Formula
AG = sodium − (chloride + bicarbonate). With potassium: AG = sodium + potassium − (chloride + bicarbonate). Albumin-corrected AG = AG + 2.5 × (4.4 − albumin in g/dL).
The anion gap describes a difference between routinely measured positive and negative ions. It is not a direct measurement of every acid in blood. Sodium, chloride, and bicarbonate must come from compatible samples and use the same concentration units. Because these ions are monovalent, their numerical values in mmol/L and mEq/L are equal. Bicarbonate is often represented by total carbon dioxide on a chemistry panel; check the laboratory report before entering it.
Worked example
With sodium 140, chloride 104, and bicarbonate 24 mEq/L, the gap is 140 − (104 + 24) = 12 mEq/L. Including potassium of 4 gives 16 mEq/L. At albumin 3 g/dL, the corrected gap without potassium is 12 + 2.5 × 1.4 = 15.5 mEq/L.
How to use this calculator
- Choose the electrolyte unit shown on the report. Enter sodium, chloride, and bicarbonate.
- Choose whether to include potassium; use a matching reference interval.
- Optionally enter albumin, choosing g/dL or g/L. The calculator divides g/L by ten.
- Select Calculate, check the displayed formula result, and compare with the reporting laboratory’s interval.
When not to use this
Do not use the gap alone to diagnose an acid–base disorder or to exclude a serious illness. Low albumin can lower an uncorrected gap; other unmeasured ions, analytical interference, and sample problems can also change it. Albumin correction is an approximation and does not replace clinical assessment, blood gas interpretation, or direct measurement of substances of concern. A negative result can occur and should prompt verification of inputs and the laboratory report, rather than automatic deletion of the value.
Reference ranges
| Measure | Context |
|---|---|
| Without potassium | Often around 8–12 mEq/L |
| With potassium | Often around 12–16 mEq/L |
| Albumin correction reference | 4.4 g/dL selected for this calculator |
These example intervals are reference only and are not universal cutoffs. Some modern analyzers report lower intervals. Adding potassium changes both the number and the interval used for comparison. Figge’s correction relates the gap to a selected reference albumin; 4.4 g/dL here is a stated convention, not a universal normal albumin concentration. Use unrounded values for independent arithmetic checks and round only the final result.
References
Frequently asked questions
Should potassium be included?
Either convention can be used, but the result must be compared with a reference interval defined using that same convention. Many laboratories omit potassium.
Why does albumin matter?
Albumin is an important unmeasured anion. Lower albumin can reduce the observed gap, which is why an optional approximate correction is offered.
Are mmol/L and mEq/L interchangeable here?
For the monovalent electrolytes in this formula, yes. This equality does not apply to every substance measured in these units.
Can the gap be negative?
Yes. Verify the entries and laboratory measurements. This calculator preserves negative arithmetic results without assigning a diagnosis.
Does a normal gap rule out illness?
No. The gap is one piece of information and must be interpreted alongside symptoms, other measurements, and professional judgment.
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