What is a normal anion gap (electrolyte imbalance) range?

Normal Anion Gap Range

The normal anion gap range is 3-12 mmol/L when calculated using modern ion-selective electrode methodology, though the specific reference range depends on your laboratory's measurement technique. 1

Understanding the Calculation

• The anion gap is calculated using the formula: Na+ + K+ - Cl- - HCO3- 2, 3
• If calculated without K+, add 4 mmol/L to the result; if potassium status is unknown, add 2 mmol/L 2
• The anion gap represents the difference between unmeasured anions (primarily albumin) and unmeasured cations in serum 1, 4

Modern Reference Ranges

• With ion-selective electrode methodology (current standard): 3-12 mmol/L 1
• Alternative reported ranges using modern techniques: 5-12 mmol/L or 3-11 mmol/L 1
• The older reference range of 8-16 mmol/L is obsolete and should not be used, as it was based on outdated flame photometry techniques 1

Critical Thresholds for Clinical Action

Elevated Anion Gap

• Anion gap >27 mmol/L: Strongly suggests severe metabolic acidosis requiring immediate intervention (particularly in toxic ingestions like ethylene glycol) 2, 3
• Anion gap 23-27 mmol/L: Consider urgent evaluation and possible intervention 2, 3
• Anion gap >24 mmol/L: Rare finding that should prompt investigation for metabolic acidosis 1
• Anion gap >28 mmol/L: Associated with 20.4% mortality in ethylene glycol poisoning cases 2, 3

Low Anion Gap

• Anion gap <3 mmol/L: Considered abnormally low and warrants investigation 1, 5
• Anion gap <2 mmol/L: Very rare finding that should prompt quality control checks and evaluation for hypoalbuminemia, hyperglobulinemia, or paraproteinemia 1

Important Clinical Caveats

• Albumin correction is essential: Severe hypoalbuminemia significantly lowers the anion gap since albumin is a major unmeasured anion 6
• Baseline low anion gap can mask high anion gap acidosis: A patient with chronic hypoalbuminemia may have a "normal" anion gap despite concurrent metabolic acidosis 6, 5
• Laboratory-specific ranges matter: Always verify your institution's specific reference range, as the central 90% range for elevated anion gap is now 13-20 mmol/L (lower than historical values of 19-28 mmol/L) 1
• Quality control indicator: If your laboratory reports high incidence of anion gaps >24 mmol/L or <2 mmol/L, check electrolyte measurement quality control 1