Anion Gap Calculator

About the Anion Gap Calculator

The anion gap is the difference between the main measured cation in blood, sodium, and the main measured anions, chloride and bicarbonate: Na⁺ − (Cl⁻ + HCO₃⁻). The negative charges it leaves out — mostly albumin, phosphate and other unmeasured anions — make the normal gap. When an acid such as lactate or a ketone builds up, its anion widens the gap, so a high anion gap points to a high-anion-gap metabolic acidosis.

This calculator gives the anion gap with the working, the gap corrected for albumin (Figge’s formula, because a low albumin hides a high gap), the gap with potassium if you enter it, and for a high gap the delta gap and delta ratio, which hint at a second metabolic disorder. Every reference value — the laboratory’s range, the normal gap for the delta and the normal albumin — can be changed to match your report. Nothing you type leaves your browser.

Without a pass, the free preview shows the anion gap in full with what it means and the first rows of the working table; a Pro pass unlocks the rest of the table, every other figure and Copy result.

Common uses

• Working out the anion gap from a chemistry panel that does not print it.
• Correcting the anion gap for a low albumin in hospitalised or critically ill patients.
• Checking a high-anion-gap acidosis for a second metabolic disorder with the delta ratio.
• Teaching and revising how the anion gap is built and why its normal range varies.

The formulas

• Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), all in mmol/L.
• With potassium = Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻), the original definition; most textbooks now leave potassium out because its changes are small, but including it helps when potassium is far from normal.
• Albumin-corrected anion gap (Figge) = anion gap + 2.5 × (normal albumin − measured albumin) with albumin in g/dL, or + 0.25 × the difference in g/L. The anion gap falls by about 2.3–2.5 mmol/L for every 1 g/dL fall in albumin.
• Delta gap = anion gap − normal anion gap, and delta ratio = delta gap ÷ (24 − HCO₃⁻): how far the gap has risen compared with how far the bicarbonate has fallen. The calculator uses the albumin-corrected gap when you enter albumin.

Why your laboratory’s range matters

The normal anion gap depends on how the laboratory measures the electrolytes. Textbook ranges run from 3–11 mmol/L with ion-selective electrodes up to 8–16 mmol/L in other textbooks, and the normal range in a population can span 8–10 mmol/L. Analysers with ion-selective electrodes report higher chloride, so their anion gaps are lower. A review of the anion gap concludes that the clinician must know the mean and the range of their own laboratory, and that a baseline gap of the same person, corrected for albumin, is the most accurate comparison for spotting a new acidosis. This page uses 8–12 mmol/L only when you do not enter your laboratory’s range.

Reading the delta ratio

If one acid is added, each mmol of anion that widens the gap should use up about one mmol of bicarbonate, a ratio near 1. In practice the ratio is close to 1 in ketoacidosis and averages about 1.5 in lactic acidosis, so a range of values counts as one disorder:

• below 0.4 — the bicarbonate has fallen much more than the gap has risen: a normal-anion-gap (hyperchloraemic) acidosis is likely the main process;
• 0.4 to below 0.8 — a high-anion-gap and a normal-anion-gap acidosis together are likely;
• 0.8 to 2 — fits a high-anion-gap acidosis on its own;
• above 2 — the bicarbonate is higher than the gap explains: a metabolic alkalosis as well, or a high bicarbonate from before, such as a compensated chronic respiratory acidosis.

These cut-offs are rules of thumb and the sources differ — one review uses 0.8 and 1.2 — so treat the delta ratio as a clue to look further, not as a diagnosis.

Sources

• Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved? — Clin J Am Soc Nephrol
• Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine — Clin J Am Soc Nephrol
• Figge J et al. Anion gap and hypoalbuminemia — Crit Care Med
• Breathing and balance: clinical insights and management strategies of respiratory acid-base disorders — Nutr Clin Pract: the delta ratio with a normal gap of 12 and bicarbonate of 24
• Analysis of acid-base disorders in an ICU cohort using a computer script — Intensive Care Med Exp: delta ratios below 0.4 and above 2
• Acid-base disorders in patients with chronic obstructive pulmonary disease: a pathophysiological review — J Biomed Biotechnol: a delta ratio of 1–2 in a pure acidosis
• Hyperglycemic crises in adults with diabetes: a consensus report — Diabetes Care: the anion gap in diabetic ketoacidosis

Limitations

• The anion gap is one clue: it cannot tell a metabolic acidosis from the compensation for a respiratory alkalosis without the pH and PaCO₂ — use the ABG / Acid-Base Interpreter for a blood gas.
• Normal ranges depend on the laboratory and analyser; results compared with another laboratory’s range can mislead.
• A normal anion gap does not rule out lactic acidosis or another high-anion-gap acidosis, especially at a low albumin.
• Accepts sodium 100–200, chloride 50–150, bicarbonate 2–60 and potassium 1–10 mmol/L, and albumin 0.5–7 g/dL (5–70 g/L).

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