American Board of Internal Medicine (ABIM) Certification Practice Exam

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What is the formula for calculating the albumin-corrected anion gap?

Anion Gap + 2.5 × Normal Albumin
Anion Gap + 2.5 × [Measured Albumin - Normal Albumin]
Anion Gap + 2.5 × [Normal Albumin - Measured Albumin]
Anion Gap - 2.5 × Measured Albumin

The correct answer is: Anion Gap + 2.5 × [Normal Albumin - Measured Albumin]

The formula for calculating the albumin-corrected anion gap is designed to adjust the anion gap based on the serum albumin level. In cases of hypoalbuminemia, the total protein in the blood may be reduced, which can affect the interpretation of the anion gap in metabolic acidosis. The correct formulation involves subtracting the measured albumin from the normal albumin level and multiplying the difference by 2.5. This approach effectively adjusts the anion gap to reflect the lower levels of albumin since albumin contributes negatively charged particles in the extracellular fluid. When albumin levels drop, the anion gap may appear falsely widened. By using the formula that subtracts the measured albumin from the normal albumin, clinicians can better assess the true acid-base status of the patient. This adjustment acknowledges that for every 1 g/dL decrease in albumin below the normal range, the anion gap can underestimate the actual unmetabolized anions present, thus necessitating this correction to provide a more accurate clinical picture.