American Board of Internal Medicine (ABIM) Certification Practice Exam

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What is the formula for calculating the urine anion gap?

UCl - (UNa + UK)
(UNa + UK - UCl)
UCl + (UNa - UK)
UK - (UCl + UNa)

The correct answer is: (UNa + UK - UCl)

The urine anion gap is an important tool in clinical practice used to assess the presence of unmeasured anions in the urine, particularly in the investigation of metabolic acidosis. The formula for calculating the urine anion gap is derived from the principle of electroneutrality, which states that the total concentration of cations must equal the total concentration of anions in a given fluid. The correct formula for calculating the urine anion gap is the difference between the sums of sodium (Na) and potassium (K) concentrations minus the chloride (Cl) concentration in the urine. Thus, the calculation can be expressed as the sum of sodium and potassium concentrations subtracted by the chloride concentration. This helps to determine the renal handling of ammonium, particularly in cases of metabolic acidosis. A positive urine anion gap generally suggests that the kidneys are not effectively excreting ammonium, which could indicate a renal cause of the acidosis, while a negative gap could suggest a non-renal cause. Through this understanding, it becomes clear why the given answer accurately reflects this calculation and is the preferred approach in interpreting the urinary excretion of ions.