Anion Gap Calculator

Frequently asked questions

What is the anion gap?

The anion gap (AG) estimates the unmeasured anions in blood from routine electrolytes: AG = Na⁺ − (Cl⁻ + HCO₃⁻), all in mEq/L. Because the body must stay electrically neutral, the difference between the measured cation (sodium) and the measured anions (chloride + bicarbonate) reflects anions the basic panel does not measure — mostly albumin, plus phosphates, sulfates and organic acids. It is mainly used to sort out the causes of metabolic acidosis. This page is educational and does not provide medical advice.

What is a normal anion gap range?

It depends on the assay: classic teaching uses 8–12 mEq/L, modern ion-selective electrodes commonly report 3–11 mEq/L, and older methods reported 8–16 mEq/L. If potassium is included in the formula, the band shifts up by roughly 4 (for example 10–20 mEq/L). Because of this variability, always interpret your result against YOUR lab's reference range, not a number from the internet. This calculator lets you switch bands but defaults to the classic 8–12.

What is the corrected (albumin-adjusted) anion gap?

Albumin carries most of the unmeasured-anion charge, so low albumin lowers the raw gap and can mask a real elevation. The standard correction is: corrected AG = AG + 2.5 × (4.0 − albumin in g/dL), after Figge et al. (Crit Care Med 1998). Example: a raw AG of 10 with albumin 2.0 g/dL looks normal but corrects to 15 mEq/L — high. Whenever albumin is low, the corrected value is the more honest one to interpret.

What does a high anion gap mean?

A high anion gap suggests unmeasured anions — most often extra acid, i.e. a high-gap metabolic acidosis. Students memorize the educational causes with MUDPILES: Methanol, Uremia (kidney failure), Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Which one applies depends on the history, lactate, ketones, kidney function and toxicology — never on the gap number alone. This is educational information, not a diagnosis.

What does a low anion gap mean?

A low anion gap (below the reference band) is uncommon. Educational associations include low albumin (hypoalbuminemia), laboratory error or an unmeasured cation load such as lithium, bromide, or cationic paraproteins in multiple myeloma. Because it is rare, a low value is worth verifying — repeat the sample before reading much into it. Discuss any unexpected result with your clinician.

What are the delta gap and delta ratio?

They check whether a high-gap acidosis is the ONLY thing going on. Delta gap = (AG − 12) − (24 − HCO₃⁻): between −6 and +6 suggests a pure high-gap acidosis; below −6 suggests an added normal-gap (hyperchloremic) acidosis; above +6 suggests an added metabolic alkalosis. Delta ratio = (AG − 12)/(24 − HCO₃⁻): 0.8–2.0 pure high-gap, 0.4–0.8 mixed high+normal gap, above 2.0 high-gap plus alkalosis, below 0.4 normal-gap alone. The 12 and 24 are conventional reference points. These are teaching tools for mixed disorders, not diagnoses.

Should potassium be included in the anion gap formula?

Both versions exist. The common bedside formula omits potassium: AG = Na⁺ − (Cl⁻ + HCO₃⁻). Some nephrologists prefer AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻) because potassium swings widely in kidney disease. Including potassium raises the result by about 4 mEq/L, so the reference band shifts up too (e.g. 10–20 instead of 8–12). Whichever you use, compare against the matching reference range. This calculator offers both.

What is the osmolar gap and how is it calculated?

The osmolar gap is measured osmolality minus calculated osmolarity, in mOsm/kg. Calculated osmolarity = 2 × Na⁺ + glucose/18 + BUN/2.8 (glucose and BUN in mg/dL), plus ethanol/4.6 if ethanol was ingested. A gap under 10 is usual; 10 or more suggests unmeasured osmoles — classically the toxic alcohols methanol, ethylene glycol or isopropanol, or unaccounted ethanol. It is a screening clue, not a confirmation, and must be correlated clinically and toxicologically.

Is this anion gap calculator a diagnostic tool?

No. It is an educational calculator: it does the arithmetic (anion gap, albumin correction, delta gap/ratio, osmolar gap) and shows textbook-style interpretations. It cannot diagnose metabolic acidosis, diabetic ketoacidosis, kidney failure or poisoning, and it does not replace clinical judgment, a physical exam, or your lab's own reference ranges. Never disregard professional medical advice or delay seeking it because of a number on this page — discuss your results with your doctor.

How do I calculate the anion gap by hand? (worked example)

Take sodium 140, chloride 100 and bicarbonate 24 mEq/L: AG = 140 − (100 + 24) = 140 − 124 = 16.0 mEq/L — above the classic 8–12 band (high). With albumin 2.0 g/dL, the corrected gap = 16 + 2.5 × (4.0 − 2.0) = 21.0 mEq/L. Delta gap = (16 − 12) − (24 − 24) = 4.0 mEq/L, inside ±6, consistent with a pure high-gap pattern on these teaching thresholds.