Laboratory & Electrolytes

Anion Gap (with Albumin Correction)

Calculate anion gap and albumin-corrected anion gap for acid-base analysis.

When to use: Calculate anion gap as the first step in any acid-base disorder analysis. Elevated AG (>12) narrows the differential to unmeasured anions (lactate, ketones, uraemic acids, toxins). Always calculate albumin-corrected AG in critically ill, malnourished, or hepatic patients, because hypoalbuminaemia lowers baseline AG and can mask a true HAGMA. Pair with the delta-delta ratio to identify mixed acid-base disorders.

Calculator

Serum Sodium (Na⁺)(mEq/L)

Serum Chloride (Cl⁻)(mEq/L)

Serum Bicarbonate (HCO₃⁻)(mEq/L)

Serum Albumin(g/dL)Normal ~4.0 g/dL; needed for corrected AG in hypoalbuminaemia

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Formula

AG = Na⁺ − (Cl⁻ + HCO₃⁻) [Normal: 8–12 mEq/L] Corrected AG = AG + 2.5 × (4.0 − albumin g/dL) High AG >12: MUDPILES | Normal AG: HARDUP | Low AG <8: hypoalbuminaemia, myeloma

Key Points for NEET PG

Normal anion gap: 8–12 mEq/L. AG = Na − (Cl + HCO₃).
High-AG metabolic acidosis (HAGMA) mnemonic: MUDPILES — Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
Normal-AG metabolic acidosis (NAGMA) mnemonic: HARDUP — Hyperalimentation, Addison disease, Renal tubular acidosis, Diarrhoea, Ureteral diversion, Pancreatic fistula.
For every 1 g/dL drop in albumin below 4, the AG falls by ~2.5 mEq/L — corrected AG unmasks HAGMA in hypoalbuminaemic patients.
Delta-delta ratio = (AG − 12) / (24 − HCO₃): <0.4 suggests NAGMA; 0.4–0.8 mixed HAGMA + NAGMA; 1–2 pure HAGMA; >2 HAGMA + metabolic alkalosis.

References

Emmett M, Narins RG. Clinical use of the anion gap · Medicine (1977)

Feldman M et al. Effectiveness of the anion gap and delta/delta gap in clinical diagnosis of complex acid-base disorders · Am J Kidney Dis (2020)

For educational purposes only. Not for clinical decision-making without professional oversight.

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