Cystatin C is a sensitive marker for detecting a reduced glomerular filtration rate when assessing chronic kidney disease in patients with rheumatoid arthritis and secondary amyloidosis

Objective: Chronic kidney disease is a predictor of end-stage renal disease, and evaluating the glomerular filtration rate (GFR) is necessary to make a definite diagnosis. We assessed the utility of serum cystatin C (cysC) for identifying a reduced GFR in patients who have rheumatoid arthritis (RA) with secondary amyloidosis. Methods: Fifty patients with RA and secondary amyloidosis (mean age 60.9 +/- 11.2 years; 45 women) were evaluated. The revised 24-h creatinine clearance (r24-hC(Cr)), which was determined by multiplying the original value by 0.719, was used as a reference for the GFR. The screening potential of the serum cysC and some estimates of the GFR calculated from the serum cysC (cysC-eGFR: eGFR(Hoek) and eGFR(Rule)) for detecting a reduced GFR (r24-hC(Cr) < 60 mL/min/1.73 m(2)) were analysed. Results: Both cysC-eGFRs were strongly correlated with the r24-hC(Cr) (eGFR(Hoek), r = 0.846, p < 0.001; eGFR(Rule), r = 0.820, p < 0.001). The difference between the average eGFR(Rule) (37.1 +/- 31.2 mL/min/1.73m(2)) and average r24-hC(Cr) (35.3 +/- 30.9 mL/min/1.73 m(2)) was small, whereas eGFR(Hoek) and sCr-eGFR were higher than eGFR(Rule) and r24-hC(Cr). In receiver operating characteristic (ROC) curve analyses of a reduced GFR, serum cysC gave a greater area under the curve (AUC = 0.958) than the sCr-eGFR (0.939-0.942). The specificity and positive predictive value (PPV) reached 100% when serum cysC > 1.365 mg/L was used. Conclusions: Serum cysC can identify a reduced GFR more accurately than sCr-eGFRs. Serum cysC > 1.09 mg/L (i.e. eGFR(Rule) < 60 mL/min/1.73 m(2)) could be a marker of a reduced GFR, and serum cysC > 1.365 mg/L would strongly suggest a reduced GFR in patients who have RA with secondary amyloidosis.