Calculate the anion gap from sodium, chloride, and bicarbonate levels, with an optional albumin-corrected value for patients with low albumin.
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The anion gap is one of the most widely used calculated values in clinical chemistry, derived from three routine blood chemistry results: sodium, chloride, and bicarbonate. It's calculated by subtracting the sum of chloride and bicarbonate from sodium, and the result provides a quick, indirect estimate of unmeasured anions — mostly proteins, phosphates, sulfates, and organic acids — that aren't part of the standard chemistry panel. A typical reference range is about 8 to 12 mEq/L, though this varies somewhat by laboratory method. The anion gap is most useful as a starting point for classifying the cause of a metabolic acidosis: an elevated anion gap points toward the accumulation of unmeasured acids (such as ketones, lactate, or toxins), while a normal anion gap acidosis points toward a different set of causes, such as bicarbonate loss through the gut or kidneys. Because serum albumin is itself a major unmeasured anion, low albumin levels can artificially lower the calculated anion gap; this calculator also offers an albumin-corrected value that adjusts for that effect, giving a more reliable picture in patients with hypoalbuminemia.
Every routine metabolic panel reports sodium, chloride, and bicarbonate as separate numbers, but combining them into a single calculated value — the anion gap — turns those three results into one of the most useful screening tools in clinical chemistry. The body's blood plasma must maintain electrical neutrality, meaning the total concentration of positively charged ions (cations) must equal the total concentration of negatively charged ions (anions). Routine lab panels measure only the major cation (sodium) and the two major anions (chloride and bicarbonate), leaving out a range of other charged particles — including albumin, phosphate, sulfate, and various organic acids — that also contribute to that balance. The anion gap approximates the size of this "unmeasured" portion by subtracting the measured anions from the measured cation.
A typical reference range for the anion gap is roughly 8 to 12 mEq/L, though the exact figure printed on any given lab report can vary meaningfully depending on the specific chloride and bicarbonate assay a laboratory uses. Older methods, common through much of the twentieth century, tended to produce higher chloride readings and correspondingly higher anion gap values, with reference ranges as wide as 12 to 16 mEq/L still appearing in some older textbooks and a handful of labs. Most modern automated chemistry analyzers report values in the lower 8-to-12 range used by this calculator. Because of this variability, comparing a result to the specific reference range printed on the lab report it came from is always more reliable than relying on a single universal number.
Clinically, the anion gap's main use is as a first branch point when evaluating a metabolic acidosis — a state in which blood pH is abnormally low due to excess acid or loss of base. An elevated anion gap generally signals that some kind of acid has accumulated in the blood beyond what chloride and bicarbonate alone can explain, whether from the ketones produced during diabetic ketoacidosis or starvation, lactate from tissue hypoperfusion or certain medications, retained organic acids from kidney failure, or various toxic ingestions. Because the list of possible causes is broad and some are medical emergencies, an unexpectedly high anion gap on a lab report always warrants prompt evaluation by a physician rather than self-interpretation.
A normal anion gap in the setting of a metabolic acidosis points toward an entirely different set of causes, generally involving either a direct loss of bicarbonate — through prolonged diarrhea, for example, or certain kidney conditions — or an impaired ability of the kidneys to excrete acid. In this scenario, chloride typically rises to compensate for the falling bicarbonate, which is why this pattern is sometimes described by clinicians as a hyperchloremic acidosis; the anion gap itself stays within its normal range because the acid-base disturbance doesn't involve unmeasured anions.
One frequently overlooked wrinkle is that albumin is itself a significant contributor to the unmeasured anions the anion gap is meant to approximate, since albumin carries a net negative charge at physiological pH. When a patient's albumin is abnormally low — common in malnutrition, liver disease, nephrotic syndrome, or critical illness — the calculated anion gap can be falsely low, potentially masking a clinically significant elevated-anion-gap process. The albumin-corrected anion gap addresses this by adding back approximately 2.5 mEq/L for every 1 g/dL that albumin falls below a reference value of 4.0 g/dL, restoring a more accurate picture of the underlying acid-base and electrolyte status. As with every calculation on this page, the result is intended purely as an educational reference point — interpreting it in the context of a specific patient's full clinical picture is a task for a qualified physician.
The anion gap is a calculated value from routine blood chemistry (sodium minus the sum of chloride and bicarbonate) that estimates the difference between unmeasured anions and cations in the blood, and is commonly used as a first step in evaluating the cause of a metabolic acidosis.
A typical reference range is about 8 to 12 mEq/L, though the exact normal range depends on the laboratory's specific measurement method and can vary somewhat between institutions.
Older laboratory methods for measuring chloride and bicarbonate tended to produce higher anion gap values (commonly cited as 12–16 mEq/L), while most modern automated analyzers produce the lower 8–12 mEq/L range used by this calculator; always compare your result to the reference range printed on your own lab report.
A high anion gap most often reflects the accumulation of unmeasured acids in the blood, as seen in conditions like diabetic ketoacidosis, lactic acidosis, kidney failure, or certain toxic ingestions — the specific cause requires clinical evaluation by a physician.
Albumin is a negatively charged protein that normally contributes to the unmeasured anions in the calculation, so low albumin can falsely lower the anion gap; the albumin-corrected anion gap adds back an estimated 2.5 mEq/L for every 1 g/dL that albumin falls below a reference value of 4.0 g/dL to correct for this effect.