Calculates the serum anion gap, corrects it for a low albumin, and shows whether it is low, usual or high.
Result
Enter the sodium, chloride and bicarbonate.
Sources
3 studies
3
studies
445
citations
3,359
PubMed papers
Evidence
Known limitations
A review of the uses and limits of the gap: the normal value varies widely with the method and between people; low values most often mean laboratory error or low albumin; high values most often mean metabolic acidosis. The rise in the gap and the fall in bicarbonate are not reliably 1:1, so the delta ratio should not be used alone.
Clin J Am Soc Nephrol: Journal Metrics Score 2.8, Q1 · cited 445 times
In nine healthy subjects and 152 critically ill patients (265 measurements), 49% of the patients had an albumin below 20 g/L. Each 1 g/L fall in albumin made the gap underestimate the unmeasured anions by 0.25 mmol/L (r² = 0.94), giving the correction: gap + 0.25 × (normal − observed albumin in g/L), or 2.5 per g/dL.
Crit Care Med: Journal Metrics Score 1.1, Q3 · cited 411 times
Using modern electrodes, the anion gap reference range had shifted down to 3 to 11 mmol/L in one laboratory, mainly from higher chloride values. The authors warn that the traditional 8 to 16 range may not apply to new instruments.
Arch Intern Med
The key review has been cited 445 times, and 3,359 PubMed articles mention the anion gap in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 9, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
High anion gap · Usual range with a low bicarbonate · Low anion gap
Look for the unmeasured acid. Likely a normal gap (hyperchloremic) acidosis; confirm with a blood gas. Usually low albumin or a laboratory error.
Next steps
Avoid common errors
5 common errors to avoid
and 3 more
When the gap misleads
1Low albumin
Albumin is the main unmeasured anion. Each 1 g/dL fall lowers the gap by about 2.5 mmol/L, so a critically ill patient with an albumin of 2.0 g/dL can have a high gap acidosis with a gap that looks normal. Enter the albumin.
2Your laboratory’s range
Newer analyzers can read chloride higher than older ones; in one laboratory (Winter 1990) this lowered the reference range from 8 to 16 to 3 to 11 mmol/L. A gap of 14 may be high in one laboratory and normal in another.
3Potassium included or not
Some laboratories add potassium to the sodium, which raises the gap by about 4 mmol/L. This calculator leaves it out; compare like with like.
4Bicarbonate from the blood gas
Use the measured total CO2 from the chemistry panel taken with the sodium and chloride. The bicarbonate a blood gas analyzer calculates can differ.
5A normal gap does not exclude acidosis
A mixed picture, such as ketoacidosis treated with saline, can bring the gap back toward normal while the bicarbonate stays low.
Formula
Sodium − (chloride + bicarbonate); add 2.5 for every 1 g/dL of albumin below 4.0.
Na − (Cl + HCO3)
Sodium − (chloride + bicarbonate); add 2.5 for every 1 g/dL of albumin below 4.0.
Formula and ranges
| Value | Formula (mmol/L) |
|---|---|
| Anion gap | sodium − (chloride + bicarbonate) |
| Albumin-corrected gap | gap + 2.5 × (4.0 − albumin in g/dL) |
| Delta ratio | (gap − 12) / (24 − bicarbonate), using the corrected gap when albumin is given |
| Result | mmol/L | Meaning |
|---|---|---|
| Low anion gap | below 3 mmol/L | Below 3 mmol/L: most often a laboratory error or a low albumin; less often a paraprotein, or lithium, bromide or iodide. |
| Within the usual range | 3 to 12 mmol/L | Within the usual range of most current analyzers (check your laboratory’s own range). If a blood gas confirms a metabolic acidosis, a low bicarbonate with a normal gap points to a normal anion gap (hyperchloremic) acidosis, as in diarrhea or renal tubular acidosis; compensation for a respiratory alkalosis can look the same. |
| High anion gap | above 12 mmol/L | Above 12 mmol/L: suggests excess unmeasured anions, most often from a metabolic acidosis such as lactic acidosis, ketoacidosis, kidney failure or a toxic ingestion. |
Emmett and Narins 1977
Winter 1990
Figge 1998
Units
Test cases
3 published cases, each also a test case
3 published cases; the calculator reproduces every result.
Worked examples
A 51-year-old man taking empagliflozin and metformin had weakness, vomiting and ketonuria with a glucose of only 160 mg/dL. Sodium was 135, chloride 103 and bicarbonate 9 mmol/L, with a venous pH of 7.16. The report gives an anion gap of 23.
“The calculated anion gap was 23.”
A high gap with near-normal glucose: the ketoacidosis would be missed if the glucose alone were used.
Entered as: Sodium: 135 mmol/L (mEq/L); Chloride: 103 mmol/L (mEq/L); Bicarbonate: 9 mmol/L (mEq/L).
Source: Serrano Reyes JC et al., Cureus 2026 (PubMed · Free full text · DOI)
A 35-year-old woman with recurrent hypokalemic paralysis (potassium 2.7 mmol/L) had a sodium of 138, chloride of 115 and bicarbonate of 14 mmol/L, with an inappropriately alkaline urine (pH 6). The report’s table gives a serum anion gap of 9 mmol/L; primary Sjögren’s syndrome turned out to be the cause.
