What is the correct equation to calculate the anion gap?

The anion gap is a valuable tool in clinical medicine, particularly in the evaluation of metabolic acidosis. The correct equation for calculating it expresses the difference between the measured cations and measured anions in the serum.

The anion gap is calculated by subtracting the sum of the primary measured anions (chloride and bicarbonate) from the major cation in plasma, which is sodium (Na). This relationship helps to highlight whether there is an excess of unmeasured anions in the plasma, which can indicate specific types of metabolic derangements.

The formula is defined as:

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

This correctly takes into account the primary contributors to the serum's cationic and anionic balance, allowing for an assessment of any discrepancies.

The other proposed equations do not accurately reflect the standard calculation method and thus would not yield the correct anion gap figure. For instance, including potassium as a contributing factor or changing the position of the ions fundamentally alters the meaning and utility of the calculation, potentially leading to misinterpretation of a patient's acid-base status.