Printed on 3/17/2026
For informational purposes only. This is not medical advice.
The anion gap is a calculated value from routine electrolytes that helps differentiate causes of metabolic acidosis. It represents the difference between measured cations (sodium) and measured anions (chloride + bicarbonate). An elevated anion gap suggests the presence of unmeasured acids (e.g., lactate, ketoacids, toxins). The albumin-corrected anion gap adjusts for low albumin, which can mask a true elevation.
Formula: AG = Na⁺ − (Cl⁻ + HCO₃⁻); Corrected AG = AG + 2.5 × (4.0 − Albumin)
Input sodium, chloride, and bicarbonate from the basic metabolic panel. Optionally enter serum albumin for the corrected calculation.
The anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). If albumin is entered, the albumin-corrected AG = AG + 2.5 × (4.0 − albumin).
Compare to normal range (3–12 mEq/L, varies by lab). Elevated AG suggests MUDPILES; normal AG suggests GI or renal bicarbonate loss. Use the [ABG Interpreter](/tools/abg-interpreter) to assess concurrent respiratory compensation and overall acid-base status. Use the result to guide further workup.
Emergency physicians, intensivists
First step in evaluating any patient with low bicarbonate or low pH. The anion gap immediately narrows the differential diagnosis.
Endocrinologists, hospitalists
Track anion gap closure during DKA treatment. A closing gap indicates effective insulin therapy and ketone clearance. Use [Sodium Correction](/tools/sodium-correction) to assess true sodium status in hyperglycemic DKA patients.
Toxicologists, emergency medicine
Elevated anion gap with high osmolar gap suggests toxic alcohol ingestion (methanol, ethylene glycol). Early recognition is critical for antidote therapy.
Intensivists, critical care nurses
Daily anion gap calculation helps track acid-base status in critically ill patients, detect new metabolic derangements, and assess response to treatment. Calculate [SOFA Score](/tools/sofa-score) or [APACHE II](/tools/apache-ii-score) for comprehensive ICU severity assessment.
Nephrologists, primary care
Patients with advanced CKD develop uremic acidosis with elevated anion gap. Serial monitoring helps assess disease progression. Track renal function with [eGFR Calculator](/tools/egfr-calculator) or [Creatinine Clearance](/tools/creatinine-clearance).
Pharmacists, hospitalists
Certain medications (metformin, nucleoside analogs, propofol) can cause lactic acidosis. Anion gap elevation may be the first sign.
Hypoalbuminemia is extremely common in hospitalized patients. Each 1 g/dL decrease in albumin below 4.0 masks approximately 2.5 mEq/L of anion gap. An 'normal' AG of 10 with albumin of 2.0 is actually 15 when corrected. Similarly, use [Corrected Calcium](/tools/corrected-calcium) to account for albumin effects on calcium interpretation.
The traditional normal AG of 8–12 was based on older assays. Modern ion-selective electrodes often yield a lower normal range (3–10). Check your lab's reference range to avoid over-interpretation.
Delta gap = (AG − 12) − (24 − HCO₃). A ratio near 1 suggests pure anion gap acidosis. <1 suggests concurrent non-AG acidosis. >2 suggests concurrent metabolic alkalosis. This reveals mixed acid-base disorders.
In most clinical settings, lactic acidosis (from hypoperfusion, sepsis, or medication) is the most common cause of elevated anion gap. Check a lactate level early in the workup.
Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. In practice, DKA and lactic acidosis are far more common than toxic ingestions.
If you suspect methanol or ethylene glycol, calculate the osmolar gap (measured − calculated serum osmolality). Elevated AG + elevated osmolar gap strongly suggests toxic alcohol ingestion.
Hyperchloremic (normal AG) metabolic acidosis: think GI bicarbonate loss (diarrhea), renal tubular acidosis, or iatrogenic (NS resuscitation, carbonic anhydrase inhibitors).
In DKA, recheck the anion gap every 2–4 hours. As ketones are metabolized with insulin, the gap closes. A persistent or widening gap suggests inadequate insulin or concurrent lactic acidosis.
