Anion Gap Calculator

Calculate anion gap for metabolic acidosis evaluation. Essential tool for medical professionals in acid-base disorder assessment.

Standard Formula
With Albumin Correction
Advanced (K+)

Standard Anion Gap Formula: AG = Na⁺ - (Cl⁻ + HCO₃⁻)

Where: AG = Anion Gap (mEq/L), Na⁺ = Sodium, Cl⁻ = Chloride, HCO₃⁻ = Bicarbonate

mEq/L
Normal: 135-145 mEq/L
mEq/L
Normal: 98-106 mEq/L
mEq/L
Normal: 22-28 mEq/L

Albumin-Corrected Anion Gap Formula: AGcorrected = AG + 2.5 × (4.5 - Albumin)

Albumin correction is important in hypoalbuminemia, as albumin is a major unmeasured anion.

mEq/L
mEq/L
mEq/L
g/dL
Normal: 3.5-5.0 g/dL

Anion Gap with Potassium: AGK = (Na⁺ + K⁺) - (Cl⁻ + HCO₃⁻)

Some clinicians include potassium for a more comprehensive assessment, especially in severe electrolyte disturbances.

mEq/L
mEq/L
Normal: 3.5-5.0 mEq/L
mEq/L
mEq/L
Calculating...

Understanding Anion Gap

The anion gap is a calculated value used to assess acid-base disorders, particularly metabolic acidosis. It represents the difference between measured cations (positively charged ions) and measured anions (negatively charged ions) in the blood.

Clinical Utility of Anion Gap:

  • Metabolic Acidosis Evaluation: Differentiates between high anion gap and normal anion gap metabolic acidosis
  • Differential Diagnosis: Helps identify underlying causes of acid-base disorders
  • Treatment Monitoring: Tracks response to therapy in acid-base disturbances
  • Mixed Disorders Detection: Identifies mixed acid-base disorders

Anion Gap Interpretation

Anion Gap Classification Clinical Significance Common Causes (MUDPILES)
< 6 mEq/L Low Anion Gap May indicate hypoalbuminemia, multiple myeloma, or laboratory error Hypoalbuminemia, multiple myeloma, lithium toxicity, hypercalcemia
6-12 mEq/L Normal Anion Gap Normal range (may vary slightly by laboratory) Normal acid-base status or hyperchloremic metabolic acidosis
13-20 mEq/L Mildly Elevated Suggests accumulation of unmeasured anions Early metabolic acidosis, mild ketoacidosis, renal impairment
21-30 mEq/L Moderately Elevated Significant metabolic acidosis present Diabetic ketoacidosis, lactic acidosis, renal failure
> 30 mEq/L Severely Elevated Severe metabolic acidosis, often life-threatening Severe lactic acidosis, advanced renal failure, toxic ingestions

MUDPILES Mnemonic for High Anion Gap Metabolic Acidosis

MUDPILES is a common mnemonic for remembering causes of high anion gap metabolic acidosis:

  • M - Methanol intoxication
  • U - Uremia (renal failure)
  • D - Diabetic ketoacidosis
  • P - Paraldehyde intoxication
  • I - Iron, Isoniazid, or Ibuprofen overdose
  • L - Lactic acidosis
  • E - Ethylene glycol intoxication
  • S - Salicylate (aspirin) overdose

Normal Anion Gap Metabolic Acidosis (Hyperchloremic)

When anion gap is normal but metabolic acidosis is present, it suggests hyperchloremic metabolic acidosis. Common causes include:

1

Gastrointestinal bicarbonate loss: Diarrhea, pancreatic fistula, ureteral diversion

2

Renal tubular acidosis: Type 1 (distal), Type 2 (proximal), Type 4 (hypoaldosteronism)

3

Drug-induced: Acetazolamide, topiramate, amphotericin B

4

Dilutional acidosis: Rapid saline infusion

5

Other causes: Total parenteral nutrition, Addison's disease

Delta Ratio and Gap-Gap Ratio

In complex acid-base disorders, additional calculations help differentiate mixed disorders:

Delta Ratio (Δ/Δ): ΔAG / ΔHCO₃⁻ = (Measured AG - 12) / (24 - Measured HCO₃⁻)

  • < 0.4: Suggests hyperchloremic normal AG metabolic acidosis
  • 0.4-0.8: Suggests mixed high AG and normal AG metabolic acidosis
  • 0.8-1.2: Suggests pure high AG metabolic acidosis
  • > 1.2: Suggests mixed high AG metabolic acidosis and metabolic alkalosis

Gap-Gap Ratio: (AG - 12) / (24 - HCO₃⁻)

Similar to delta ratio but uses different normal values. Interpretation is comparable to delta ratio.

Albumin Correction

Albumin is the major unmeasured anion in plasma. In hypoalbuminemia, the anion gap is artificially low and should be corrected:

Albumin-Corrected Anion Gap Formula: Corrected AG = Calculated AG + 2.5 × (4.5 - Measured Albumin in g/dL)

For every 1 g/dL decrease in albumin, anion gap decreases by approximately 2.5 mEq/L.

Clinical Applications

  • Emergency Medicine: Rapid assessment of toxic ingestions and metabolic emergencies
  • Critical Care: Monitoring acid-base status in ICU patients
  • Nephrology: Evaluating renal tubular acidosis and renal failure
  • Endocrinology: Managing diabetic ketoacidosis
  • Toxicology: Identifying specific intoxications based on anion gap pattern

Clinical Note: Anion gap should always be interpreted in the context of the patient's clinical condition, other laboratory values, and arterial blood gas results when available. Normal ranges may vary between laboratories. Always consider mixed acid-base disorders.

Frequently Asked Questions

The normal anion gap is typically 8-12 mEq/L, but this can vary between laboratories (range 6-16 mEq/L). The most commonly used reference range is 8-16 mEq/L. It's important to use the reference range provided by your laboratory when interpreting results.

Albumin is the major unmeasured anion in plasma. In hypoalbuminemia (low albumin levels), the anion gap is artificially low because there are fewer unmeasured anions. Without correction, this can mask a high anion gap metabolic acidosis. For every 1 g/dL decrease in albumin below 4.5 g/dL, the anion gap decreases by approximately 2.5 mEq/L.

A low anion gap (<6 mEq/L) may indicate: (1) Hypoalbuminemia (most common cause), (2) Multiple myeloma or other paraproteinemias, (3) Hypercalcemia or hypermagnesemia, (4) Lithium intoxication, (5) Bromide intoxication, or (6) Laboratory error. It's important to correlate with clinical findings and other laboratory values.

The anion gap helps differentiate between two main types of metabolic acidosis: (1) High anion gap metabolic acidosis (MUDPILES causes) where unmeasured acids accumulate, and (2) Normal anion gap (hyperchloremic) metabolic acidosis where chloride increases to maintain electroneutrality when bicarbonate is lost. This distinction is crucial for determining the underlying cause and guiding appropriate treatment.

Some clinicians include potassium in the anion gap calculation, especially when potassium levels are significantly abnormal. The formula then becomes: AG = (Na⁺ + K⁺) - (Cl⁻ + HCO₃⁻). Including potassium increases the normal range to approximately 12-20 mEq/L. This approach may provide a more accurate assessment in severe electrolyte disturbances, but the standard formula (without potassium) is more commonly used in clinical practice.