Which laboratory finding most directly suggests a reduced glomerular filtration rate (GFR)?

Question

BlackTom AI · Accepted Answer

Serum creatinine is an inverse marker of glomerular filtration rate (GFR): when GFR falls, serum creatinine rises because less creatinine is being cleared by the kidneys.

1) Relevant knowledge points and concepts
- GFR measures kidney filtration capacity and is approximated clinically by clearance of endogenous substances (creatinine, cystatin C).  
- Creatinine is produced at an approximately constant rate from muscle and is primarily cleared by glomerular filtration; therefore steady-state serum creatinine (Scr) is inversely related to GFR.  
- Other labs (serum calcium, platelet count) are not direct measures of GFR.

2) Step-by-step reasoning
- At steady state, rate of creatinine production (P) = rate of creatinine excretion = GFR × Scr (ignoring minor tubular secretion).  
- Rearranging gives Scr = P / GFR. Thus if GFR decreases while P stays constant, Scr increases.

3) Calculation with intermediate steps (illustrative)
- Assume P = 120 mg/day and approximate concentration units so that when GFR = 120 mL/min Scr = 1.0 mg/dL.  
- If GFR falls to 60 mL/min (a 50% reduction): Scr_new = P / GFR_new. With the inverse relation, Scr doubles to ≈2.0 mg/dL.  
- This demonstrates quantitatively that decreased GFR produces an elevated serum creatinine.

4) Why the correct answer is correct
- Elevated serum creatinine most directly indicates reduced renal clearance and therefore reduced GFR. It’s the standard, readily available laboratory marker used to infer changes in GFR.

5) Why the other options are incorrect
- a. Elevated serum calcium only: calcium can be abnormal for many reasons (hyperparathyroidism, malignancy) and is not a direct measure of GFR.  
- b. Decreased serum creatinine: this would suggest higher clearance, lower muscle mass, or lab error — not reduced GFR.  
- d. Decreased platelet count: thrombocytopenia reflects hematologic or splenic processes, not glomerular filtration.

6) Conclusion
Elevated serum creatinine most directly suggests a reduced GFR; therefore the correct answer is c. Elevated serum creatinine.

Which laboratory finding most directly suggests a reduced glomerular filtration rate (GFR)?Single choice

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