Serum cystatin C is not a better marker of creatinine or digoxin clearance than serum creatinine

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Abstract
Aims To assess whether cystatin C, a new serum marker of renal function, is a better index of creatinine or digoxin clearance than serum creatinine in older people. Methods Twenty-two volunteers over the age of 65 years (mean 73±5) were recruited from a healthy elderly volunteer database. None of the volunteers was taking digoxin or other medication known to interfere with digoxin kinetics or assay. Digoxin was infused at a dose of 7–10 µg kg−1 and blood samples were taken over the following 48 h and assayed for serum digoxin. Serum cystatin C, creatinine and creatinine clearance were measured and a calculated creatinine clearance was estimated using the Cockcroft Gault formula. Digoxin clearance was calculated using a pharmacokinetic software package. All values were log transformed to normalize their distribution. Results Of the 22 volunteers enrolled into the study, 18 completed the study. Serum cystatin C ranged between 0.72 and 1.89 mg l−1 and serum creatinine ranged from 69.6 to 153.9 µmol l−1. Measured creatinine clearance ranged from 38 to 123 ml min−1 and calculated creatinine clearance from 29.5 to 88.0 ml min−1. Digoxin clearance ranged from 51.0 to 103.5 ml min−1. Cystatin C correlated extremely well with creatinine (r=0.93, Pr=0.67, P=0.002, 95% CI 0.3, 0.87). Neither serum cystatin C nor serum creatinine correlated with digoxin clearance (r=0.25, P=0.31, 95% CI −0.25, 0.64 and r=0.44, P=0.068, 95% CI −0.03, 0.75, respectively). Measured creatinine clearance, however, did correlate well with digoxin clearance (r=0.55, P=0.018, 95% CI 0.11, 0.81). Conclusions Serum cystatin C and serum creatinine show very similar correlations with creatinine and digoxin clearances. Serum cystatin C does not offer any advantages in this respect. It remains to be seen whether cystatin C offers any advantage over creatinine in elderly people in other respects.