Urinary free light chains may help to identify infection in patients with elevated systemic inflammation due to rheumatic disease.

The risk of infection in patients with rheumatic diseases is elevated, but a clear marker to differentiate the cause of the systemic inflammation is missing. We assessed the ability urinary immunoglobulin free light chains (FLCs) to indicate the presence of infection in patients with rheumatic disease. We performed a retrospective analysis of patients with rheumatic disease attending the Georg-August University Hospital in Goettingen, Germany, from January 2011 to December 2013. Subjects were included if they had urine levels of κ and λ FLCs available. A reference group of patients without autoimmune disease, but with documented infection, was constructed. A total of 1500 patients had their urinary FLCs quantified during the study period. Of the 382 patients with rheumatic disease, 172 (45%) displayed no systemic inflammation, 162 (42%) had inflammation due to the underlying disease activity, and 48 (13%) had inflammation due to a confirmed infection. Urinary FLC concentrations were much higher in patients with rheumatic diseases and infection (κ 68.8 ± 81.8 mg/L, λ 31.4 ± 53.5 mg/L) compared to those with inflammation due to rheumatic disease activity (κ 22.7 ± 26.3 mg/L, λ 8.1 ± 9.1 mg/L, κ p < 0.001, λ p = 0.004). Urinary κ FLCs demonstrated good ability to predict infection, with a sensitivity of 63% and specificity of 84%. Urinary λ FLCs gave similar values, with a sensitivity of 65% and specificity of 81%. FLCs may be useful for distinguishing inflammation due to rheumatic disease activity from that due to the additional presence of infection. The ability to quantify these proteins in urine provides a simple alternative to the use of blood.