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Blood gases

Anion gap

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The calculated difference between measured cations and anions, Na - (Cl + HCO3), representing unmeasured anions (mainly albumin, phosphate, sulphate, organic acids). It separates raised-gap from normal-gap metabolic acidosis.

Reference interval

Conventional (mEq/L)SI (mmol/L)
Adults8–12 mEq/L8–12 mmol/L

Specimen: Serum (calculated from Na, Cl and HCO3) · Conventional × 1 = SI

Causes of high values

High anion gap metabolic acidosis: ketoacidosis (diabetic, alcoholic, starvation), lactic acidosis, renal failure, toxic alcohols (methanol, ethylene glycol), salicylate poisoning, paracetamol (pyroglutamic acid). Also laboratory error and some paraproteins.

Causes of low values

Hypoalbuminaemia (the most common cause), multiple myeloma with cationic paraproteins, hypercalcaemia, hypermagnesaemia, lithium or bromide intoxication, laboratory error.

Pre-analytical and method notes

The commonly quoted interval is 8 to 12 mEq/L when potassium is excluded, but ranges from about 4 to 12 in some laboratories using ion-selective electrodes, so use the local interval. Correct for albumin by adding about 2.5 mEq/L per 1 g/dL fall below 4 g/dL; the formula that includes potassium has a higher normal range. Reference intervals vary by laboratory, method and population; use the interval printed on the report.

Related calculators

References

  1. Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
  2. Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
  3. Brenner and Rector's The Kidney. Elsevier, 11th edition (2019) — Tier 3 (textbook)

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