Which laboratory value is most sensitive for detecting early kidney impairment after contrast administration?

• A. Serum sodium
• B. Bilirubin
• C. Serum creatinine and estimated GFR
• D. Alanine aminotransferase

Answer: (C)

In this scenario, the test is looking for a measure that reflects the kidneys’ ability to filter blood. After contrast exposure, the first reliable signal of reduced filtration is a change in creatinine-based estimates of kidney function. Serum creatinine is produced at a steady rate and cleared by the kidneys; when the glomerular filtration rate drops, creatinine accumulates in the blood and the estimated GFR, calculated from creatinine (along with age, sex, and race), decreases. This combination is the most sensitive indicator among the given options for detecting early kidney impairment caused by contrast.

The other values aren’t as informative for early nephrotoxicity: bilirubin and alanine aminotransferase reflect liver function, and serum sodium is a general electrolyte marker that does not specifically track acute kidney injury from contrast.