What is the formula to calculate the anion gap?

The anion gap is a critical laboratory value used to help identify the cause of metabolic acidosis. It helps assess the balance of electrolytes in the plasma and indicates the presence of unmeasured anions. The correct calculation of the anion gap is determined by subtracting the sum of chloride (Cl) and bicarbonate (HCO3) from sodium (Na). This is represented mathematically as:

Anion Gap = Sodium (Na) - (Chloride (Cl) + Bicarbonate (HCO3))

This formula is essential for determining the anion gap, which assists in diagnosing conditions such as diabetic ketoacidosis, renal failure, and poisoning from substances like methanol and ethylene glycol.

Analyzing the other choices helps clarify the elements of the calculation. The first option incorrectly adds chloride and bicarbonate to sodium. The third option proposes a division, which does not reflect the necessary subtraction needed to find the anion gap. The fourth option also adds chloride and subtracts bicarbonate from sodium, which deviates from the standard formula. Thus, the accurate calculation using subtraction is paramount for obtaining the correct anion gap value.