Anion gap
Value calculated from electrolytes that gives clues about the acid-base balance
Also known as
Definition
The anion gap is not measured directly but calculated from the electrolytes in a blood sample. It describes the difference between positively charged particles (sodium, sometimes potassium) and negatively charged particles (chloride, bicarbonate). The value helps assess whether the blood is too acidic (acidosis) or not acidic enough (alkalosis) and whether the electrolytes are out of balance.
| Parameter | Value |
|---|---|
| Unit | mmol/l (mEq/l) |
| Reference Range | No universal range: it depends on whether the lab includes potassium and how it measures the electrolytes. Normal values therefore vary considerably between labs. The range printed on your report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
For monovalent ions such as sodium or chloride, 1 mEq/l equals exactly 1 mmol/l; the numbers are the same in both units.
↓ What a low value means
A low anion gap is rare. To rule out a measurement error, the test is often repeated first. The most common cause is a low albumin level; low albumin in turn can be linked to low protein intake, kidney, liver or heart disease and some cancers. Discuss the result with your doctor.
↑ What a high value means
A raised anion gap can indicate that the blood is more acidic than usual (acidosis). Possible links include dehydration, very heavy physical exertion, kidney disease, diabetes with ketoacidosis, and poisoning, for example with salicylates (too much aspirin) or antifreeze. The value alone is not a diagnosis; your doctor investigates the cause using your medical history and further tests.
When to test
The anion gap is calculated from the results of an electrolyte or metabolic panel, such as a basic or comprehensive metabolic panel. It is requested specifically when symptoms could fit acidosis or alkalosis. No separate preparation is needed; if other values are measured from the same sample, fasting may be required.
Frequently asked questions
How is the anion gap calculated? +
The usual formula is sodium minus (chloride plus bicarbonate). Some labs include potassium. Because formulas and methods vary, a value can only be compared with the reference range of the lab that reported it.
Why does albumin matter? +
Albumin influences the anion gap even though it does not appear in the formula. According to MedlinePlus, a low albumin level is the most common cause of a low anion gap.
Do I need to fast for this value? +
Not for the anion gap itself. If other values are measured from the same blood sample, your practice may ask you to fast for several hours.
Is an abnormal value enough for a diagnosis? +
No. Normal values vary widely and there are many possible causes. Your doctor assesses the value together with symptoms, medical history and further tests.
Anion gap
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.