Anion gap formula study sheet

Part 1 · The formula

Anion gap = Na − (Cl + HCO3)

1. 1.Take sodium (Na+), the main measured positive ion.
2. 2.Add the two measured negative ions it is compared with: chloride (Cl−) and bicarbonate (HCO3−, reported as total CO2 on many panels).
3. 3.Subtract that sum from the sodium: anion gap = Na − (Cl + HCO3).
4. 4.Some textbooks and labs add potassium to the sodium side. Check which version your course uses, and always use the same version for the whole problem.

Why a "gap" at all?

Body fluid carries equal total positive and negative charge. The panel measures only some of the ions, so the difference between the measured positives and negatives stands in for the ions the panel does not measure.

What fills the gap

Mostly negatively charged proteins (albumin above all), plus phosphate, sulfate and small organic acids that the electrolyte panel does not report.

Albumin matters

Albumin is the largest unmeasured anion, so a low albumin lowers the calculated gap. Courses teach this as a reason to read the gap next to the albumin value.

Units

All three values are in mEq/L (or the same number in mmol/L, since these ions carry a single charge), so the gap is in mEq/L too.

Ranges vary

Labs and textbooks publish different reference ranges, and the range depends on the method and on whether potassium is included. On any worksheet, the range your course or lab prints is the one to use.

Study use only

This sheet is for classroom practice with made-up numbers. It cannot be used for a real person or your own results.

Part 2 · Worked examples

Four worked practice problems

Invented numbers for classroom practice. Work each one on paper first, then open the steps.

• Problem 1

  Na 140, Cl 104, HCO3 24 (mEq/L)

  Show the steps

• Problem 2

  Na 138, Cl 100, HCO3 18 (mEq/L)

  Show the steps

• Problem 3

  Na 142, Cl 108, HCO3 26 (mEq/L)

  Show the steps

• Problem 4

  Na 136, Cl 98, HCO3 20, K 4 (mEq/L)

  Show the steps

Part 3 · Glossary

The terms on the sheet

Na+

Sodium, the main positive ion in the fluid outside cells

Cl−

Chloride, the main measured negative ion outside cells

HCO3−

Bicarbonate, the buffer ion; many panels report it as total CO2

K+

Potassium, left out of the gap in most courses and added in some

mEq/L

Milliequivalents per litre, the usual unit for these ions

BMP

Basic metabolic panel, the panel that holds the three ions

Unmeasured anion

A negative ion that the panel does not report, such as albumin or phosphate

Albumin

The most plentiful blood protein, and the biggest unmeasured anion