What is the anion gap formula?

The standard anion gap is AG = Na+ - (Cl- + HCO3-), using serum concentrations in mEq/L. The normal range is 8-12 mEq/L when serum albumin is approximately 4.0 g/dL. The formula works because sodium is the dominant cation, chloride and bicarbonate are the dominant measured anions, and the "gap" represents unmeasured anions such as albumin, phosphate, sulfate and organic acids.

Why do I need to correct the anion gap for albumin?

Albumin is a negatively charged protein that contributes roughly 2.5 mEq/L per g/dL to the anion gap. In hypoalbuminaemia (common in critically ill patients) the raw anion gap is artificially low and can hide a high-AG metabolic acidosis. The Figge-Jabor-Kazda-Fencl correction adds 2.5 x (4.0 - measured albumin) to the raw AG, restoring it to what it would be at a normal albumin of 4.0 g/dL.

What does the delta ratio tell you?

The delta ratio (also called delta-delta or delta gap) is (AG - 12) / (24 - HCO3-). It estimates whether the rise in the anion gap is fully explained by the fall in bicarbonate. A ratio of 1-2 indicates a pure HAGMA with expected compensation. Below 1 suggests an additional normal-AG (hyperchloraemic) acidosis is present. Above 2 suggests a concurrent metabolic alkalosis or a chronically elevated baseline bicarbonate.

What is the MUDPILES mnemonic?

MUDPILES is a memory aid for the common causes of high-AG metabolic acidosis: Methanol, Uraemia (renal failure), Diabetic ketoacidosis or alcoholic ketoacidosis, Propylene glycol, Iron or Isoniazid toxicity, Lactic acidosis, Ethylene glycol and Salicylates (aspirin overdose). An elevated osmolar gap alongside a high anion gap strongly suggests toxic alcohol ingestion (methanol or ethylene glycol).

What is the osmolar gap and when is it clinically important?

The osmolar gap is the difference between the directly measured serum osmolality and the osmolality calculated from the standard formula: 2xNa + glucose/18 + BUN/2.8. A gap below 10 mOsm/kg is normal. An elevated osmolar gap in the context of a high anion gap metabolic acidosis is a red flag for toxic alcohol ingestion — the osmotically active alcohol raises the measured osmolality before it is metabolised into acidic metabolites that subsequently raise the anion gap.

Is this calculator suitable for clinical use?

This tool is designed for educational purposes and rapid mental-arithmetic verification. It implements the same standard formulas used in published medical references and clinical decision-support systems. However, it does not replace laboratory-reported values, clinical assessment or professional medical judgement. Always verify results against your institution's laboratory reference ranges and the full clinical picture.

Anion Gap Calculator

The anion gap is one of medicine’s most widely used bedside calculations. Three numbers from a basic metabolic panel — sodium, chloride and bicarbonate — reveal whether the blood’s acid-base chemistry is normal or hiding a dangerous metabolic acidosis. This calculator does the arithmetic instantly, adds an albumin correction when serum albumin is low, computes the delta ratio to classify mixed disorders, and optionally calculates the osmolar gap to screen for toxic alcohol ingestion. Everything runs in your browser — no patient data is ever transmitted.

How it works

Standard anion gap

Plasma is electrically neutral: total cations equal total anions. The major measured cation is sodium (Na+). The major measured anions are chloride (Cl-) and bicarbonate (HCO3-). The remaining anions — albumin, phosphate, sulfate and organic acids — are “unmeasured” and make up the gap:

AG = Na+ - (Cl- + HCO3-)

Normal value: 8–12 mEq/L (assuming albumin ~4.0 g/dL). A value above 12 indicates an accumulation of unmeasured anions — the hallmark of high-AG metabolic acidosis (HAGMA).

Albumin-corrected anion gap (Figge correction)

Albumin is negatively charged and contributes approximately 2.5 mEq/L per g/dL to the anion gap. When albumin is low (e.g. 2.0 g/dL in a critically ill patient), the raw AG falls by ~5 mEq/L and can mask a HAGMA entirely. The correction restores the AG to what it would be at a normal albumin of 4.0 g/dL:

AG_corrected = AG + 2.5 x (4.0 - albumin)

Delta ratio (delta-delta)

When HAGMA is present, the delta ratio asks whether the bicarbonate fell by exactly as much as the anion gap rose — as expected in a pure HAGMA. It is defined as:

Delta ratio = (AG - 12) / (24 - HCO3-)

Delta ratio	Interpretation
Less than 0.4	Pure hyperchloraemic (normal-AG) acidosis
0.4 to 1.0	Mixed HAGMA and normal-AG acidosis
1.0 to 2.0	Pure HAGMA with appropriate compensation
Greater than 2.0	HAGMA plus concurrent metabolic alkalosis

Calculated osmolality and osmolar gap

Calculated Osm = 2 x Na + Glucose/18 + BUN/2.8

The osmolar gap is the measured osmolality minus the calculated osmolality. A gap above 10 mOsm/kg is elevated and, alongside a high anion gap, raises strong suspicion for toxic alcohol ingestion (methanol or ethylene glycol).

Worked example

A 42-year-old presents confused with vomiting. ABG and labs show:

Na+ = 140, Cl- = 100, HCO3- = 12 mEq/L
Albumin = 2.5 g/dL
Glucose = 90 mg/dL, BUN = 14 mg/dL
Measured osmolality = 310 mOsm/kg

Step 1 — Raw AG: 140 - (100 + 12) = 28 mEq/L (markedly elevated)

Step 2 — Albumin correction: 28 + 2.5 x (4.0 - 2.5) = 28 + 3.75 = 31.75 mEq/L (even higher — the low albumin was partially masking the gap)

Step 3 — Delta ratio: (31.75 - 12) / (24 - 12) = 19.75 / 12 = 1.65 (pure HAGMA — the bicarbonate fell proportionally to the AG rise)

Step 4 — Osmolar gap: Calculated Osm = 2 x 140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290. Gap = 310 - 290 = 20 mOsm/kg (elevated — suspect toxic alcohol or other osmole)

The combination of a very high anion gap and an elevated osmolar gap in this context is a medical emergency requiring urgent toxicology review.

This calculator is for educational use only. It does not constitute medical advice and should never replace clinical assessment, laboratory reference ranges or professional judgement.