Renal function estimated from serum creatinine is overestimated in patients with rheumatoid arthritis because of their muscle atrophy.

Abstract We evaluated the reliability of serum creatinine concentration (Scr) to estimate renal function in patients with rheumatoid arthritis (RA). To quantify muscle volume (study 1) the lean body mass (LBM) in 25 women RA patients and 10 controls was measured using dual X-ray absorptiometry (DEXA). The 60-min creatinine clearance (Ccr60) and 60-min urinary excretion of creatinine (Ucr60) were also determined. The Ucr60 and LBM of the extremities, which were significantly correlated (r = 0.757, P < 0.0001), were lower in patients with long-standing and advanced RA than in controls. In study 2, the 24-h creatinine clearance (Ccr24) and 24-h urinary excretion of creatinine (Ucr24) were determined retrospectively in 82 women RA patients and 120 controls with normal Scr. The Ccr of long-standing and advanced RA patients was significantly lower than that of the controls, although the Scr of the long-standing RA patients was significantly lower than that of the advanced RA patients. The upper limit of the normal Scr for RA patients was calculated as being approximately 10% lower than that for controls. Thus, the renal function estimated from Scr may be overestimated in patients with long-standing and advanced RA because of their muscle atrophy.