Anion Gap Calculator
Use this anion gap calculator to calculate AG from sodium, chloride, and bicarbonate (HCO3), with optional potassium when your lab includes it.
Last reviewed: February 26, 2026
Clinical disclaimer
Most labs report the anion gap without potassium unless specified.
How it works
The anion gap estimates unmeasured anions by comparing measured cations and anions in serum. The standard formula excludes potassium:
AG = Na - (Cl + HCO3). Some labs include potassium: AG = Na + K - (Cl + HCO3).
- Use sodium, chloride, and bicarbonate (total CO2) from the same lab draw.
- Enter values in mEq/L to match most basic metabolic panel units.
- Toggle K+ only if your lab includes it in the reported anion gap.
Interpretation and caveats
Anion gap reference ranges differ by lab and assay method. Mild elevations may reflect laboratory variation, while higher values can suggest unmeasured anions (for example, lactate or ketones) in the appropriate clinical setting.
- Use your lab's specific reference range when deciding if the gap is elevated.
- Low albumin lowers the anion gap; consider correcting if albumin is low.
- Total CO2 from a BMP may differ slightly from ABG bicarbonate.
- Always interpret with acid-base status, clinical presentation, and trend data.
This calculator does not adjust for albumin or lactate. If you need corrected values, compute them separately and document the correction method.
Rapid bedside workflow
- Verify all chemistry inputs (Na, Cl, bicarbonate, and optional K) come from the same draw and unit system.
- Match the potassium inclusion setting to your lab reporting convention before interpreting value severity.
- Interpret the gap with clinical context, lactate/ketone considerations, and acid-base data.
- If albumin is low, apply correction separately and document method used.
- Trend serial values after treatment changes to confirm expected trajectory.
Reassessment cadence
- Active acid-base management: repeat chemistry panels per protocol in the same treatment block.
- Stable improving patient: continue serial reassessment at routine interval with trend-focused interpretation.
- Unexpected worsening symptoms or trajectory discordance: immediate repeat labs and escalation.
Escalation triggers
- Rising anion gap with worsening perfusion, mental status change, or hemodynamic instability.
- Persistent high gap despite expected treatment response.
- Major discrepancy between clinical status and laboratory trend.
- Unclear laboratory convention or uncertain input integrity in high-risk cases.
Shift handoff checklist
- Exact formula mode used (with or without potassium) and input timestamp.
- Current gap value, clinical interpretation, and differential focus.
- Albumin/lactate/ketone context and any correction method applied.
- Next lab timing and escalation trigger threshold for covering team.
Related calculators
References
Frequently asked questions
Most labs report the anion gap without potassium. If your lab includes K+ in the reported anion gap, toggle it on here so your calculation matches the lab reference range.
Use the serum bicarbonate (total CO2) reported on a BMP/CMP. Do not use arterial pH values or calculated bicarbonate from ABG unless that is how your lab reports it.
Reference ranges vary by lab and age. Many labs list an anion gap around 8-12 mEq/L when potassium is excluded; including K+ raises the range by roughly 3-5 mEq/L. Always use your local lab reference.
Hypoalbuminemia lowers the anion gap. A common correction is: corrected AG = AG + 2.5 x (4 - albumin g/dL) when albumin is below 4 g/dL.
Document formula convention (K included or excluded), lab timestamp, key differential considerations, and planned recheck timing. This makes serial interpretation safer across shifts.