Anion Gap Calculator
Calculate the anion gap, correct it for albumin, and get the delta ratio that tells you whether a second acid-base disorder is hiding behind the first one.
Chemistry panel
Optional but important. A low albumin can hide a real gap completely.
Anion gap
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Enter valuesWhat the anion gap actually measures
Plasma is electrically neutral. The cations and the anions have to balance. We routinely measure one major cation, sodium, and two major anions, chloride and bicarbonate. The difference between them is not a real gap in nature, it is the space occupied by the anions we did not measure: albumin above all, then phosphate, sulfate, and any acid the patient has accumulated.
So when the gap goes up, it means an acid has been added and its conjugate base is sitting in that unmeasured space. Lactate. Ketones. The retained anions of renal failure. A toxic alcohol metabolite. That is the entire clinical value of the number: it tells you an acid has been added rather than a base having been lost.
What counts as normal
With modern ion-selective electrodes, a normal anion gap is roughly 8 to 12 mEq/L, and many use a single cutoff of 12. Older texts and some labs quote up to 16, which came from earlier chloride measurement methods that read lower.
This matters more than it sounds. If you carry a threshold of 16 in your head and your lab runs a normal of 8 to 12, you will miss real gap acidoses in the 13 to 15 range. Look up what your own laboratory reports as the reference interval and use that number. This calculator flags above 12.
Some older formulas add potassium to the sodium. Both versions exist and both are internally consistent, but their normal ranges differ by about 4. Pick one, and it should be the one your institution uses. This calculator uses the sodium-only version, which is the standard in nearly all US hospitals.
Why you have to correct for albumin
Albumin is the single largest unmeasured anion. It is what most of the normal gap actually is. So in a patient whose albumin has fallen, the baseline gap falls with it, and a gap that reads as normal may be concealing a substantial acid load.
The correction is 2.5 mEq/L of gap for every 1 g/dL the albumin sits below 4.0.
What this looks like in practice
A septic cirrhotic with an albumin of 1.8. Sodium 138, chloride 108, bicarbonate 18. The measured gap is 12, which reads as reassuringly normal.
Correct it: 12 + 2.5 x (4.0 - 1.8) = 12 + 5.5 = 17.5. That is a clearly elevated gap, and in this patient it is almost certainly lactate. The uncorrected number would have sent you looking in the wrong direction.
Any patient sick enough to need an anion gap is usually sick enough to have a low albumin. Correct it every time.
The delta ratio, and why it finds the second disorder
In a pure anion gap acidosis, every point the gap rises should be matched by a point the bicarbonate falls, because the added acid consumed that bicarbonate. The delta ratio compares those two movements.
| Delta ratio | What it means |
|---|---|
| Under 0.4 | The bicarbonate has fallen much further than the gap has risen. There is a coexisting normal gap acidosis. |
| 0.4 to 1.0 | A mixed gap and non-gap acidosis. |
| 1.0 to 2.0 | A pure anion gap acidosis. This is the expected range in uncomplicated DKA or lactic acidosis. |
| Over 2.0 | The gap has risen much further than the bicarbonate has fallen. Something is holding the bicarbonate up: a coexisting metabolic alkalosis, or a chronic respiratory acidosis with renal compensation. |
The classic case is the patient in DKA who has also been vomiting for two days. The ketoacidosis drives the gap up while the vomiting drives a metabolic alkalosis that props the bicarbonate up. The pH can be nearly normal and the bicarbonate unremarkable, and the delta ratio over 2 is what tells you there are two things going on.
Causes of an elevated anion gap
GOLDMARK has replaced the old MUDPILES mnemonic because it is more accurate about what actually causes gaps:
Ethylene glycol (antifreeze) and propylene glycol, which is the solvent in intravenous lorazepam and can accumulate in anyone on a prolonged infusion. Look for an osmolal gap alongside the anion gap.
5-oxoproline, from chronic therapeutic acetaminophen use, classically in a malnourished or chronically ill woman. Easy to miss because the acetaminophen level is normal.
The one that matters most on the wards. Sepsis, shock of any kind, mesenteric ischemia, seizure, metformin. Check a lactate on every unexplained gap.
Short bowel syndrome with bacterial overgrowth. Standard lactate assays do not detect it, so a normal lactate does not rule it out.
Visual symptoms plus a gap plus an osmolal gap. A true emergency.
Salicylate poisoning classically gives a mixed picture: an anion gap metabolic acidosis together with a primary respiratory alkalosis. If you see both, check a level.
Retained phosphate, sulfate and organic anions. The gap is usually modest, in the mid to high teens, and it tracks the creatinine.
Diabetic, alcoholic and starvation. Beta-hydroxybutyrate is the dominant ketone and urine ketone dipsticks measure acetoacetate, so a negative urine dip does not exclude it. Send a serum beta-hydroxybutyrate.
Causes of a normal gap (hyperchloremic) acidosis
Here bicarbonate has been lost rather than acid gained, and chloride rises to fill the space. The mnemonic is HARDASS, but the practical list is short:
- Diarrhea. The most common cause by a wide margin.
- Renal tubular acidosis. Types 1, 2 and 4, each with a different potassium and a different urine pH.
- Large-volume normal saline. Resuscitating with several liters of 0.9 percent saline reliably produces a hyperchloremic acidosis. This is iatrogenic and common, and it is one reason balanced crystalloids have gained ground.
- Acetazolamide and other carbonic anhydrase inhibitors.
- Ureteral diversions and high-output fistulas.
To separate gastrointestinal loss from renal loss, calculate the urine anion gap: urine sodium plus urine potassium minus urine chloride. A negative value means the kidney is excreting ammonium properly, so the bicarbonate is being lost from the gut. A positive value means the kidney is the problem.
Causes of a low anion gap
Uncommon, and usually a laboratory clue rather than a diagnosis in itself.
- Hypoalbuminemia. By far the most frequent explanation, and the reason to correct.
- Paraproteinemia. Multiple myeloma with a cationic paraprotein. A persistently low gap in an older patient with anemia and renal impairment deserves a serum protein electrophoresis.
- Lithium, bromide or iodide, which are cations or interfere with the chloride assay.
- Severe hypercalcemia or hypermagnesemia.
- Laboratory error. Always consider it before you chase a zebra.
Mistakes people make with the anion gap
1. Not correcting for albumin, and therefore missing a real gap in exactly the patients most likely to have one.
2. Not calculating a gap at all when the pH is normal. A gap acidosis plus a metabolic alkalosis produces a normal pH and a normal bicarbonate.
3. Treating a normal gap as reassurance. A non-gap acidosis is still an acidosis.
4. Ruling out ketoacidosis on a negative urine ketone dipstick, which misses beta-hydroxybutyrate.
5. Forgetting to recheck it. The gap is a treatment response marker. In DKA, the gap closing is what tells you the insulin is working, not the glucose falling.
Related tools: the ABG calculator for the full acid-base interpretation, and the clinical calculators index for the osmolal gap and the corrected sodium.
This is a teaching tool, not a clinical decision system. Reference ranges vary by laboratory. Confirm every result against your own institution's values and the patient in front of you.