This is the first report from Saudi Arabia studying the normal reference intervals in adult Saudi subjects and evaluating serum cystatin C as a prospective marker for the assessment of the glomerular filtration rate (GFR). Three hundred healthy adult Saudi subjects including 156 males (52%) and 144 females (48%), with a mean age of 31.21 ± 9.82 years were prospectively studied to establish normal reference ranges for cystatin C. A total of 68.34% of the study patients were in the age-group of 21-40 years. The mean serum cystatin C in the 300 healthy subjects was 0.751 ± 0.11 mg/L (0.50 - 1.09), increasing gradually with age: it was 0.738 ± 0.11 mg/L (0.51 - 1.09) in the age-group 21 - 30 years and 0.807 ± 0.12 (0.51 - 1.09) among subjects who were > 50 years of age. The mean serum cystatin C in females (0.778 ± 0.118 mg/L) was significantly hig-her than in males (0.726 ± 0.095 mg/L) (p < 0.0001). The serum cystatin C level was within the defined reference range of 0.53 - 0.95 mg/L in 95% of the subjects with a mean value of 0.74 ± 0.097 mg/L, and was falling within the 95% confidence interval of 0.73865 - 0.7637 mg/L, and with 98.84% area under the curve (AUC). All the other renal function markers (urea, serum crea-tinine, calculated GFR, BMI) among the studied subjects were within the normal reference ranges for adult Saudi population. The serum cystatin C level had a significant correlation with the body mass index (BMI) (r = 0.155; p = 0.007) and a correlation with serum creatinine as well (r = 0.009; p = 0.873). It showed a negative correlation with calculated GFR as per Cockroft-Gault equation (r = - 0.101; p = 0.083).
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