In renal insufficiency, which statement about serum creatinine is true as GFR declines?

• A. Serum creatinine decreases
• B. Serum creatinine shows no change
• C. Serum creatinine increases
• D. Serum creatinine fluctuates unpredictably

Answer: (C)

The main idea is that serum creatinine tracks kidney function inversely: as GFR falls, the kidney’s ability to clear creatinine diminishes, so blood levels rise. Creatinine is produced at a fairly constant rate by muscle, and in a healthy kidney most of it is filtered and excreted; when filtration declines in renal insufficiency, less creatinine is cleared and its concentration in the blood increases. This rising creatinine is the reason clinicians use it to estimate GFR. Early in disease, creatinine might still appear normal because remaining nephrons compensate, but with continuing decline in GFR the serum creatinine trend is upward. The other statements don’t fit this pattern: creatinine would not consistently decrease or stay unchanged as GFR drops, and while it can vary with factors like hydration, the characteristic course in renal insufficiency is an increase in serum creatinine.