Health & Medicine · Clinical Scores · Renal Function
Anion Gap Calculator
Calculates the anion gap and albumin-corrected anion gap from serum electrolyte and albumin values to detect metabolic acidosis.
Calculator
Formula
AG is the anion gap in mEq/L. [Na⁺] is serum sodium in mEq/L. [Cl⁻] is serum chloride in mEq/L. [HCO₃⁻] is serum bicarbonate (or total CO₂) in mEq/L. Albumin is serum albumin in g/dL. The correction factor 2.5 accounts for the anionic charge of albumin, and 4.0 g/dL is the normal reference albumin concentration. The corrected anion gap adjusts for hypoalbuminemia, which artificially lowers the measured anion gap.
Source: Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Critical Care Medicine. 1998;26(11):1807-1810. Also referenced in Harrison's Principles of Internal Medicine, 21st Edition.
How it works
The anion gap is based on the principle of electrochemical neutrality — the total positive charges in serum must equal the total negative charges. Not all ions are routinely measured, so the gap between measured cations (primarily sodium) and measured anions (chloride and bicarbonate) reflects the contribution of unmeasured anions. In healthy individuals, this gap is primarily accounted for by albumin, phosphate, sulfate, and organic acids, producing a normal anion gap of roughly 8–12 mEq/L (or 3–11 mEq/L in modern laboratories using ion-selective electrodes).
The standard formula is AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻]). A high anion gap indicates an accumulation of unmeasured anions, which is classically associated with conditions such as lactic acidosis, diabetic ketoacidosis (DKA), uremia, salicylate toxicity, and methanol or ethylene glycol poisoning — collectively remembered by the mnemonic MUDPILES or GOLDMARK. Because albumin contributes roughly 2.5 mEq/L of anionic charge per 1 g/dL of concentration, patients with hypoalbuminemia (common in critically ill or malnourished patients) will have a spuriously low anion gap. The albumin-corrected anion gap adjusts for this by adding 2.5 × (4.0 − measured albumin) to the raw anion gap, preventing clinicians from missing a true elevated anion gap in patients with low albumin.
Clinicians interpret the anion gap alongside arterial blood gas results, serum lactate, ketones, osmolar gap, and clinical history. The delta-delta ratio (change in AG relative to change in bicarbonate) further distinguishes whether a mixed acid-base disorder is present. Anion gap calculation is a foundational skill in internal medicine, emergency medicine, nephrology, and critical care, and is integrated into virtually all acid-base analysis frameworks.
Worked example
A 58-year-old diabetic patient presents with nausea, vomiting, and altered mental status. Laboratory results show:
- Serum Sodium: 138 mEq/L
- Serum Chloride: 98 mEq/L
- Serum Bicarbonate: 10 mEq/L
- Serum Albumin: 2.5 g/dL
Step 1 — Calculate the standard anion gap:
AG = 138 − (98 + 10) = 138 − 108 = 30 mEq/L
Step 2 — Calculate the albumin-corrected anion gap:
Corrected AG = 30 + 2.5 × (4.0 − 2.5) = 30 + 2.5 × 1.5 = 30 + 3.75 = 33.75 mEq/L
Interpretation: Both the standard and corrected anion gaps are markedly elevated (normal is approximately 8–12 mEq/L), strongly suggesting a high anion gap metabolic acidosis. In this clinical context, diabetic ketoacidosis (DKA) is the most likely diagnosis. The albumin correction reveals that the true degree of unmeasured anion accumulation is even greater than the raw number suggests due to the patient's hypoalbuminemia.
Limitations & notes
The anion gap is a screening tool, not a definitive diagnostic test. Several important limitations must be kept in mind. First, laboratory reference ranges vary between institutions depending on whether older flame photometry or modern ion-selective electrode (ISE) analyzers are used — ISE methods tend to yield a lower normal range (3–11 mEq/L vs 8–12 mEq/L). Always interpret results in the context of your institution's reference values. Second, the formula only includes sodium as the measured cation and does not account for potassium; some older texts use a four-variable formula (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻), adding approximately 4 mEq/L to the result. Third, the albumin correction formula is a linear approximation and may be imprecise when albumin levels are very low (below 1.5 g/dL) or in patients with paraproteinemias (e.g., multiple myeloma), where abnormal immunoglobulins act as unmeasured cations and can falsely lower the anion gap. Fourth, a normal anion gap does not rule out metabolic acidosis — hyperchloremic (non-anion gap) metabolic acidosis, as seen in renal tubular acidosis or gastrointestinal bicarbonate loss, is characterized by a normal AG with a low bicarbonate. Always interpret the anion gap alongside the full clinical picture and arterial blood gas values.
Frequently asked questions
What is a normal anion gap value?
The normal anion gap ranges from approximately 8–12 mEq/L using older flame photometry methods, or 3–11 mEq/L with modern ion-selective electrode analyzers. An anion gap above 12 mEq/L (or the lab-specific upper limit) is considered elevated and raises concern for high anion gap metabolic acidosis. Always use your laboratory's reference range.
Why do we correct the anion gap for albumin?
Albumin is the primary unmeasured anion in healthy individuals, contributing approximately 2–2.5 mEq/L per 1 g/dL of serum albumin. In patients with hypoalbuminemia — common in critically ill, cirrhotic, malnourished, or nephrotic patients — the measured anion gap is artificially lowered, potentially masking a clinically significant high anion gap acidosis. The Figge correction (adding 2.5 × [4.0 − albumin]) restores the AG to what it would be at a normal albumin of 4 g/dL.
What conditions cause a high anion gap metabolic acidosis?
Common causes include lactic acidosis (sepsis, shock, ischemia), diabetic ketoacidosis (DKA), alcoholic or starvation ketoacidosis, uremia (end-stage renal disease), and toxic ingestions such as methanol, ethylene glycol, propylene glycol, and salicylates. The mnemonics MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates) and GOLDMARK are used for memorization.
What is the delta-delta ratio and how does it relate to anion gap?
The delta-delta ratio (or delta ratio) compares the rise in anion gap above normal (delta AG) to the fall in bicarbonate below normal (delta HCO₃⁻). A ratio between 1 and 2 suggests a pure high anion gap metabolic acidosis. A ratio below 1 suggests a concurrent non-anion gap acidosis, while a ratio above 2 suggests a concurrent metabolic alkalosis. This is a powerful tool for identifying mixed acid-base disorders.
Should potassium be included in the anion gap formula?
Some older clinical guidelines and textbooks use the four-variable formula: AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻), which typically yields a normal range of 12–16 mEq/L. Most modern clinical practice and guidelines use only sodium as the measured cation, giving a normal range of 8–12 mEq/L, because potassium concentration is relatively low and stable in most patients. The key is consistency — always use the formula corresponding to your institution's reference range.
Last updated: 2025-01-15 · Formula verified against primary sources.