The measurement of serum cystatin C is useful for which of the following?

• A. Detecting an early decrease in kidney function
• B. Calculating creatinine clearance
• C. Diagnosing end-stage renal disease
• D. Monitoring dialysis patients

Answer: (A)

Cystatin C rises in the blood as GFR falls, and it does so earlier than creatinine in many people. Because it is produced at a constant rate and is freely filtered by the glomerulus with minimal influence from muscle mass, it serves as a sensitive marker of early changes in kidney function. This makes it especially helpful for detecting a subtle, initial decline in GFR before creatinine-based measures show impairment. It isn’t typically used to calculate creatinine clearance (that relies on timed urine collection), and while it can be used to estimate GFR, it’s not the standard tool for diagnosing end-stage renal disease or for routine dialysis monitoring, where other markers and metrics are preferred.