Anion Gap Calculator
The serum anion gap from a basic metabolic panel — with the optional albumin correction that keeps a low albumin from hiding a high-gap acidosis.
Written by Apex Respiratory Editorial Team
Enter to apply the albumin correction.
Enter sodium, chloride, and bicarbonate to calculate the anion gap. Albumin is optional.
Reading the gap
The anion gap estimates the unmeasured anions in serum and is the first fork in working up a metabolic acidosis. A high gap points to added acid — lactate, ketones, toxic alcohols, salicylate, or uremia (MUDPILES). A normal gap points to bicarbonate loss — diarrhea or renal tubular acidosis (HARDASS).
The reference range is lab- and analyzer-specific; this tool uses the widely taught 8–12 mEq/L for the potassium-excluded formula. Because albumin is the dominant unmeasured anion, a low albumin lowers the measured gap and can mask a real acidosis — enter the albumin to apply the correction (add ~2.5 mEq/L per 1 g/dL below 4.0 g/dL).
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162-174.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-1810.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Acid-base balance.