Synovial fluid calprotectin may be a relevant serum biomarker that can be used to differentiate septic arthritis from other inflammatory arthritides, such as rheumatoid arthritis (RA) and pseudogout, according to results of a prospective study published in Rheumatology.

The investigators sought to determine whether calprotectin and α-defensins could differentiate septic arthritis from other inflammatory arthritic disorders. The study included patients who presented with acute monoarthritis with an inflammatory synovial count of >2000/mm3 and >80% neutrophils. Researchers prospectively collected synovial fluid containing a predominance of neutrophils from patients with septic arthritis, pseudogout, and RA. Neutrophil-related bactericidal proteins, calprotectin, and human neutrophil α-defensins levels were evaluated in synovial fluid. Biomarkers with P values ≤.05 and demographic parameters were included in a multivariable model.

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A total of 74 patients were included in the analysis, 26 of whom had septic arthritis, 28 of whom had pseudogout, and 20 of whom had RA. Patients with septic arthritis were more likely to be men and of a younger age, and to exhibit higher synovial neutrophil counts. Calprotectin levels were significantly increased in patients with septic arthritis. Among the 26 patients with septic arthritis, 23 individuals had positive synovial fluid cultures and 6 had positive blood cultures (3 patients had both). The bacteria identified included Streptococcus species in 10 patients, Staphylococcus species in 9 patients, Pseudomonas aeruginosa in 2 patients, and Neisseria gonorrhoeae, Enterococcus faecalis, Enterobacter cloacae, Citrobacter koseri, and Klebsiella oxytoca in 1 patient each.

Calprotectin level in joint aspirate was the only biomarker that significantly differentiated septic arthritis from nonseptic inflammatory arthritides, with 76% sensitivity, 94% specificity, and a positive likelihood ratio of 12.2 at the threshold for calprotectin of 150 mg/L (odds ratio, 1.05; 95% CI, 1.02-1.10; P <.01).

The investigators concluded that the use of synovial fluid calprotectin as a biomarker for septic arthritis should be tested in an independent cohort to pinpoint its utility in daily practice.

Reference

Baillet A, Trocmé C, Romand X, et al. Calprotectin discriminates septic arthritis from pseudogout and rheumatoid arthritis [published online March 28, 2019]. Rheumatology (Oxford). doi:10.1093/rheumatology/kez098