🧪 Anion Gap Calculator

Blood must stay electrically neutral: positive ions (sodium, potassium) balance negative ones (chloride, bicarbonate). But routine labs do not measure every anion, so the measured positives always exceed the measured negatives. That difference, the anion gap, represents the unmeasured anions: mostly albumin, plus phosphates, sulfates, and any pathological acids.

A normal gap is 8 to 12. A high gap means unmeasured acids have accumulated: lactate, ketoacids, toxins. This calculator computes the gap, corrects it for low albumin (which falsely lowers it), and applies the delta ratio to detect mixed disorders.

How to use this calculator

Take sodium, chloride, and bicarbonate from the basic metabolic panel, plus albumin if available.

The formulas

The gap is simple subtraction. The albumin correction matters because albumin is the main unmeasured anion: when it is low, the gap shrinks and can look normal despite real acidosis.

The delta ratio refines the delta gap: under 0.4 suggests pure normal-gap acidosis alongside, 0.4 to 0.8 a combined high and normal gap acidosis, 0.8 to 2 a pure high-gap acidosis, and above 2 a high-gap acidosis plus metabolic alkalosis.

Worked example

Sodium 140, chloride 105, bicarbonate 22, albumin 4.0. The gap is 140 - (105 + 22) = 13, just above the normal 8 to 12. Albumin is normal, so no correction needed: a mildly high gap worth noting.

Now the same electrolytes with albumin 2.0, common in critical illness. Corrected gap = 13 + 2.5 x (4.0 - 2.0) = 13 + 5 = 18. The uncorrected 13 looked borderline; the corrected 18 is clearly high. Missing the correction is one of the most common acid-base errors in the ICU.

Causes of a high gap

The MUDPILES mnemonic organizes them: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates. In practice, lactate and ketoacids dominate, and the osmolar gap sorts the toxic alcohols.

A low gap (under 8) is rarer: lab error, severe hypoalbuminemia (correct it first), multiple myeloma's cationic proteins, or lithium and bromide interference. For the full acid-base workup around the gap, use our ABG interpreter and arterial pH calculator; for bicarbonate replacement math, see bicarbonate deficit.

Normal gap acidosis

A normal gap with low bicarbonate means bicarbonate was lost rather than consumed by new acids: diarrhea, renal tubular acidosis, ureteral diversions, or large volumes of normal saline (whose chloride replaces bicarbonate). The urine anion gap and urine pH sort renal from gut causes.

Treatment differs by type: high-gap acidosis needs the acid source fixed (insulin, perfusion, dialysis), while normal-gap acidosis often just needs the losses stopped and bicarbonate repleted. The gap is what tells you which problem you have.

The osmolar gap: catching toxic alcohols

When the anion gap is high and lactate, ketones, and kidney failure do not explain it, the osmolar gap hunts toxic alcohols. Measured osmolality minus calculated osmolality (2 x Na + glucose/18 + BUN/2.8) should be under 10. Methanol and ethylene glycol raise it early, before they metabolize into the acids that raise the anion gap.

The classic double-gap pattern, high osmolar gap plus high anion gap, is toxic alcohol ingestion until proven otherwise, and it is an emergency: fomepizole and often dialysis. A high anion gap with a normal osmolar gap points instead toward lactate, ketoacids, or uremia.

The urine anion gap

In normal-gap metabolic acidosis, the urine anion gap (Na + K - Cl in urine) separates gut losses from kidney causes. A negative urine gap means the kidneys are excreting ammonium properly, pointing to diarrhea or other extrarenal bicarbonate loss. A positive gap means the kidneys are failing to acidify urine, pointing to renal tubular acidosis.

It is a niche test but a beautiful one: a single urine electrolyte panel answering "kidney or gut" in a patient with unexplained normal-gap acidosis. Like the serum gap, it is a subtraction that reveals what direct measurement cannot see.

Salicylate poisoning: the classic trap

Aspirin overdose produces a high anion gap acidosis plus a respiratory alkalosis: the salicylate stimulates the brain's breathing center directly. The pH may look deceptively normal while both disorders rage, and the delta ratio plus the clinical story (tinnitus, hyperventilation, confusion) unmasks it.

Treatment is urinary alkalinization and often hemodialysis, and the trap is underestimation: salicylate levels can keep rising as pills dissolve, so a "mild" early level means little. Any suspected ingestion with an unexplained high gap deserves toxicology input fast.

Lactate: the most common high gap

Lactic acidosis is the high-gap acidosis clinicians see most. Type A comes from poor tissue perfusion: shock, cardiac arrest, severe hypoxemia. Type B comes from metabolism gone wrong without hypoperfusion: metformin accumulation, malignancy, liver failure, or even intense exercise and seizures.

The treatment split follows the type: restore perfusion for type A, fix the underlying cause for type B. Bicarbonate drips treat the number, not the disease, and are reserved for extreme cases. When the gap is high and the cause is unclear, lactate is the first test to check.

For informational purposes only, not medical advice. Consult a healthcare professional.