Last reviewed: March 2026
This is an educational reference tool only. Acid-base interpretation requires integration with the full clinical picture, blood gas results, and clinical context.
The default normal AG of 12 mmol/L is a historical value. Many modern analysers report a lower reference range (6–10 mmol/L without K). Always verify your laboratory's specific reference range before interpreting results.
The anion gap (AG) is a calculated value derived from routine electrolyte measurements — sodium, chloride, and bicarbonate. It represents the difference between the measured cations and measured anions in serum, reflecting the concentration of unmeasured anions such as albumin, phosphate, sulphate, and organic acids.
In clinical practice, the anion gap is used primarily to classify metabolic acidosis. An elevated AG points to the accumulation of an unmeasured acid (e.g., lactate, ketoacids, toxins), while a normal AG suggests a different pathophysiological mechanism (e.g., renal tubular acidosis, diarrhoea).
The most useful mnemonic for causes of high-AG metabolic acidosis is MUDPILES: Methanol, Uraemia, Diabetic ketoacidosis (DKA), Propylene glycol / Paracetamol, Isoniazid / Iron, Lactic acidosis, Ethylene glycol, Salicylates.
The standard anion gap calculation and its albumin correction are computed using well-established formulas.
Enter the patient's serum sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻) in mmol/L. Optionally add potassium (K⁺) to compute the AG with K⁺, and albumin (g/dL) to generate the albumin-corrected AG.
The calculator also accepts glucose and BUN (blood urea nitrogen) to compute the osmolal gap (when serum osmolality is available) — an additional tool for detecting occult toxins such as methanol and ethylene glycol.
Physiological plausibility validation is applied to all inputs. Values falling outside expected clinical ranges (e.g., sodium below 100 or above 180 mmol/L) trigger inline field-level warnings, reducing the risk of transcription errors entering the calculation silently.
Results are displayed with interpretation guidance: an AG below 8 mEq/L prompts consideration of hypoalbuminaemia and reduced unmeasured anions; an AG above 12 mEq/L (or the lab's upper limit) suggests a high-gap acidosis requiring further investigation of the MUDPILES causes.
Session history records all calculations with timestamps and supports CSV export for documentation in clinical notes.
Normal AG (8–12 mEq/L) in a patient with metabolic acidosis suggests a hyperchloraemic (non-gap) metabolic acidosis. Common causes include: renal tubular acidosis (RTA), diarrhoea (loss of bicarbonate), ureteral diversion, and saline administration.
Elevated AG (>12 mEq/L) indicates accumulation of unmeasured acids. The MUDPILES mnemonic covers the major causes. Lactic acidosis (from sepsis, hypoperfusion, or metformin toxicity) and diabetic ketoacidosis are the most common in clinical practice.
Low AG (<6 mEq/L) should prompt measurement or verification of albumin, as hypoalbuminaemia is by far the most common reason for an unexpectedly low gap. A low albumin 'masks' the gap — the corrected AG is typically normal or elevated.
The delta-delta ratio ((AG − 12) ÷ (24 − HCO₃)) is a secondary calculation used to detect a mixed high-gap and non-gap acidosis, or a concurrent metabolic alkalosis, in patients with an elevated AG. A ratio below 1 suggests a co-existing non-gap process; above 2 suggests a concurrent metabolic alkalosis.
The osmolal gap is the difference between the measured serum osmolality and the calculated osmolality. Calculated osmolality = 2 × Na⁺ + Glucose (mmol/L) + Urea (mmol/L), or in US units: 2 × Na⁺ + [Glucose (mg/dL) ÷ 18] + [BUN (mg/dL) ÷ 2.8].
A gap above 10–20 mOsm/kg suggests the presence of an unmeasured osmotically active substance, most importantly: ethanol, methanol, ethylene glycol, isopropanol, or propylene glycol. A normal osmolal gap does not exclude methanol or ethylene glycol toxicity in the late stages when the parent alcohol has been metabolised.
Standard clinical interpretation guide for anion gap and albumin-corrected anion gap. Always confirm your institution's reference range, as values vary by methodology.
| Anion Gap (mEq/L) | Interpretation | Common Causes |
|---|---|---|
| <6 mEq/L | Low AG | Hypoalbuminaemia, multiple myeloma, lithium toxicity |
| 6–8 mEq/L | Low-normal AG | May still indicate mild hypoalbuminaemia — check albumin |
| 8–12 mEq/L | Normal AG | Normal, or non-gap metabolic acidosis (RTA, diarrhoea) |
| 12–20 mEq/L | Mildly elevated | Early high-gap acidosis, DKA, mild lactic acidosis |
| 20–30 mEq/L | Moderately elevated | Significant DKA, lactic acidosis, renal failure |
| >30 mEq/L | Markedly elevated | Severe lactic acidosis, toxic alcohol ingestion, salicylates |
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Usman Shaoukat is the founder and editor of BestMedCal. He is a computer and web specialist who researches every BestMedCal tool from published standards and medication-safety references, including NIST, the U.S. FDA, NCBI and the American Academy of Pediatrics. His focus is dose-to-volume calculations and unit conversions. Every formula and worked example is checked against the sources listed on each page. BestMedCal has no clinicians on staff: our tools check arithmetic and explain methods; they do not recommend doses. If you spot an error, contact us and we will review it.