What is an anion gap and why is it important?

The anion gap is a calculated value from a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) that helps identify the cause of metabolic acidosis (too much acid in the body). It's calculated as [Na+ - (Cl- + HCO3-)]. Normal anion gap is 8-12 mEq/L (or 3-11 if albumin is included in calculation). An elevated anion gap indicates accumulation of unmeasured acids in the blood from conditions like diabetic ketoacidosis, lactic acidosis, kidney failure, or poisoning. Normal anion gap acidosis suggests bicarbonate loss (diarrhea, kidney issues).

What causes a high anion gap?

High anion gap (>12 mEq/L) indicates metabolic acidosis from acid accumulation. Common causes remembered by MUDPILES mnemonic: Methanol poisoning, Uremia (kidney failure), Diabetic ketoacidosis, Propylene glycol/Paraldehyde, Iron/Isoniazid, Lactic acidosis (sepsis, shock, exercise), Ethylene glycol, Salicylates (aspirin overdose). These conditions produce acids that increase the gap between measured cations and anions. Elevated anion gap is a medical concern requiring prompt evaluation to identify and treat the underlying cause.

What causes a low anion gap?

Low anion gap (<6 mEq/L) is less common and can result from: hypoalbuminemia (low albumin reduces unmeasured anions), multiple myeloma or other paraproteinemias (abnormal proteins), or laboratory error. Low albumin is the most common cause—albumin normally contributes to unmeasured anions, so when it's low, the gap decreases. Some medications (lithium, bromide) can also lower the gap. Very low anion gaps warrant investigation but are generally less clinically urgent than high anion gaps.

How is anion gap different from anion gap corrected for albumin?

Standard anion gap assumes normal albumin levels. Since albumin is an unmeasured anion, low albumin artificially lowers the calculated anion gap, potentially masking metabolic acidosis. The albumin-corrected anion gap adds 2.5 mEq/L for every 1 g/dL that albumin is below 4.0 g/dL. For example, if standard anion gap is 10 but albumin is 2.0 g/dL (2 below normal 4.0), corrected gap = 10 + (2 × 2.5) = 15, indicating metabolic acidosis that was masked. Always check albumin levels when interpreting anion gap.

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Anion Gap

Calculate anion gap from blood electrolyte levels.

Anion Gap

-- mEq/L

Enter values

Normal Range: 8 – 12 mEq/L

How to Use This Calculator

Obtain Blood Test Results

You need a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) from your healthcare provider. The test measures sodium (Na+), chloride (Cl-), and bicarbonate (HCO3-) levels in your blood.

Enter Electrolyte Values

Input your sodium, chloride, and bicarbonate levels exactly as shown on your lab results. Values should be in mEq/L (milliequivalents per liter). Double-check numbers for accuracy.

Enter Albumin (Optional)

If available from your test results, enter your albumin level to get a corrected anion gap calculation. This is important if your albumin is low, as it affects gap interpretation.

Review and Discuss Results

View your calculated anion gap and whether it's in the normal range. Do not self-diagnose—abnormal results require evaluation by your healthcare provider who will consider your symptoms, medical history, and other test results to determine the cause and appropriate treatment.