Anion Gap Calculator

What is the anion gap?

The anion gap is a value calculated from electrolytes measured in a routine blood test, usually a basic or comprehensive metabolic panel. Blood always stays electrically neutral, so the positive charges (cations) and negative charges (anions) must balance. A lab measures the main cation, sodium, and the two main anions, chloride and bicarbonate. The difference between them is the "gap", which represents negatively charged substances that are not routinely measured, such as albumin, phosphate, sulfate and organic acids.

Doctors mainly use the anion gap to investigate metabolic acidosis, a condition in which the blood becomes too acidic, and to help narrow down its possible cause.

Anion gap formula

This calculator uses the standard formulas:

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

Anion gap with potassium = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)

Albumin-corrected anion gap = Anion gap + 2.5 × (4.0 − albumin in g/dL)

Example: With sodium 140, chloride 104 and bicarbonate 24 mEq/L, the anion gap is 140 − (104 + 24) = 12 mEq/L, which is normal. If potassium is 4.0 mEq/L, the gap with potassium is 16 mEq/L. If albumin is low at 2.0 g/dL, the corrected gap becomes 12 + 2.5 × (4.0 − 2.0) = 17 mEq/L, which is high. This example shows why albumin correction matters.

Normal anion gap range

Anion gap (mEq/L)	Interpretation
Below 8	Low
8 – 16	Typical normal range
Above 16	High (elevated)

Normal ranges differ between laboratories. Many modern analysers give lower values, so some labs use a range closer to 3–11 mEq/L. When potassium is included, the normal range is a few points higher. Your own lab report is the most reliable reference.

Why correct the anion gap for albumin?

Albumin is the largest unmeasured anion in the blood. When albumin is low, as can happen in liver disease, kidney disease, malnutrition or serious illness, the anion gap drops too. This can hide a high anion gap acidosis and make the result look normal. Adjusting for albumin gives a more accurate picture, which is why doctors often use the corrected value in hospital patients.

What causes a high anion gap?

A high anion gap usually points to extra acids in the blood. Common causes include diabetic ketoacidosis, alcoholic or starvation ketoacidosis, lactic acidosis (for example from severe infection or shock), advanced kidney failure, and poisoning with substances such as methanol, ethylene glycol (antifreeze) or high doses of aspirin (salicylates). Many medical students remember these causes with memory aids such as MUDPILES or GOLDMARK.

What causes a low anion gap?

A low anion gap is rare. The most common cause is low albumin. Other causes include laboratory error, high levels of certain abnormal proteins (as in multiple myeloma), and very high calcium, magnesium or lithium levels.

What does a normal anion gap acidosis mean?

Sometimes a person has metabolic acidosis but a normal anion gap. This usually happens when the body loses bicarbonate, for example through severe diarrhoea, or when the kidneys cannot handle acid properly, as in renal tubular acidosis. In these cases, chloride typically rises to replace the lost bicarbonate, which is why this is also called hyperchloraemic acidosis.

Important note

The anion gap is just one piece of the puzzle. Doctors interpret it together with blood gas results, glucose, kidney function, symptoms and medical history. Do not use this calculator to diagnose or treat any condition. If you have concerns about your results, speak with your doctor.

Frequently Asked Questions

What is a normal anion gap?

A typical normal anion gap is about 8 to 16 mEq/L when calculated without potassium. Some laboratories use a lower range, such as 3 to 11 mEq/L, so always check the range on your own report.

What does a high anion gap mean?

A high anion gap often suggests metabolic acidosis caused by extra acids in the blood, such as from ketoacidosis, lactic acidosis, kidney failure or certain poisonings. A doctor needs to confirm the cause with further tests.

Should I include potassium in the anion gap?

Most laboratories and doctors calculate the anion gap without potassium, because potassium levels are small and fairly stable. The potassium-included version is shown for reference and has a slightly higher normal range.

What is the corrected anion gap?

The corrected anion gap adjusts the result for low albumin. For every 1 g/dL that albumin falls below 4.0, about 2.5 mEq/L is added to the anion gap. This prevents low albumin from hiding a high anion gap.

Is a low anion gap dangerous?

A low anion gap on its own is not usually an emergency. It is most often due to low albumin or a lab error, but occasionally it can point to other conditions, so it is worth discussing with your doctor.