Plasma cystatin C determinations in a healthy elderly population

Plasma cystatin C measurement has been previously shown to be a better indicator of changes in glomerular filtration rate (GFR) than plasma creatinine. The available literature on reference intervals for cystatin C concentration encompasses only paediatric and adult populations up to 60 years of age, therefore we set out to determine an elderly reference range. Blood was taken from 401 subjects (65-101 years) and cystatin C and creatinine concentrations measured using commercially available methodologies. The availability of height and weight measurements allowed the additional calculation of predicted creatinine clearances using the Cockcroft and Gault formulae. Whilst no notable gender difference in cystatin C values was observed (female, 1.48 mg/l; male, 1.53 mg/l), concentrations rose with increasing age (60-79 years, 1.39 mg/l; >80 years, 1.70 mg/l). Conversely, there was a significant (P<0.0001) gender difference in creatinine values (female, 99 micromol/l; male, 120 micromol/l) but none between age groups (60-79 years, 105 micromol/l; >80 years, 113 micromol/l). Calculated GFR determinations resulted in a predicted creatinine clearance range of 21-81 ml/min per 1.73 m2 (n=361). There was no significant difference between gender (male, 18-88 ml/min per 1.73 m2; female, 24-69 ml/min per 1.73 m2), but a very significant 20% decrease in predicted GFR per decade. Sex-related reference intervals for creatinine were established (female, 66-149 micromol/l; male, 71-204 micromol/l); whilst age-related reference intervals were established for both cystatin C (60-79 years, 0.93-2.68 mg/l; >80 years, 1.07-3.35 mg/l) and predicted creatinine clearance (60-79 years, 27-89 ml/min per 1.73 m2; >80 years=18-55 ml/min per 1.73 m2). Plasma cystatin C measurement offers a simple, more sensitive screening assay for early changes in GFR and reflects the decreasing GFR that occurs with increasing age.