This private blood test measures serum cystatin C, a precision filtration marker used to assess kidney function (eGFR) independently of muscle mass. The catalogue test price is £135.63 plus a £50 phlebotomy fee, with results in 5 days.
Standard creatinine-based eGFR can look falsely abnormal in muscular athletes, bodybuilders, people taking creatine supplements, or patients with low muscle mass; cystatin C avoids that muscle-mass bias.
Written by Mr Stephen Lingam. Reviewed by Dr Mohammad Bakhtiar (GMC 4694470) on 18/10/2026.
Creatinine comes from muscle breakdown, so heavy strength training, high-protein diets or creatine use can make creatinine eGFR appear reduced even when kidney filtration is completely normal.
No. Cystatin C is unaffected by dietary protein or creatine supplementation, and no fasting is needed.
Yes. Combining Cystatin C with Urea and Electrolytes and a urine albumin-to-creatinine ratio in one visit gives a complete kidney assessment under a single £50 phlebotomy fee.
Results are normally available in 5 working days.