Cystatin C for estimation of glomerular filtration rate in patients with spinal cord injury

Background Serum creatinine is not a satisfactory marker of glomerular filtration rate (GFR) in patients with spinal cord injury (SCI) who have varying degrees of muscle atrophy. In contrast to serum creatinine, serum cystatin C, a 13-kDa protein, is not affected by muscle mass and is therefore potentially a useful marker of GFR in patients with SCI. In addition, cystatin C is not dependent on sex or age and is not secreted by the renal tubule. Aim We assessed serum cystatin C as a surrogate marker of GFR in SCI patients. Methods Cystatin C was analysed using a particle-enhanced immunonephelometric assay (Dade Behring) in serum samples sent for routine measurement of creatinine (64 patients) and creatinine clearance (27 patients) from patients in the Spinal Unit of the Austin Health. We compared these results with serum cystatin C of 57 non-SCI patients who had had a creatinine clearance measurement during the study period. Results In patients with SCI, the reciprocal of cystatin C had a stronger correlation (r = 0.48, P < 0.01) with creatinine clearance than the reciprocal of serum creatinine (r = 0.25, P < 0.19). Further, the value of serum creatinine was much lower for a given creatinine clearance in SCI patients than in non-SCI patients; the serum cystatin C concentrations were equivalent. Conclusion The serum cystatin C is a convenient and more reliable surrogate marker of GFR than serum creatinine and will enable early detection of renal impairment. We need to confirm this finding with a larger study, including comparison with an accepted gold standard for GFR.