture.
Worked example
A patient has a measured anion gap of 12 mmol/L with an albumin of 2 g/dL. The correction adds 2.5 × (4 − 2) = 5 mmol/L, giving a corrected gap of 17 mmol/L — a genuinely high anion gap that the low albumin had concealed.
Critical actions
Correction adds ~2.5 mmol/L per 1 g/dL fall in albumin below 4. Albumin in g/dL (divide g/L by 10).
Pearls / pitfalls
- The correction matters most in hypoalbuminaemic patients, where an uncorrected gap can look falsely normal.
- Use albumin in g/dL (divide a g/L result by 10) and a reference albumin of about 4 g/dL.
- The correction is an approximation; calcium, phosphate and magnesium disturbances can also affect the gap.
- Always interpret with bicarbonate, lactate and the clinical context rather than the corrected gap in isolation.
Evidence & validation
The correction factor of roughly 2.5 mmol/L per g/dL of albumin was described by Figge and colleagues from physico-chemical analysis of plasma. It is widely taught and used in acid–base assessment, particularly in critical care and nephrology.
Frequently asked questions
Why does low albumin lower the anion gap?
Albumin is negatively charged and is the largest contributor to the unmeasured anions in the gap. When albumin falls, the measured anion gap falls too, which can mask a high-anion-gap acidosis.
How much correction does each gram of albumin make?
Approximately 2.5 mmol/L is added to the measured anion gap for every 1 g/dL the albumin is below about 4 g/dL. This is an established approximation rather than an exact figure.
When is the correction most useful?
It is most useful in hypoalbuminaemic patients, who are common in hospital and critical care. In these patients an uncorrected gap may appear normal while a clinically important acidosis is present.
Does the corrected gap replace measuring lactate or ketones?
No. The corrected gap flags a raised gap, but you still need to identify the cause, which often means measuring lactate, ketones and renal function and considering toxins.
References
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174.
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