“Arterial blood gas analysis showed metabolic acidosis in the presence of a normal serum anion gap.”
The acidosis is hyperchloremic: chloride rose as bicarbonate fell, so the gap stayed normal.
Entered as: Sodium: 138 mmol/L (mEq/L); Chloride: 115 mmol/L (mEq/L); Bicarbonate: 14 mmol/L (mEq/L).
Source: Ehsan N et al., Cureus 2026 (PubMed · Free full text · DOI)
A 73-year-old man with stage 3 chronic kidney disease using brinzolamide eye drops had a sodium of 131 and chloride of 107 mmol/L, a blood gas bicarbonate of 8.9 mmol/L, and an albumin of 2.7 g/dL. The report gives an anion gap of 15.1 mmol/L and a gap of 18.3 corrected with 2.5 mmol/L per 1 g/dL fall in albumin.
“The serum anion gap was 15.1 mmol/L; corrected for hypoalbuminemia (adding 2.5 mmol/L per 1 g/dL fall in albumin), the corrected anion gap was 18.3 mmol/L.”
The calculator gives 18.35 for the corrected gap, shown as 18.4; the report rounds it down to 18.3. Its delta ratio of 0.42 matches the calculator’s. The report used the blood gas bicarbonate; the calculator expects the chemistry-panel total CO2. Most of the fall in bicarbonate was a normal-gap (hyperchloremic) component, which the authors attributed mainly to drug-induced renal tubular acidosis.
Entered as: Sodium: 131 mmol/L (mEq/L); Chloride: 107 mmol/L (mEq/L); Bicarbonate: 8.9 mmol/L (mEq/L); Albumin: 2.7 g/dL.
Source: Chao MV et al., BMC Nephrol 2026 (PubMed · Free full text · DOI)
FAQ
7 questions and answers
and 5 more answers
Questions
Anion gap = sodium − (chloride + bicarbonate), all in mmol/L (the same numbers as mEq/L). A sodium of 135, chloride of 103 and bicarbonate of 9 give 135 − (103 + 9) = 23 mmol/L. Some laboratories add potassium to the sodium, which raises the gap by about 4; compare your result with a range calculated the same way.
It depends on the analyzer. The older range of 8 to 16 mmol/L came from earlier methods; with modern ion-selective electrodes, Winter and colleagues found 3 to 11 mmol/L in one laboratory, because chloride now reads higher. This calculator treats 3 to 12 as the usual range, but your laboratory’s own range is the one to use.
Add 2.5 mmol/L for every 1 g/dL that albumin is below 4.0 g/dL: corrected gap = gap + 2.5 × (4.0 − albumin). With albumin in g/L, use 0.25 × (40 − albumin). Figge and colleagues derived the factor in critically ill patients, where low albumin often hides a high gap.
Acids whose anions are not measured: lactic acidosis, ketoacidosis (diabetic, alcoholic or starvation), kidney failure, and toxins such as methanol, ethylene glycol and salicylate. A high gap does not always mean acidosis; laboratory error, metabolic alkalosis, high phosphate and some paraproteins can also raise it.
If the blood gas confirms a metabolic acidosis, a low bicarbonate with a normal gap means chloride has risen as bicarbonate fell (hyperchloremic acidosis). Common causes are diarrhea, renal tubular acidosis, large volumes of saline, and carbonic anhydrase inhibitors such as acetazolamide.
Most often a laboratory error or a low albumin. Less often it points to a paraprotein (as in IgG myeloma), or to lithium, an unmeasured cation, or to bromide or iodide, which can make chloride read falsely high.
The rise in the gap divided by the fall in bicarbonate: (gap − 12) / (24 − bicarbonate). It is used to look for a second acid–base disorder alongside a high gap acidosis. The calculator shows it in the working when the gap is high, but Kraut and Madias caution that it varies between patients and should not be used on its own. The calculator uses the albumin-corrected gap when albumin is entered. 12 and 24 are the traditional normal values; if your laboratory’s normal gap is lower, the ratio will read low.
Further reading
7 references with PubMed and DOI links
7
references
3
free full text
From Kraut JA et al., Clin J Am Soc Nephrol 2007 to the newest case reports.
References
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 9, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
This calculator supports, and does not replace, clinical judgment. It is meant for healthcare professionals and is not medical advice for patients. Report an error through the contact page.
clinical grade of aneurysmal subarachnoid hemorrhage, I to V.
total calcium adjusted for albumin, in mg/dL or mmol/L.
stroke severity from 15 bedside items, 0 to 42.
MAP from systolic and diastolic pressure, against the 65 mm Hg target.
eye, verbal and motor scores for preverbal children, 3 to 15.
oxygenation ratio with the Berlin ARDS severity ranges.
serum sodium corrected for high glucose, Katz or Hillier factor.
urea nitrogen to creatinine ratio, in US or SI units.
estimated creatinine clearance with actual, ideal or adjusted weight.
cardiac output and index from oxygen consumption and saturations.
ANC from the white cell count, neutrophils and bands.
For the sodium in hyperglycemic crises, often checked alongside the gap.
Another quick ratio from the same chemistry panel.
Which published the Kraut and Madias review, with its current metrics.