Anion Gap Calculator — Serum Anion Gap & Delta Ratio

About

The serum anion gap (AG) represents the difference between measured cations (sodium) and measured anions (chloride and bicarbonate) in the serum. It reflects the concentration of unmeasured anions — normally albumin, phosphate, sulfate, and organic acids. The concept was first introduced by Gamble in the 1920s and later refined by Emmett and Narins in 1977 who established the differential diagnosis for elevated AG metabolic acidosis using the MUDPILES mnemonic. An elevated AG indicates the presence of pathological unmeasured anions such as lactate, ketones (beta-hydroxybutyrate, acetoacetate), toxic alcohols (methanol, ethylene glycol), uremic toxins, or salicylates. The albumin-corrected AG accounts for the effect of hypoalbuminemia, which reduces AG by approximately 2.5 mEq/L per 1 g/dL drop in albumin — a crucial adjustment in critically ill patients where low albumin is common and can mask a significant AG elevation. The delta ratio (ΔAG/ΔHCO3) helps identify mixed acid-base disorders by comparing the change in AG to the change in bicarbonate. A pure high AG metabolic acidosis yields a delta ratio of 0.8–1.0, while values outside this range suggest a concomitant metabolic alkalosis or normal AG acidosis. The AG remains a cornerstone of acid-base interpretation, used daily in ICUs and emergency departments worldwide. Evidence level: Grade B, supported by extensive physiological validation.

Formula

AG = Na - (Cl + HCO3) | Corrected AG = AG + 2.5 × (4 - Albumin) | Delta Ratio = (AG - 12) / (24 - HCO3)

The anion gap is calculated as AG = Na⁺ − (Cl⁻ + HCO₃⁻). All values are in mEq/L. A normal AG is 8–12 mEq/L, using older laboratories with different measurement methods — modern ion-selective electrode methods often yield a lower normal range of 3–10 mEq/L, so clinicians must know their laboratory's normal range. When albumin is low, the AG should be corrected: Corrected AG = Measured AG + 2.5 × (4 − albumin in g/dL). For example, a patient with Na 140, Cl 105, HCO₃ 15 has an AG of 140 − (105 + 15) = 20 mEq/L (elevated). If albumin is 2.0 g/dL: Corrected AG = 20 + 2.5 × (4 − 2) = 20 + 5 = 25 mEq/L. The delta ratio is calculated as (AG − 12) / (24 − HCO₃). It compares the excess AG (AG above normal) to the bicarbonate deficit (HCO₃ below normal). A ratio of 0.8–1.0 confirms a pure high AG metabolic acidosis — the rise in unmeasured anions fully accounts for the fall in HCO₃⁻. A ratio <0.4 suggests a normal AG acidosis (e.g., diarrhea, renal tubular acidosis) where the AG is not elevated but HCO₃⁻ is low. A ratio >1.0 suggests a concurrent metabolic alkalosis or pre-existing compensated respiratory acidosis. The delta ratio is less reliable when the initial HCO₃⁻ is very high or very low, or in chronic acid-base disorders. For interpretation, the AG should always be assessed in context of the patient's clinical presentation and the serum electrolyte pattern.

Score Interpretation

The anion gap is essential for acid-base interpretation in both emergency and critical care settings. The Kidney Disease: Improving Global Outcomes (KDIGO) guidelines emphasize the AG in the evaluation of acute kidney injury and metabolic acidosis. The Surviving Sepsis Campaign recommends calculating the AG as part of initial sepsis evaluation — an elevated AG with lactate elevation signals tissue hypoperfusion requiring urgent resuscitation. In diabetic ketoacidosis (DKA), the AG is a more sensitive marker of treatment response than blood glucose or beta-hydroxybutyrate levels — resolution of DKA is defined by a normalized AG (≤12 mEq/L). The American Diabetes Association recommends monitoring AG every 2–4 hours during DKA treatment. In toxicology, an elevated AG with an osmolal gap is the hallmark of methanol and ethylene glycol poisoning, guiding the use of fomepizole and hemodialysis. The delta ratio helps clinicians distinguish between pure and mixed acid-base disorders, which is crucial for appropriate management. For example, a patient with DKA who develops vomiting has a delta ratio >1.0 due to concurrent metabolic alkalosis — treating the alkalosis is as important as treating the DKA. A low AG (<8 mEq/L) may indicate hypoalbuminemia, but when persistent without explanation, it should prompt investigation for multiple myeloma, as paraproteins are positively charged and reduce the AG. The AG also aids in detecting laboratory errors — a negative AG is almost always artefactual. In clinical decision-making, the albumin-corrected AG is particularly important in the ICU where hypoalbuminemia is present in up to 70% of patients. Without correction, a significant AG elevation can be missed, leading to delayed diagnosis of conditions such as lactic acidosis or uremic acidosis.

Reference Ranges