Basic formula: Na+ - (Cl- + HCO3-)


The anion gap is used to identify errors in the measurement of electrolytes (sodium, chloride, bicarbonate, and potassium most notably), detect paraproteins (IgG for example), and evaluate for suspected acid-base disorders – the latter being the most common and essential use.1

An anion gap (AG or AGAP) is calculated from the results of a comprehensive metabolic panel (CMP), and represents the difference between measured positively charged ions (cations) and measured negatively charged ions (anions) and can be categorized into three types: serum, plasma, and urine anion gaps. The primary use of this is to classify cases of metabolic acidosis—a condition characterized by lower-than-normal blood pH—into those with and without unmeasured anions in the plasma. While the human body maintains electrical neutrality and technically does not possess a true anion gap, this calculation helps identify discrepancies in that balance. It's important to pay particular attention to changes in albumin and bicarbonate concentrations when interpreting the math.1, 2

The anion gap is clinically significant for several reasons, including:

  • Acid-base balance assessment: It is primarily used to evaluate acid-base disturbances, particularly metabolic acidosis. A marked increase in anion gap (> 25 mmol/L) is strong evidence of metabolic acidosis.3

  • Diagnostic tool: It also helps by distinguishing between normal and high anion gap metabolic acidosis3, as well as alkalosis.

  • Indicator of unmeasured ions: An increased anion gap usually indicates an increase in unmeasured anions, often due to an increase in organic acids.4

  • Early detection: It can help identify serious conditions such as diabetic ketoacidosis, lactic acidosis, or certain poisonings before other symptoms become apparent.4

  • Monitoring critical illness: In ICU patients, the anion gap can be affected by factors like hypoalbuminemia, which may mask a high anion gap acidosis.1

The normal anion gap range is typically 4 to 12 mEq/L, though this can vary depending on the measurement method.3 Deviations from this range can indicate various medical conditions and help guide treatment decisions.5, 6

For reference (and because I have them), my anion gap results from my comprehensive metabolic panels for my last two hospitalizations (both myocardial infarctions) were:

Although my total protein was normal, my albumin levels were low from the 21st of December onward whereas my potassium, sodium, chloride, and carbon dioxide readings were all normal for the same period, if any are curious.


1https://www.ncbi.nlm.nih.gov/books/NBK539757/
2https://en.wikipedia.org/wiki/Anion_gap
3https://acutecaretesting.org/en/articles/clinical-aspects-of-the-anion-gap
4https://pmc.ncbi.nlm.nih.gov/articles/PMC3894543/
5https://medlineplus.gov/lab-tests/anion-gap-blood-test/
6https://my.clevelandclinic.org/health/diagnostics/22041-anion-gap-blood-test

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