Which nitrogenous waste is measured in urine to assess kidney function?

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

Answer: (A)

Creatinine is measured in urine to gauge kidney function because it is produced at a fairly constant rate from muscle metabolism and is filtered by the kidneys with minimal reabsorption. This makes the amount cleared from the blood each minute a good proxy for the glomerular filtration rate (GFR). By comparing creatinine levels in urine and blood over a collection period, clinicians estimate GFR and assess how well the kidneys are filtering.

Urea, while also a waste product, is more variable because its levels depend on protein intake, hydration, and liver function, so its urinary clearance is less reliable for estimating GFR. Uric acid and ammonia are not standard measures for assessing overall kidney filtration capacity; they relate more to stone risk or other metabolic processes rather than a direct, consistent measure of renal filtration.