Utility of estimated glomerular filtration rate using cystatin C and its interpretation in patients with rheumatoid arthritis under glucocorticoid therapy

Background: Patients with rheumatoid arthritis (RA) often have reduced muscle mass. Estimated glomerular filtration ratio using the serum cystatin C concentration (eGFRcys) is more accurate than eGFR using the serum creatinine (eGFRcreat) because cystatin C is not influenced by muscle mass, but glucocorticoid therapy may affect serum cystatin C concentration. Methods: Fifty patients with RA were included in this study. Renal inulin clearance (CM) was measured and compared with eGFRcreat, eGFRcys, or the mean of eGFRcreat and eGFRcys (eGFRavg). Results: The mean creatine kinase (CM) concentration was low (36.8 +/- 24.4 U/l).The eGFRcreat and eGFRcys regression lines were significantly different from y = x. The mean eGFRcreat value was significantly higher than CM and that of eGFRcys was lower than CM. The difference between eGFRcys and CM was negatively correlated with daily PSL dose. The mean eGFRcys value of patients taking < 10 mg PSL was not different from an and the eGFRcys regression line was not different from y = x. Conclusion: eGFRcys of patients taking a daily PSL dose >= 10 mg was inaccurate, while eGFRcys was underestimated. eGFRcys was more accurate than eGFRcreat or eGFRavg for patients taking a daily PSL dose of < 10 mg.