About

The anion gap quantifies unmeasured anions in plasma by computing the difference between routinely measured cations (Na⁺) and anions (Cl⁻ + HCO₃⁻). A normal value falls between 8 and 12 mEq/L. Misinterpretation leads to missed diagnoses of lactic acidosis, diabetic ketoacidosis, toxic ingestions, or renal tubular acidosis. Hypoalbuminemia artificially lowers the gap by approximately 2.5 mEq/L per 1 g/dL deficit, masking dangerous elevations in critically ill or malnourished patients.

This calculator applies the Figge correction for albumin and computes the delta-delta ratio (ΔΔ) to detect concurrent non-anion gap metabolic acidosis or metabolic alkalosis superimposed on a high-AG state. Limitations: the tool assumes steady-state electrolyte concentrations and does not account for paraproteinemias, lithium toxicity, or severe hyperlipidemia, all of which alter unmeasured ion fractions. Pro tip: always correlate with venous blood gas and lactate before anchoring to a single AG value.

Formulas

The standard anion gap measures the difference between the principal measured cation and measured anions in serum:

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

When potassium is included in the calculation:

AG_K = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)

The albumin-corrected anion gap adjusts for the reduced anionic charge contributed by low albumin levels using the Figge correction factor:

AG_corrected = AG + 2.5 × (4.0 − Albumin)

The delta-delta ratio identifies mixed acid-base disturbances by comparing the change in anion gap to the change in bicarbonate from their respective normal values:

ΔΔ = AG − 1224 − HCO₃⁻

Where Na⁺ = serum sodium (mEq/L), K⁺ = serum potassium (mEq/L), Cl⁻ = serum chloride (mEq/L), HCO₃⁻ = serum bicarbonate (mEq/L), Albumin = serum albumin (g/dL). The normal anion gap reference of 12 and normal bicarbonate of 24 are standard clinical benchmarks used across emergency and critical care medicine.

Reference Data

Condition	Typical AG Range	Key Features	Common Causes
Normal	8 - 12 mEq/L	Balanced unmeasured anions	Healthy state
Diabetic Ketoacidosis (DKA)	> 20 mEq/L	Ketone accumulation, hyperglycemia	Insulin deficiency, Type 1 DM
Lactic Acidosis	> 15 mEq/L	Lactate > 4 mmol/L	Sepsis, shock, hypoxia, metformin
Uremic Acidosis	15 - 25 mEq/L	Phosphate, sulfate retention	Chronic kidney disease (GFR < 15)
Methanol Poisoning	> 20 mEq/L	Formic acid, osmol gap elevated	Ingestion of methanol
Ethylene Glycol Poisoning	> 20 mEq/L	Oxalic acid, calcium oxalate crystals	Antifreeze ingestion
Salicylate Toxicity	15 - 25 mEq/L	Mixed respiratory alkalosis + metabolic acidosis	Aspirin overdose
Starvation Ketoacidosis	12 - 18 mEq/L	Mild ketone elevation	Prolonged fasting, pregnancy
Normal AG Metabolic Acidosis (NAGMA)	8 - 12 mEq/L	Hyperchloremia, normal AG	RTA, diarrhea, NS infusion
Renal Tubular Acidosis Type 1	Normal	Urine pH > 5.5, hypokalemia	Distal tubule H⁺ secretion failure
Renal Tubular Acidosis Type 2	Normal	Proximal HCO₃⁻ wasting	Fanconi syndrome, myeloma
Renal Tubular Acidosis Type 4	Normal	Hyperkalemia, low renin/aldosterone	Diabetic nephropathy, ACEi/ARBs
Pyroglutamic Acidosis	> 15 mEq/L	5-oxoproline accumulation	Chronic acetaminophen use
D-Lactic Acidosis	> 15 mEq/L	Standard lactate assay normal	Short bowel syndrome
Isopropanol Ingestion	Normal	Osmol gap elevated, no acidosis	Rubbing alcohol ingestion
Toluene Inhalation	Normal or elevated	Distal RTA pattern, hypokalemia	Glue sniffing
Hypoalbuminemia Effect	Falsely low	AG decreases ~2.5 per 1 g/dL drop	Cirrhosis, nephrotic syndrome, sepsis
ΔΔ Ratio < 1	Variable	Concurrent NAGMA + HAGMA	Mixed disorder
ΔΔ Ratio 1 - 2	Elevated	Pure HAGMA	Single acid-base disturbance
ΔΔ Ratio > 2	Elevated	Concurrent metabolic alkalosis + HAGMA	Vomiting + DKA

Frequently Asked Questions

Albumin carries a net negative charge at physiological pH (~7.4). Each 1 g/dL decrease in albumin below the normal 4.0 g/dL reduces the measured anion gap by approximately 2.5 mEq/L. In critically ill patients with albumin of 2.0 g/dL, the uncorrected AG could appear normal at 10 while the corrected value reveals a true gap of 15, indicating a masked high-AG metabolic acidosis.

A ΔΔ ratio < 1 indicates a concurrent non-anion gap metabolic acidosis (NAGMA) alongside the HAGMA. The bicarbonate dropped more than the AG rose, suggesting an additional hyperchloremic process such as diarrhea or RTA. A ratio > 2 indicates a concurrent metabolic alkalosis. The bicarbonate is higher than expected for the degree of AG elevation, commonly seen in patients with vomiting combined with DKA. A ratio between 1 and 2 suggests a pure HAGMA.

Most clinical laboratories and textbooks use the formula without K⁺ because potassium concentrations are tightly regulated within 3.5 - 5.0 mEq/L and contribute minimally to the gap. When included, the normal reference range shifts upward to 10 - 16 mEq/L. Some institutions in Europe and Australia routinely include potassium. Always compare your result against the reference range that matches the formula used.

A negative anion gap is uncommon but clinically significant. It occurs in severe hypoalbuminemia, lithium intoxication (lithium is an unmeasured cation), IgG myeloma (cationic paraproteins), or laboratory error such as bromide interference with chloride assays. If the calculated AG is < 0, verify the specimen and consider ordering serum protein electrophoresis and lithium levels.

Large-volume normal saline (0.9% NaCl) infusion causes hyperchloremic metabolic acidosis. The excess chloride raises Cl⁻ relative to Na⁺, narrowing the anion gap while lowering bicarbonate. This produces a normal-AG (NAGMA) acidosis. In a patient with concurrent DKA receiving aggressive saline resuscitation, the AG may normalize while acidosis persists due to the iatrogenic hyperchloremia. Balanced crystalloids like lactated Ringer's avoid this artifact.

MUDPILES is a diagnostic mnemonic for causes of high anion gap metabolic acidosis: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Each condition generates organic acids or unmeasured anions that widen the gap beyond 12 mEq/L. Some authors extend this to GOLDMARK (Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis) for more precise categorization.