Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 26-year-old man is evaluated during a follow-up visit after presenting to an urgent care clinic for back pain 1 week ago. Laboratory studies at that time were significant for a serum creatinine level of 1.4 mg/dL (123.8 µmol/L); other laboratory studies, including urinalysis, were normal. A urine albumin-creatinine ratio obtained in preparation for this visit is 10 mg/g. He is a personal trainer, and his daily exercise regimen includes weightlifting. He states that his back pain has resolved. He occasionally takes ibuprofen; the last use was 1 week ago. He takes no over-the-counter supplements.

On physical examination today, vital signs are normal. BMI is 29. The patient is muscular, without signs of obesity. There is no muscle tenderness.

Which of the following is the most appropriate management?

A. Avoid all NSAID medications
B. Measure the serum creatine kinase level
C. Measure the serum cystatin C level
D. Schedule a kidney biopsy

MKSAP Answer and Critique

The correct answer is C. Measure the serum cystatin C level.

Measurement of the serum cystatin C level is appropriate for this patient. Cystatin C may be preferable to creatinine to assess kidney function in individuals with higher muscle mass. An increase in muscle mass would be expected to result in an increase in serum creatinine level in the absence of change in kidney function. This muscular man with a BMI of 29 has increase in muscle mass. Because serum creatinine is derived from the metabolism of creatinine produced by muscle, a significant increase in muscle mass would be expected to increase serum creatinine. An elevation in serum creatinine could also occur with creatine supplements, which he is not taking. This patient has a normal urinalysis and no proteinuria, all of which indicate no evidence of underlying kidney disease. Cystatin C, which is cleared by the kidney, is produced by all nucleated cells; therefore, levels are less dependent on muscle mass. Cystatin C can also be used for more accurate glomerular filtration rate estimation in these patients as a component of the Chronic Kidney Disease Epidemiology Collaboration equation.

Although NSAIDs can cause acute kidney injury, the remote and infrequent use by this patient is unlikely to have any effect on serum creatinine. The hemodynamic effects of NSAIDs will disappear within 24 hours of stopping the medication, and interstitial nephritis from NSAIDs is unlikely to present with occasional dosing and is usually associated with proteinuria. The adverse effects of renal fibrosis associated with NSAIDs are only seen with extensive and long-term use.

Creatine kinase levels can be measured to evaluate for the presence of rhabdomyolysis. Rhabdomyolysis significant enough to cause kidney injury would be expected to result in myoglobinuria reflected by heme-positive urine in the absence of red cells. No blood was seen on urinalysis.

In the absence of other changes suggesting glomerular or interstitial disease, a kidney biopsy is not necessary.

Key Point

• Increased muscle mass can result in an increase in serum creatinine level in the absence of change in kidney function.

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