Anion Gap Calculator - Acid-Base Balance Assessment

Calculate serum anion gap and albumin-corrected AG. Essential tool for evaluating metabolic acidosis and electrolyte disorders.

Sodium Na⁺ (mEq/L)

Normal Range: 135-145 mEq/L

Chloride Cl⁻ (mEq/L)

Normal Range: 95-105 mEq/L

Bicarbonate HCO₃⁻ (mEq/L)

Normal Range: 22-28 mEq/L

Albumin (g/dL) - Optional

Normal Range: 3.5-5.0 g/dL

Calculate albumin-corrected anion gap

Frequently Asked Questions (FAQ)

Why is calculating the anion gap important?

The anion gap is a key diagnostic tool for metabolic acidosis: (1) Differential Diagnosis: Distinguishes high AG from normal AG metabolic acidosis, narrowing the diagnostic scope. (2) Detect Mixed Disorders: Identifies complex acid-base disturbances through delta gap analysis. (3) Toxicology Screening: Elevated AG is an important clue for certain drug toxicities. (4) Assess Severity: Higher AG typically indicates more severe conditions. (5) Monitor Treatment: Dynamic AG monitoring helps evaluate treatment efficacy.

When should albumin-corrected anion gap be used?

Albumin-corrected AG should be used in: (1) Albumin < 4.0 g/dL (hypoalbuminemia). (2) Albumin > 4.5 g/dL (hyperalbuminemia, rare). (3) Liver cirrhosis, nephrotic syndrome, malnutrition. (4) Critically ill patients (often have hypoalbuminemia). (5) Multiple trauma or post-major surgery patients. Uncorrected AG may miss high AG metabolic acidosis, while corrected AG more accurately reflects acid-base status.

What is the normal reference range for anion gap?

Traditionally, the normal anion gap range is 8-16 mEq/L (average ~12 mEq/L). Important considerations: (1) Different laboratories may vary slightly (typically 6-14 or 8-16 mEq/L). (2) Modern automated analyzers measure chloride more precisely, slightly lowering normal AG values. (3) Albumin levels affect AG and correction should be considered. (4) AG > 20 mEq/L almost always indicates metabolic acidosis. (5) AG > 30 mEq/L usually indicates severe metabolic acidosis.

Does an elevated anion gap always indicate acidosis?

The most common cause of elevated AG is metabolic acidosis, but not the only one: (1) Metabolic Acidosis (most common): DKA, lactic acidosis, uremia. (2) Metabolic Alkalosis: AG may be mildly elevated in severe metabolic alkalosis. (3) Laboratory Error: Pseudohyponatremia or hyperchloremia, hyperlipidemia. (4) Dehydration: Hemoconcentration causing elevated ion concentrations. (5) Massive Antibiotic Infusion: Penicillins, carbenicillin. Therefore, elevated AG requires clinical context and other blood gas analysis results for accurate interpretation.

What is the clinical significance of the MUDPILES mnemonic?

MUDPILES is a mnemonic for causes of high anion gap metabolic acidosis: M - Methanol toxicity, U - Uremia (renal failure), D - Diabetic Ketoacidosis (DKA), P - Paraldehyde (now rare), I - INH (isoniazid), Iron poisoning, L - Lactic acidosis, E - Ethanol, Ethylene glycol, S - Salicylates (aspirin). This mnemonic helps clinicians quickly recall and evaluate potential causes of elevated AG in emergency settings, guiding targeted diagnostic tests and treatments.

How is the delta gap (delta-delta) used in clinical practice?

The delta gap evaluates mixed acid-base disorders: Delta Gap = Measured AG - 12. Expected HCO₃⁻ = 24 - Delta Gap. Interpretation: (1) Measured HCO₃⁻ ≈ Expected HCO₃⁻: Pure high AG metabolic acidosis. (2) Measured HCO₃⁻ > Expected HCO₃⁻: High AG metabolic acidosis + metabolic alkalosis. (3) Measured HCO₃⁻ < Expected HCO₃⁻: High AG metabolic acidosis + normal AG metabolic acidosis. This approach helps identify complex mixed disorders that require comprehensive management.

Medical Disclaimer

The results provided by this calculator are for reference only and cannot replace professional medical diagnosis. The interpretation of anion gap requires combining clinical symptoms, other laboratory tests, and blood gas analysis results. The diagnosis of acid-base balance disorders must be performed by medical professionals. If you have health problems or need to interpret laboratory results, please consult a doctor. Seek immediate medical attention in case of emergency.

The anion gap is a bookkeeping check on the electrolytes in blood. Measured cations and measured anions never balance exactly, because routine panels do not measure every charged particle present; the residual difference is the anion gap. A gap that has widened is one of the fastest ways to narrow down the cause of a metabolic acidosis at the bedside, because it separates acidosis caused by an accumulating acid from acidosis caused by losing bicarbonate.

