Anion Gap Calculator
Last reviewed: by Jayson Minagawa, BSN, RN
Free anion gap calculator. Enter sodium, chloride, and bicarbonate (and optionally albumin for correction) — returns the calculated anion gap and notes whether it's normal, elevated, or low. Used in metabolic acidosis workup. Clinical reference only.
Anion gap is the first thing the ICU team asks about in any acidosis. An elevated gap points to lactic acidosis, ketoacidosis, salicylates, methanol, ethylene glycol, or uremia (MUDPILES mnemonic). A normal gap with acidosis points to GI losses, RTA, or saline-related hyperchloremia. Getting the calculation right matters because it changes the differential.
— Jayson Minagawa, BSN, RNCalculate
The anion gap formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻). Normal range varies by lab: 3-12 mEq/L is widely cited (older labs reported 8-16 because of older potassium-inclusive formulas). Some labs include K⁺ (AG = Na+K − Cl − HCO3); know your lab's reference range. Anion gap reflects unmeasured anions (lactate, ketones, sulfates, phosphates, organic acids).
Albumin correction
Albumin is the major unmeasured anion in healthy plasma. In hypoalbuminemic patients, the calculated AG underestimates the true AG. Corrected AG = AG + 2.5 × (4 − measured albumin in g/dL). If albumin is 2.0, the correction adds 5 to the AG. This matters because critical illness frequently lowers albumin, masking a real anion-gap acidosis.
MUDPILES — anion-gap metabolic acidosis differential
M Methanol. U Uremia. D Diabetic ketoacidosis (DKA, also alcoholic + starvation ketoacidosis). P Propylene glycol (lorazepam infusion toxicity, antifreeze). I Iron / INH (isoniazid). L Lactic acidosis (sepsis, shock, mitochondrial). E Ethylene glycol. S Salicylates. The most common in hospitalized patients is lactic acidosis from sepsis or hypoperfusion.
Non-anion-gap metabolic acidosis
If AG is normal but HCO3 is low (acidosis present), think: GI bicarb losses (severe diarrhea, ileostomy), renal tubular acidosis (RTA types 1, 2, 4), saline-induced hyperchloremic acidosis (large-volume NS resuscitation), pancreatic fistula. Mnemonic: HARDUP (Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureteroenteric fistula, Pancreatic fistula).
Reading the gap at the bedside: how nurses use this number
The anion gap is one of the few lab values where the calculation matters as much as the raw result. When I review labs at the start of shift, I always run the math even if the metabolic panel comment doesn't flag it: there are scenarios where bicarbonate is low normal (22–24) and chloride is mid-normal (105–108), but a calculated AG of 18–20 still tells you something is wrong. Sepsis, occult lactic acidosis from a hidden bowel ischemia, early DKA in a Type 2 patient with relative insulin resistance — these can all show with a "normal-looking" BMP that hides a high anion gap.
Albumin correction is non-optional in critical care. Most ICU patients have albumin in the 2.0–2.8 range from inflammation, third-spacing, or malnutrition. At an albumin of 2.0, a measured AG of 12 corrects to 17 — that's not normal, that is a real anion-gap process being masked. I add the correction in my head every time, and I document it as "AG corrected to X for albumin Y." This is the kind of small clinical thinking that catches septic shock and DKA earlier than waiting for lactate to come back.
The delta gap and mixed acid-base disorders
The delta-delta ratio compares change in AG to change in bicarbonate. The reasoning: in a pure anion-gap acidosis (like uncomplicated DKA), every additional unmeasured acid that consumes a bicarbonate molecule should raise the AG by the same amount it lowers HCO3. So (AG − 12) should approximately equal (24 − HCO3). When the ratio is 1.0–2.0, you have a pure anion-gap acidosis. Below 1.0 suggests a concurrent non-anion-gap acidosis (e.g., DKA + diarrhea, or DKA after large-volume saline resuscitation has added a hyperchloremic component). Above 2.0 suggests a concurrent metabolic alkalosis (e.g., DKA with vomiting). On a typical med-surg floor, the delta-delta is rarely calculated, but in the ICU it is part of every formal acid-base workup.
Common pitfalls and what changes management
Three things to watch out for when interpreting an anion gap. First: the normal range your lab reports. Older texts and some legacy labs cite 8–16 mEq/L, which assumes K is included in the formula. Modern labs typically use the K-excluded formula (Na − Cl − HCO3) and quote 3–12. Always check your lab's reference range or you will misclassify normal patients as acidotic. Second: spurious results. Severe hyperlipidemia, hyperproteinemia, hyperglycemia (which falsely lowers measured Na), and bromide toxicity (falsely raises measured Cl) all distort the calculated AG. Third: the AG never tells you the cause; it only narrows the differential. Always pair the gap with lactate, beta-hydroxybutyrate, urinalysis ketones, salicylate level, and clinical context. The AG is a screening tool that points the workup, not a diagnosis on its own.
Frequently asked
Why does normal anion gap exist?
Even healthy plasma has a small population of unmeasured anions — primarily albumin, plus phosphates, sulfates, and organic acids. The 3-12 mEq/L 'normal' range reflects this baseline.
Should I use the K-inclusive or K-exclusive formula?
Most modern labs use the K-EXCLUSIVE formula (Na − Cl − HCO3) because potassium contributes minimally to AG and is more variable. Some older texts use Na + K − Cl − HCO3, which raises the normal range to ~8-16.
What's a delta-delta?
Delta-delta = (AG − 12) ÷ (24 − HCO3). It checks whether the change in AG matches the change in HCO3 (suggesting pure anion-gap acidosis, ratio ~1.0) or whether there's a co-existing non-AG acidosis or metabolic alkalosis. Used in the ICU when the picture is mixed.