Is Anion Gap Included on Standard Laboratory Tests?
Yes, the anion gap is routinely calculated and reported on standard electrolyte panels, as it is derived from commonly measured serum electrolytes (sodium, chloride, and bicarbonate) that are part of basic metabolic panels. 1
How Anion Gap is Obtained
The anion gap is a calculated value, not a directly measured laboratory test, using the formula: Na⁺ + K⁺ - Cl⁻ - HCO₃⁻ (or more commonly without potassium: Na⁺ - Cl⁻ - HCO₃⁻). 1
When standard electrolyte panels are ordered (basic metabolic panel or comprehensive metabolic panel), the laboratory automatically calculates and reports the anion gap from the measured sodium, chloride, and bicarbonate values. 2
Most clinical laboratories now use ion-selective electrode methodology, which has resulted in a lower reference range of 3-11 mmol/L (or 5-12 mmol/L in some studies), compared to the older reference range of 8-16 mmol/L that many clinicians and textbooks still incorrectly reference. 3
Clinical Availability and Reporting
The anion gap appears automatically on standard chemistry reports whenever a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) is ordered, making it universally available without requiring a separate test order. 4
In blood gas analysis, the anion gap is also calculated and reported from whole blood specimens analyzed on blood gas analyzers and point-of-care chemistry devices. 5
The calculation serves dual purposes: as a quality control measure to verify the accuracy of electrolyte measurements, and as a clinical tool to evaluate acid-base disorders. 4
Important Caveats
While the anion gap is automatically calculated, clinicians must be aware of the updated reference ranges to avoid misinterpretation—using outdated reference values (8-16 mmol/L) can lead to missing clinically significant elevations. 3
The anion gap should be corrected for hypoalbuminemia when present, as severe hypoalbuminemia significantly lowers the calculated anion gap and can mask concurrent high anion gap metabolic acidosis. 6, 2
Certain conditions can produce falsely low or even negative anion gaps (<2 mmol/L), including laboratory error, specimen contamination, hyperglobulinemia (such as IgG multiple myeloma), lithium or barium intoxication, and severe hypoalbuminemia. 6, 3, 5