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Anion Gap Calculator

Calculate the anion gap to help evaluate acid-base disorders and determine the cause of metabolic acidosis.

Enter sodium level in mEq/L or mmol/L
Enter chloride level in mEq/L or mmol/L
Enter bicarbonate level in mEq/L or mmol/L

How to Use This Calculator

  1. Enter the patient's sodium (Na+) level in mEq/L or mmol/L
  2. Enter the patient's chloride (Cl-) level in mEq/L or mmol/L
  3. Enter the patient's bicarbonate (HCO3-) level in mEq/L or mmol/L
  4. Click Calculate to see the anion gap result and interpretation

Formula Used

Anion Gap = [Na+] - ([Cl-] + [HCO3-])

Where:

  • Na+ = Sodium concentration in mEq/L or mmol/L
  • Cl- = Chloride concentration in mEq/L or mmol/L
  • HCO3- = Bicarbonate concentration in mEq/L or mmol/L

Example Calculation

Clinical Scenario:

A patient presents with metabolic acidosis. The following lab values are obtained:

Given:

  • Sodium (Na+) = 140 mEq/L
  • Chloride (Cl-) = 100 mEq/L
  • Bicarbonate (HCO3-) = 22 mEq/L

Calculation:

Anion Gap = 140 - (100 + 22) = 140 - 122 = 18 mEq/L

Result: The anion gap is 18 mEq/L, which is elevated (normal range: 8-12 mEq/L), suggesting a high anion gap metabolic acidosis.

Why This Calculation Matters

Clinical Applications

  • Helps determine the cause of metabolic acidosis
  • Distinguishes between high anion gap and normal anion gap metabolic acidosis
  • Guides further diagnostic workup and treatment

Key Benefits

  • Quick assessment tool for acid-base disorders
  • Helps narrow down differential diagnoses
  • Can monitor response to treatment in certain conditions

Common Mistakes & Tips

Ensure all electrolyte values are in the same units (mEq/L or mmol/L). Mixing units will result in an incorrect anion gap calculation.

In patients with abnormal albumin levels, the corrected anion gap should be calculated: Corrected AG = Calculated AG + 2.5 × (4 - Albumin [g/dL]). This is especially important in critically ill patients.

Frequently Asked Questions

A normal anion gap is typically between 8-12 mEq/L (or 3-11 mEq/L depending on the laboratory). Values above this range suggest a high anion gap metabolic acidosis, while values below suggest a low anion gap.

Common causes include ketoacidosis (diabetic, alcoholic, starvation), lactic acidosis, renal failure, toxic ingestions (methanol, ethylene glycol, salicylates), and certain medications. The mnemonic "MUDPILES" is often used to remember the causes.

A low anion gap is less common but can be caused by hypoalbuminemia, multiple myeloma, certain paraproteinemias, bromide intoxication, and lithium toxicity. In most cases, a low anion gap is due to laboratory error or hypoalbuminemia.

References & Disclaimer

Medical Disclaimer

This calculator is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding a medical condition.

References

Accuracy Notice

This calculator provides an estimation based on the values entered. The accuracy of the result depends on the accuracy of the input values. Laboratory reference ranges may vary slightly between institutions. Always interpret results in the clinical context and consult with healthcare professionals for diagnosis and treatment decisions.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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