In patients with short bowel syndrome or gastric bypass, D-lactic acidosis can cause elevated AG that won't show up on standard L-lactate assays. Request D-lactate specifically if suspected.
A critically ill patient may have sepsis (lactic acidosis) + renal failure (uremic acidosis) + metabolic alkalosis from NG suction. Systematic analysis using delta-delta helps untangle the contributions.
The anion gap concept was introduced in the 1970s and remains a cornerstone of metabolic acidosis evaluation. The albumin correction was validated by Figge et al. (Crit Care Med 1998). Normal ranges have shifted with modern ion-selective electrode assays, and clinicians should verify their laboratory's reference range. The MUDPILES mnemonic, while imperfect, provides a useful framework for the differential diagnosis.
A normal anion gap is generally 3–12 mEq/L, though the reference range varies by laboratory and the method used to measure electrolytes. An elevated anion gap (typically >12 mEq/L) indicates the presence of unmeasured anions in the blood, most commonly organic acids. The classic causes are remembered by the mnemonic MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates.
If you provided an albumin value, the albumin-corrected anion gap adjusts for hypoalbuminemia. Each 1 g/dL decrease in albumin below 4.0 can mask approximately 2.5 mEq/L of the anion gap. In critically ill or malnourished patients with low albumin, the uncorrected anion gap may appear normal even when significant unmeasured acids are present. Always interpret the corrected value in hypoalbuminemic patients.
Calculate the anion gap whenever a metabolic acidosis is identified on [arterial blood gas](/tools/abg-interpreter) or basic metabolic panel (low bicarbonate, low pH). It is the essential next step in the workup of metabolic acidosis, as it determines the differential diagnosis: an elevated anion gap points toward acid-producing conditions (ketoacidosis, lactic acidosis, toxic ingestions, renal failure), while a normal anion gap (hyperchloremic) acidosis points toward bicarbonate-losing conditions (diarrhea, renal tubular acidosis, normal saline administration).
The anion gap should be calculated routinely in emergency department and ICU settings for patients presenting with altered mental status, suspected overdose, sepsis, diabetic emergencies, or unexplained metabolic derangements. It is also useful for monitoring the response to treatment in known anion gap acidosis.
The anion gap is an indirect calculation, not a direct measurement, and is affected by the accuracy of the underlying electrolyte measurements. Different laboratory analyzers can produce slightly different results, which is why normal ranges vary between institutions. The traditional normal range of 8–12 mEq/L was established with older assays; newer ion-selective electrode methods often yield a lower normal range of 3–10 mEq/L.
The anion gap does not identify the specific cause of acidosis — it only narrows the differential. Additional testing (lactate, ketones, osmolar gap, toxicology screen) is required to determine the etiology. A normal anion gap does not exclude serious illness, as hyperchloremic acidosis from diarrhea, RTA, or other causes can be clinically significant.
In patients with multiple concurrent acid-base disorders, the delta-delta ratio (comparing the change in anion gap to the change in bicarbonate) provides additional information but adds complexity. The anion gap is a screening tool that works best as part of a systematic acid-base analysis, not in isolation.
For related assessments, see ABG Interpreter, Corrected Calcium and A-a Gradient.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
Interpret arterial blood gas (ABG) results to identify acid-base disorders. Determines primary disorder and compensation status from pH, pCO₂, and HCO₃⁻.
ClinicalCalculate corrected calcium adjusted for albumin levels. Essential for accurate interpretation of total calcium in hypoalbuminemic patients.
ClinicalCalculate the alveolar-arterial oxygen gradient to evaluate the cause of hypoxemia. Differentiates lung pathology from hypoventilation.
ClinicalCalculate serum osmolality from sodium, glucose, and BUN. Normal range: 275–295 mOsm/kg. Osmolal gap >10 suggests toxic alcohol ingestion (methanol, ethylene glycol, isopropanol) or other unmeasured osmoles.
ClinicalCalculate expected pCO₂ for respiratory compensation in metabolic acidosis using Winter's formula: expected pCO₂ = 1.5 × [HCO₃] + 8 ± 2. Compare to actual pCO₂ to identify concurrent respiratory disorders.