What is the formula to calculate the anion gap?

The anion gap is a useful clinical tool for evaluating metabolic acidosis and helping to identify the presence of unmeasured anions in the blood. The correct formula for calculating the anion gap is derived from the concentrations of key electrolytes in the serum.

The anion gap is calculated by taking the sodium (Na) level and subtracting the sum of the chloride (Cl) and bicarbonate (HCO3) levels. This can be mathematically expressed as:

Anion Gap = Na - (Cl + HCO3)

However, since the provided options format this differently, the correct interpretation is that the anion gap can be represented as Na - Cl - HCO3. This formula highlights that when you subtract the measured anions (Cl and HCO3) from sodium, you can derive the concentration of unmeasured anions.

Understanding this helps clinicians evaluate various conditions that may cause disturbances in acid-base balance. High anion gaps are often seen in conditions such as diabetic ketoacidosis or renal failure. Hence, the correct response relates directly to the fundamental calculation needed in practice to assess the anion gap effectively.