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Calculates the albumin-corrected anion gap. In hypoalbuminemia, the measured anion gap is falsely low, potentially masking a high anion gap metabolic acidosis. This correction adjusts for the albumin contribution.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
The measured anion gap (Na − (Cl + HCO₃)) reflects the balance between unmeasured anions and unmeasured cations in the blood. Albumin is the dominant negatively charged serum protein and makes the largest contribution to the unmeasured anion pool, so hypoalbuminemia lowers the measured anion gap and can obscure a clinically important high anion gap metabolic acidosis. This calculator corrects for low albumin by adding 2.5 mmol/L to the measured anion gap for every 1 g/dL that albumin is below 4.0 g/dL. The correction is most useful in patients with low albumin from critical illness, cirrhosis, nephrotic syndrome, or malnutrition, in whom an uncorrected normal anion gap may actually represent a significant unmeasured anion state. The 2.5 mmol/L per 1 g/dL factor is an approximation rather than a precisely measured value, and the albumin–anion gap relationship varies between individuals and with pH. The corrected gap should be interpreted together with the measured anion gap, serum albumin, electrolytes, blood gas, and clinical context; it does not by itself identify a specific cause of acidosis.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-06
Formula
Keywords
Normal Range
8–12 mmol/L
| Classification | Range |
|---|---|
Low corrected anion gap | <7.1 |
Normal corrected anion gap | 8–12 |
High corrected anion gap | ≥13 |
Markedly elevated corrected anion gap | ≥20 |
Adjusts the serum anion gap for hypoalbuminemia to unmask a hidden high anion gap metabolic acidosis (HAGMA) that a low albumin would otherwise conceal.
The albumin-corrected anion gap = (Na - (Cl + HCO3)) + 2.5 x (4.0 - serum albumin). The correction reveals a high anion gap that would otherwise be masked by hypoalbuminemia.
Serum sodium in mmol/L, used in the uncorrected anion gap.
Serum chloride in mmol/L, added to bicarbonate in the anion gap.
Serum bicarbonate in mmol/L, added to chloride in the anion gap.
Serum albumin in g/dL; each 1 g/dL below 4.0 adds 2.5 mmol/L to the corrected gap.
A corrected anion gap of 8–12 mmol/L is normal. ≥13 mmol/L indicates a high corrected anion gap, and ≥20 mmol/L is markedly elevated. Each 1 g/dL drop in albumin below 4.0 g/dL lowers the measured anion gap by roughly 2.5 mmol/L.
Albumin normally contributes roughly 75% of the anion gap. In hypoalbuminemia the measured gap is falsely low, so correction is essential in critically ill patients where a HAGMA could otherwise be missed.
A 62-year-old ICU patient has sodium 140 mmol/L, chloride 105 mmol/L, bicarbonate 12 mmol/L, and albumin 3.0 g/dL.
Inputs:
Expected Result: Corrected AG = (140 − (105 + 12)) + 2.5 × (4 − 3) = 23 + 2.5 = 25.5 mmol/L — markedly elevated, consistent with a high anion gap metabolic acidosis.
Albumin is a negatively charged protein that contributes significantly to the normal anion gap. When albumin is low, there are fewer unmeasured negative charges, so the calculated gap falls even when an acidosis is present.
The most common correction adds 2.5 mmol/L to the measured anion gap for every 1 g/dL that albumin falls below 4.0 g/dL. Some sources use 2.4 or 2.8 mmol/L per g/dL depending on the population.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Interpret the corrected anion gap together with arterial blood gas and the clinical context.