The formula

Anion gap = Na+ - (Cl- + HCO3-)
Corrected AG = AG + 2.5 x (4.0 - albumin in g/dL)
Delta ratio = (AG - 12) / (24 - HCO3-)

Na+ = serum sodium in mmol/L
Cl- = serum chloride in mmol/L
HCO3- = serum bicarbonate in mmol/L
Albumin correction is applied because albumin is the largest unmeasured anion

The three-electrolyte form is the version in general clinical use; some centres add potassium, which raises the expected range by about 4 mmol/L. The albumin correction of 2.5 mmol/L per 1 g/dL below normal follows Figge et al., Journal of Laboratory and Clinical Medicine, 1998.

How the calculation works

Blood is electrically neutral, so the total positive charge equals the total negative charge. A routine panel measures one major cation, sodium, and two major anions, chloride and bicarbonate. Subtracting the measured anions from the measured cation leaves a positive residue that represents the unmeasured anions, chiefly albumin, phosphate, sulfate and organic acids, minus the unmeasured cations such as potassium, calcium and magnesium.

When an acid accumulates in the blood, its hydrogen ion is buffered by bicarbonate, which falls, while the acid anion that came with it is not measured by the panel. Chloride does not change to compensate. The result is a widened gap. If instead the body loses bicarbonate directly, through diarrhoea or renal tubular acidosis, the kidneys retain chloride to preserve neutrality, bicarbonate and chloride move in opposite directions by the same amount, and the gap stays normal.

How to read your result

Reference ranges depend on the laboratory. Older ion-selective electrode methods gave a normal range around 8 to 16 mmol/L; contemporary analysers measure chloride slightly higher and typically produce a normal range of about 3 to 11 mmol/L. Because of this drift, the gap should always be compared against the reporting laboratory range rather than a remembered figure.

The classic causes of a high gap are captured by the mnemonic GOLD MARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.
Every 1 g/dL fall in albumin below 4 g/dL lowers the measured gap by roughly 2.5 mmol/L, so an unwell patient with low albumin can have a genuinely raised gap that reads as normal.

Finding	What it points to
Raised gap with low bicarbonate	High anion gap metabolic acidosis, an acid is accumulating
Normal gap with low bicarbonate	Normal anion gap (hyperchloraemic) acidosis, bicarbonate is being lost
Raised gap with normal bicarbonate	Consider a mixed disorder, or dehydration and haemoconcentration
Low or negative gap	Usually low albumin; also consider paraproteinaemia, lithium, bromide or severe hypercalcaemia

The delta ratio and mixed disorders

In a pure high anion gap acidosis, the gap rises by about as much as bicarbonate falls, because each accumulated acid anion displaces one bicarbonate. Comparing the two changes as a ratio therefore tests whether a second acid-base disorder is hiding underneath. A ratio near 1 to 2 is consistent with a single high gap acidosis.

A ratio below 1 suggests bicarbonate has fallen further than the gap has risen, pointing to a coexisting normal gap acidosis. A ratio above 2 suggests bicarbonate is higher than the gap alone would predict, pointing to a coexisting metabolic alkalosis or a compensated chronic respiratory acidosis. The delta ratio is a screening heuristic and performs poorly at the extremes, so it is used to raise questions rather than settle them.

Limitations

Reference ranges differ substantially between laboratories because chloride methods differ. A gap must be read against the range printed on the report that produced it.
Hypoalbuminaemia masks a raised gap. Without the albumin correction, a critically ill patient with an albumin of 2 g/dL can show a normal-looking gap while a significant acid load is present.
The gap identifies a pattern, not a diagnosis. A raised gap narrows the differential but says nothing about which acid is accumulating; that requires lactate, ketones, osmolar gap, salicylate level or renal function.
Severe hyperlipidaemia or paraproteinaemia can interfere with sodium measurement by some methods, distorting the gap in either direction.
The delta ratio assumes a one-to-one exchange between the accumulating anion and bicarbonate, which holds only approximately, and it becomes unreliable when the gap is only mildly raised.
This calculator arithmetically combines values you enter. It cannot detect a transcription error, and it is not a substitute for clinical interpretation of a blood gas alongside the patient.

Frequently asked questions

What is a normal anion gap?

Roughly 3 to 11 mmol/L on contemporary analysers, or about 8 to 16 mmol/L on older methods, when calculated without potassium. Adding potassium raises the expected range by about 4 mmol/L. Always compare against the reporting laboratory range.

Why correct the anion gap for albumin?

Albumin is negatively charged and is the largest single contributor to the unmeasured anions. When albumin is low, the gap falls for reasons unrelated to acid, which can hide a real high gap acidosis. The correction adds back about 2.5 mmol/L for every 1 g/dL that albumin sits below 4 g/dL.

What does a high anion gap mean?

It means an unmeasured acid anion is accumulating, most often lactate, ketoacids, or products of kidney failure or a toxic alcohol ingestion. It narrows the differential but does not identify which acid; that needs specific tests.

Can the anion gap be negative?

Yes, though it is uncommon. A negative gap usually reflects a laboratory interference, such as bromide or iodide poisoning, severe hyperlipidaemia, or a paraprotein, or a marked increase in unmeasured cations such as in severe hypercalcaemia or lithium toxicity.

Does a normal anion gap rule out acidosis?

No. Acidosis from bicarbonate loss, such as severe diarrhoea or renal tubular acidosis, produces a normal gap because chloride rises as bicarbonate falls. A normal gap distinguishes the type of acidosis; it does not exclude one.