An increased serum level of which of the following analytes is most commonly associated with decreased glomerular filtration?

• A. Creatinine
• B. Uric acid
• C. Urea
• D. Ammonia

Answer: (A)

When the kidneys’ filtering capacity drops, waste products that are normally cleared accumulate in the blood. Creatinine fits best as a marker because it is produced at a steady rate by muscle, is freely filtered by the glomerulus, and is not reabsorbed or significantly secreted, so its blood level rises in direct proportion to declining GFR. That relationship makes serum creatinine a reliable indicator of decreased filtration and a common basis for estimating GFR with standard equations.

Urea (blood urea nitrogen) also increases with reduced GFR, but its level is heavily influenced by protein intake, hydration, and catabolic state, so it’s less specific as a GFR marker. Uric acid can rise with reduced clearance as well, but it’s not as tightly linked to GFR as creatinine. Ammonia isn’t used to assess kidney filtration because its interpretation depends more on liver function and metabolic context, not on glomerular filtration alone.