Free tool
Anion Gap Calculator.
The anion gap is the quickest way to split a metabolic acidosis into high-gap vs normal-gap — which points you at completely different cause lists. Enter the electrolytes off the BMP, optionally correct for a low albumin (which hides a real gap), and get the gap with the formula, a worked example, and a plain-English read.
Serum Anion Gap
Pull these straight off the basic metabolic panel (BMP / CMP).
Enter sodium, chloride, and bicarbonate.
How to read an anion gap
| Gap (mEq/L) | Read | What it points to |
|---|---|---|
| Low (< 8) | Low gap | Most often a low albumin (correct for it first). Also lab error, paraproteinemia (e.g., myeloma), or bromide/lithium toxicity. |
| Normal (8–12) | Normal gap | If an acidosis is present, think normal-gap (hyperchloremic) acidosis — diarrhea, RTA, saline. (HARDASS.) |
| High (> 12) | High gap | If an acidosis is present, think high-anion-gap metabolic acidosis (HAGMA) — see MUDPILES below. |
Typical normal anion gap is 8–12 mEq/L using Na − (Cl + HCO₃). Reference ranges vary by lab and analyzer (some report 3–11). If you include potassium, normal shifts up to roughly 12–16 mEq/L. The gap is a clue, not a diagnosis — interpret it alongside the pH/HCO₃ on an ABG and the whole clinical picture.
Why correct for albumin?
Albumin is the biggest unmeasured anion. When it's low (sick, malnourished, cirrhotic patients), the measured gap drops by roughly 2.5 mEq/L for every 1 g/dL below 4.0 — so a true high-gap acidosis can hide inside a "normal" gap. Corrected anion gap = measured gap + 2.5 × (4.0 − albumin). Enter an albumin above and this tool shows both.
High-gap causes — MUDPILES
| M | Methanol |
| U | Uremia (renal failure) |
| D | Diabetic (and other) ketoacidosis — DKA, alcoholic, starvation |
| P | Propylene glycol (e.g., prolonged lorazepam infusions) |
| I | Iron, Isoniazid (INH) |
| L | Lactic acidosis (sepsis, shock, ischemia, metformin) |
| E | Ethylene glycol |
| S | Salicylates (aspirin) |
A common normal-gap mnemonic is HARDASS (Hyperalimentation, Acetazolamide, RTA, Diarrhea, Addison's, Spironolactone, Saline). Use these as memory aids — confirm the cause clinically.
References
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174. (Anion gap = Na − [Cl + HCO₃]; normal range and albumin effect.)
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434–1445. (Albumin correction ≈ 2.5 mEq/L per 1 g/dL below 4.0; MUDPILES differential.)