Anion Gap Calculator
Calculate serum anion gap from sodium, chloride, and bicarbonate levels to evaluate metabolic acidosis. Includes albumin correction for critically ill patients with interpretation.
About This Calculator
What is the Anion Gap?
The serum anion gap is a calculated value derived from routinely measured electrolytes in the blood: sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻). In the body, total serum cations and anions must be electrically balanced. The anion gap represents the concentration of unmeasured anions (proteins, phosphate, sulfate, organic acids) that are not directly included in routine electrolyte panels. It is a cornerstone of acid-base physiology used by internists, nephrologists, intensivists, and emergency physicians to identify the cause of metabolic acidosis.
How is the Anion Gap Calculated?
The standard formula is: Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻), with all values expressed in mEq/L (which are numerically equivalent to mmol/L). When potassium (K⁺) is included, the formula becomes: AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). The reference range without potassium is 3-11 mEq/L; with potassium it is 5-16 mEq/L. For patients with low albumin—a common finding in critically ill patients—the anion gap should be corrected using the Figge-Jabor-Kazda-Fencl equation: Corrected AG = AG + 2.5 × (4.0 − measured albumin in g/dL). For every 1 g/dL drop in albumin below 4.0 g/dL, the anion gap decreases by approximately 2.5 mEq/L.
Clinical Interpretation
An elevated anion gap (>11 mEq/L without K, >16 mEq/L with K) indicates high anion gap metabolic acidosis (HAGMA). Common causes are recalled by the MUDPILES mnemonic: Methanol, Uremia, Diabetic Ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates. A normal anion gap (3-11 mEq/L without K, 5-16 mEq/L with K) with acidosis suggests hyperchloremic (non-gap) metabolic acidosis from bicarbonate loss (diarrhea, renal tubular acidosis). A low anion gap (<3 mEq/L without K, <5 mEq/L with K) is most commonly due to hypoalbuminemia but may also be caused by paraproteinemias (multiple myeloma) or laboratory error.
Regional Notes
India: Basic metabolic panels including sodium, potassium, chloride, and bicarbonate are routinely available across hospital laboratories in India. The same international reference ranges (anion gap 3-11 mEq/L) are used in Indian clinical practice. Hypoalbuminemia is common in malnutrition and chronic liver disease, making albumin correction particularly relevant in Indian hospital settings.
US: The anion gap is a standard component of every basic metabolic panel (BMP) and comprehensive metabolic panel (CMP) in US laboratories. The American Society of Nephrology and the National Kidney Foundation endorse the use of albumin-corrected anion gap in critically ill patients. US reference ranges match the international standard of 3-11 mEq/L.
UK: The National Health Service (NHS) includes anion gap calculation as part of routine blood gas and electrolyte analysis. British guidelines from the Renal Association and the Intensive Care Society recommend the same reference ranges and albumin correction protocols used internationally.
Important Disclaimer
This calculator provides reference information only. Anion gap interpretation should always be performed by a qualified healthcare professional as part of a comprehensive clinical assessment. It is not a substitute for professional medical advice, diagnosis, or treatment.
Frequently Asked Questions
What is the anion gap and why is it measured?
The anion gap is the difference between measured cations (positively charged ions) and measured anions (negatively charged ions) in the blood serum. It is calculated using the formula AG = Na⁺ − (Cl⁻ + HCO₃⁻). The anion gap is primarily used to evaluate metabolic acidosis and help identify its underlying cause. A high anion gap suggests the accumulation of unmeasured anions such as lactate, ketones, or toxins, while a normal gap with acidosis suggests hyperchloremic (non-gap) metabolic acidosis. This test is used in clinical practice across India, the US, and the UK for diagnosing acid-base disorders.
How is the anion gap calculated?
The standard anion gap formula is AG = Na⁺ − (Cl⁻ + HCO₃⁻), where all values are in mEq/L. When potassium is included, the formula becomes AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). The reference range without potassium is 3-11 mEq/L; with potassium it is 5-16 mEq/L. For patients with low albumin (common in critically ill patients), a corrected anion gap is calculated using the Figge-Jabor-Kazda-Fencl equation: Corrected AG = AG + 2.5 × (4.0 − measured albumin in g/dL). This correction prevents hypoalbuminemia from masking a true elevation of the anion gap.
What is a normal anion gap range?
Without potassium: normal anion gap range is 3-11 mEq/L. With potassium included: normal range is 5-16 mEq/L. Values above the normal range indicate high anion gap metabolic acidosis. Values below the normal range (less than 3 mEq/L without K, or less than 5 mEq/L with K) indicate a low anion gap, which may be caused by hypoalbuminemia, paraproteinemias (such as multiple myeloma), or laboratory error. These reference ranges are used consistently in India, the United States, and the United Kingdom.
What causes a high anion gap metabolic acidosis?
High anion gap metabolic acidosis (HAGMA) is commonly recalled using the MUDPILES mnemonic: Methanol intoxication, Uremia (renal failure), Diabetic ketoacidosis (DKA), Propylene glycol toxicity, Isoniazid or Iron overdose, Lactic acidosis (sepsis, shock, hypoxia), Ethylene glycol poisoning (antifreeze), and Salicylate (aspirin) overdose. Other causes include starvation ketosis and alcoholic ketoacidosis. Identifying the specific cause requires correlation with clinical history, serum osmolality, lactate levels, ketone testing, and toxicology screening.
What causes a normal anion gap metabolic acidosis?
Normal anion gap metabolic acidosis (NAGMA), also called hyperchloremic metabolic acidosis, occurs when there is a loss of bicarbonate with a compensatory increase in chloride. Common causes include: severe diarrhea (bicarbonate loss in stool), renal tubular acidosis (Types 1, 2, and 4), ureterosigmoidostomy or ileal conduits, early chronic kidney disease, acetazolamide therapy, and ammonium chloride ingestion. The differentiation between high gap and normal gap acidosis is a critical step in the diagnostic workup of acid-base disorders.
Why is the anion gap corrected for albumin?
Albumin is a negatively charged protein that normally contributes to the unmeasured anion pool. In hypoalbuminemia (low albumin), which is common in critically ill patients, the measured anion gap may be falsely lowered, potentially masking a true high anion gap metabolic acidosis. The corrected anion gap formula (Corrected AG = AG + 2.5 × [4.0 − Albumin]) adjusts for this effect. For every 1 g/dL drop in albumin below 4.0 g/dL, the anion gap decreases by approximately 2.5 mEq/L. Correcting the anion gap is especially important in ICU settings across India, the US, and the UK.
What does a low anion gap indicate?
A low anion gap (less than 3 mEq/L without potassium, or less than 5 mEq/L with potassium) is less common than an elevated gap. The most frequent cause is hypoalbuminemia (low albumin), which is why albumin correction is recommended. Other causes include: multiple myeloma and other paraproteinemias (positively charged paraproteins reduce the gap), bromide intoxication (bromide is measured as chloride, falsely elevating chloride), lithium toxicity, and severe hypercalcemia or hypermagnesemia. Laboratory error should also be considered. In India, US, and UK, the same differential diagnosis applies.
Can I rely on this anion gap calculator for clinical decisions?
This anion gap calculator is designed for educational and reference purposes. While it uses standard medical formulas validated in clinical practice, results should always be interpreted by a qualified healthcare professional in the context of the full clinical picture, including patient history, physical examination, arterial blood gas analysis, and additional laboratory studies. In India, the United States, and the United Kingdom, acid-base disorders are diagnosed and managed by licensed physicians. This tool is not a substitute for professional medical advice, diagnosis, or treatment.