Anion Gap Calculator (with Albumin Correction)
Anion gap calculator. Computes Na − (Cl + HCO₃) with optional albumin correction and a normal / raised band for metabolic acidosis. Educational only.
Anion Gap Calculator
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How to use the anion gap calculator
Enter the electrolytes
Type sodium, chloride and bicarbonate in mEq/L from the same basic metabolic panel. These three values give the standard anion gap.
Add albumin to correct it
Albumin is the main unmeasured anion, so a low albumin lowers the apparent gap. Enter albumin to see the corrected value, which adds about 2.5 mEq/L for every 1 g/dL below 4.0.
Optionally include potassium
Some labs include potassium: (Na + K) − (Cl + HCO₃), which shifts the normal range up by about 4. Tick the box if your reference range includes potassium.
Acknowledge, then read the band
A raised, albumin-corrected gap points toward a high-anion-gap metabolic acidosis (remembered by mnemonics such as GOLD MARK). The gap is one step in an acid–base assessment that a clinician completes.
The anion gap — accounting for the body's hidden ions
Balancing the charge sheet
Blood is electrically neutral: the positive ions (cations) and negative ions (anions) must balance. Routine chemistry measures only some of them — chiefly the cation sodium and the anions chloride and bicarbonate. The anion gap is the difference between the measured cation and the measured anions: sodium minus the sum of chloride and bicarbonate. It isn't a real gap in the body; it represents the anions we don't routinely measure — mostly albumin, with small contributions from phosphate, sulphate, and organic acids. A normal gap sits around 8 to 12 mEq/L on older assays, though many modern ion-selective electrodes run lower, around 3 to 11. The number's value is diagnostic: when a metabolic acidosis develops, calculating the gap splits the causes into two groups and points the workup in the right direction.
A high anion gap acidosis means an unmeasured acid has accumulated — lactate in shock, ketoacids in diabetic ketoacidosis, toxins like methanol or ethylene glycol, or the retained acids of kidney failure. Clinicians remember the list with mnemonics such as GOLD MARK. A normal gap acidosis, by contrast, usually reflects bicarbonate loss (diarrhoea) or renal tubular acidosis, where chloride rises to fill the space. So the gap doesn't just describe a number; it sorts a frightening differential into manageable halves.
"The anion gap measures the ions we don't measure. A raised gap says an unexpected acid is in the blood — and tells the clinician which half of the differential to chase."
Why albumin correction is essential
The single biggest pitfall is albumin. Because albumin is itself the dominant unmeasured anion, a low albumin level — extremely common in hospital patients — shrinks the apparent gap and can hide a genuine high-gap acidosis behind a deceptively "normal" number. The correction adds roughly 2.5 mEq/L to the gap for every 1 g/dL the albumin falls below 4.0, restoring the gap's sensitivity. A critically ill patient with an albumin of 2.0 g/dL and a measured gap of 11 actually has a corrected gap near 16 — clearly raised. This is why a corrected anion gap, not the raw value, should be used whenever albumin is low. Even then, the gap is only one part of a structured acid–base assessment that also uses the bicarbonate, the blood pH, the delta ratio, and the clinical context. Reference ranges differ between laboratories, so the band shown here is indicative. A raised gap is a prompt for urgent clinical evaluation — lactate, ketones, renal function, and a toxicology history — not a self-diagnosis, and the interpretation belongs with a clinician.
10 Facts About the Anion Gap
Anion gap = Na − (Cl + HCO₃).
It represents the unmeasured anions, mostly albumin.
Normal ≈ 8–12 mEq/L (3–11 on modern analysers).
A high gap points to an accumulated acid (lactate, ketones, toxins).
A normal gap acidosis usually means bicarbonate loss.
Mnemonic for high-gap causes: GOLD MARK.
Low albumin lowers the gap — correct before interpreting.
Correction: +2.5 mEq/L per 1 g/dL albumin below 4.0.
Including potassium shifts the normal range up by ~4.
It's one part of a full acid–base assessment.
Frequently asked questions
It is the difference between the measured cation sodium and the measured anions chloride and bicarbonate: Na − (Cl + HCO₃). It estimates the concentration of anions that routine tests don't measure — mostly albumin, plus phosphate, sulphate, and organic acids. The number is used to classify metabolic acidosis and to detect unmeasured acids in the blood.
Classically about 8–12 mEq/L, but modern ion-selective electrode analysers often report lower ranges, around 3–11 mEq/L. If potassium is included in the calculation, the normal range shifts up by roughly 4. Because the range is assay-dependent, you should compare your result with the reference range used by the laboratory that produced the electrolytes.
Albumin is the largest unmeasured anion, so a low albumin reduces the apparent gap and can hide a real high-gap acidosis. The correction adds about 2.5 mEq/L to the gap for every 1 g/dL the albumin is below 4.0. In hospital patients, who frequently have low albumin, using the corrected gap rather than the raw value is essential to avoid missing significant acidosis.
An accumulation of unmeasured acid. Common causes include lactic acidosis (shock, sepsis), ketoacidosis (diabetic, alcoholic, starvation), kidney failure (retained acids), and certain toxins such as methanol, ethylene glycol, and large salicylate overdoses. The mnemonic GOLD MARK (Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis) is one way clinicians recall the list.
A metabolic acidosis with a normal gap (also called hyperchloraemic acidosis) usually reflects loss of bicarbonate rather than gain of acid — most often from diarrhoea or from renal tubular acidosis. Chloride rises to maintain electrical neutrality, keeping the gap normal. Distinguishing high-gap from normal-gap acidosis is the first fork in working out the cause, which is exactly what the gap is for.
Both conventions exist. Most clinicians use the sodium-only formula, Na − (Cl + HCO₃), because potassium varies little and the normal range is well established. Some laboratories include potassium, (Na + K) − (Cl + HCO₃), which raises the normal range by about 4. What matters is consistency: use the same formula your reference range was set for. The calculator lets you toggle potassium in or out.
No. The gap is one component of a structured acid–base assessment that also uses the bicarbonate, the arterial or venous blood gas (pH, CO₂), the delta ratio, and the clinical picture. A raised gap signals that an unmeasured acid is present and directs further tests (lactate, ketones, renal function, toxicology), but the full interpretation — and any treatment — requires those additional data and clinical judgement.
Yes, though it's less common and easy to miss. A low anion gap can occur with very low albumin (the commonest reason), with certain paraproteins as in some myelomas, or with lithium, bromide, or severe hypercalcaemia. Because a low gap is often an artefact of low albumin, correcting for albumin matters here too. A persistently low corrected gap can occasionally be a clue to an underlying condition worth investigating.
No. A raised anion gap can indicate serious, time-critical conditions such as diabetic ketoacidosis, sepsis, or poisoning. This tool is educational, to show how the gap is calculated and what it signifies. If you have abnormal electrolytes or symptoms, the result needs urgent professional assessment in the context of a full acid–base evaluation — not a self-diagnosis.
No. The electrolyte and albumin values you enter are processed only in your browser. Nothing is transmitted to a server, stored, or shared, and no account is required. The calculation runs entirely on your device.
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