BIOLOGICAL VARIATION OF CYSTATIN-C - IMPLICATIONS FOR THE ASSESSMENT OF GLOMERULAR-FILTRATION RATE

To assess the inherent potential for detecting mild to moderate reduct ions in glomerular filtration rate, this study determined the biologic al variability of serum cystatin C and creatinine in 12 healthy subjec ts. After accounting for analytical variation, interindividual varianc e accounted for 93% and intraindividual variance accounted for 7% of s erum creatinine biological variation. As such, to lie outside the assa y reference interval, some subjects must exceed 13 SD from their usual mean value, whereas in others, a change of only 2 SD would be suffici ent. For cystatin C, interindividual variation explained 25% and intra individual variance explained 75% of biological variability. Therefore , the upper limit of the population reference interval for cystatin C is seldom more than 3-4 SD from the mean value of any healthy individu al. The critical difference for sequential values significant at P les s than or equal to 0.05 was calculated as 37% for serum cystatin C and 14% for serum creatinine. We conclude that cystatin C is potentially a better marker for detecting impaired renal function than serum creat inine, but serum creatinine is probably still the better marker for de tecting temporal changes of renal function in individuals with establi shed renal disease.