Laboratory error described as increasing sodium and potassium while decreasing chloride and bicarbonate would cause which effect on the calculated anion gap?

• A. A falsely increased anion gap
• B. A falsely decreased anion gap
• C. No change
• D. A random change

Answer: (A)

Anion gap is what you get when you subtract the sum of measured anions (chloride and bicarbonate) from the sum of measured cations (sodium, and sometimes potassium). If you increase the cations (sodium and potassium) while decreasing the anions (chloride and bicarbonate), the numerator gets larger and the subtraction term gets smaller. Both changes push the result upward, making the calculated anion gap larger than it should be.

For example, start with Na 140, K 4, Cl 105, HCO3 24. The gap is (140 + 4) − (105 + 24) = 15. If Na rises to 145 and K to 5, while Cl falls to 100 and HCO3 to 20, the gap becomes (145 + 5) − (100 + 20) = 30. This demonstrates a falsely increased anion gap due to the described lab changes, not a true metabolic alteration.