Septic arthritis has to be suspected in children with joint effusion and fever so as to perform joint aspiration, which will confirm the diagnosis by bacteriological methods, and to perform surgical treatment by joint lavage. Since development of current molecular methods, such as real-time PCR, Kingella kingae has become the first microbial agent of osteoarticular infections in young children, whereas Staphylococcus aureus is second. C-reactive protein (CRP) is an aid used to diagnose septic arthritis, but its elevation could be moderate. In a previous study, conducted at our hospital, 10% of children hospitalized for S. aureus or K. kingae septic arthritis had a CRP level<10 mg/L. To determine if diagnosis of septic arthritis could be made by other parameters, we analyzed the clinical and biologic features of these patients and compared them to those of children hospitalized for septic arthritis with initial CRP ≥10 mg/L. Among the 89 children with septic arthritis, 10% (n=9) had initial CRP<10 mg/L (K. kingae, n=5/63 ; S. aureus, n=4/26). Initial temperature and fibrinogen were significantly lower in the CRP<10 mg/L group than in the other (37.3°C vs. 37.9°C, P=0.039 and 4.19 vs. 5.72 g/L, P=0.003, respectively). Age, symptom duration before diagnosis, as well as leukocyte and platelet counts were similar in both groups. Two children (2/89=2.2%) with S. aureus septic arthritis had no fever, CRP elevation, or fibrinogen elevation. In the CRP-negative group, three of four children with S. aureus arthritis and one of five with K. kingae arthritis had a high CRP level (34, 40, 61, and 13 mg/L, respectively) 3 days after surgery and antibiotic treatment. One child with K. kingae septic arthritis and initial CRP<10 mg/L needed a second surgical drainage because of relapse of arthritis. In the S. aureus arthritis group, none of the children with initial CRP<10 mg/L experienced complications, while six of those with initial CRP≥10 mg/L needed a second surgical act or hospitalization in an intensive care unit. While CRP is most often>10 mg/L during septic arthritis in children, it could be negative in up to 20% of patients in different studies. However, a mild inflammatory syndrome or even a CRP<10 mg/L cannot exclude diagnosis of septic arthritis. Therefore, a first episode of monoarthritis in children has to be considered as septic arthritis and treatment should not be delayed.
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