In renal insufficiency, which of the following is used to assess glomerular filtration rate (GFR)?

• A. Urinalysis
• B. Blood urea nitrogen
• C. Renal plasma flow measure
• D. Creatinine clearance

Answer: (D)

Estimating GFR relies on a marker that is filtered by the glomeruli and not significantly altered by the tubules. Creatinine fits this well: it’s produced at a relatively steady rate, is freely filtered, and only minimally secreted by the proximal tubule. Because of that, the amount of creatinine cleared from blood into the urine over a set period roughly equals the amount filtered each minute, giving an estimate of GFR.

In practice, creatinine clearance is calculated from a timed urine collection and a serum creatinine level, using the relation that compares how much creatinine appears in the urine with how much is in the blood. This approach is more directly tied to filtration than other options: urinalysis doesn’t measure filtration rate, and blood urea nitrogen is influenced by hydration and protein intake and may not track GFR accurately. Renal plasma flow measures blood flow to the kidneys, not how much is filtered. Although measuring inulin clearance would give the true GFR, it’s impractical for routine use. Therefore, creatinine clearance provides a practical, reasonably accurate estimate of GFR in renal insufficiency.