When to Suspect and How to Diagnose Kingella kingae Osteoarthritis

To the Editors: Pediatric septic arthritis and osteomyelitis are medical emergencies that require prompt diagnosis and adequate antibiotic therapy to prevent acute complications and long-term disability. The initial antibiotic choice usually involves administering wide-spectrum antimicrobials to cover the most common etiologies, pending culture and/or molecular test results. Hence, collecting information on the local prevalence and antibiotic susceptibility patterns of pathogens causing skeletal system infections is central for guiding empiric therapy. In a recent retrospective study, Hajósi-Kalcakosz et al.1 compared the features and outcomes of culture-negative versus culture-positive osteoarticular infections in a Hungarian pediatric population. Using traditional bacteriologic cultures but no nucleic acid amplification methods, Staphylococcus aureus was detected in 69 of 97 (71.2%) patients, Streptococcus pyogenes in 12 (11.9%), and Salmonella species in 7 (6.8%).1 Despite Kingella kingae being recognized today as the prime etiology of bone and joint infections in children aged 6–48 months, the organism was not identified in any of the study patients.1 The failure to detect K. kingae in this cohort was likely due to its fastidious nature. Kingella kingae is notoriously difficult to recover in culture unless synovial fluid and bone exudates are inoculated into blood culture vials.2 Detection of the bacterium can be further enhanced by species-specific real-time polymerase chain reaction assays.2,3 Because the authors did not employ sensitive laboratory methods, one might have expected the fraction of culture-negative patients to be high. However, a pathogen was identified in 70 of 97 (72.5%) children, within the range reported in studies that routinely used molecular technology.2–5 This counterintuitive result can be explained by the strategy for recruiting patients to the cohort. The presence of fever was a criterion for defining both septic arthritis and osteomyelitis and, thus, for enrollment in the study.1 In a report by Roversi et al.,4 comprising 702 children with septic arthritis, osteomyelitis or spondylodiscitis who underwent sensitive nucleic acid amplification testing, a pathogen was identified in 423 (60.2%). Kingella kingae was the most common etiologic agent, detected in 174 of 423 (41.1%) patients with bacteriologically proven infection.4 Of those, 128 (73.6%) were afebrile on admission.4 Because of the absence of fever, mild symptomatology and moderate elevation of acute-phase reactant levels of K. kingae, the familiar Kocher and Caird diagnostic algorithms would overlook septic arthritis in a large fraction of children.5 Ruling out a skeletal infection because of the absence of fever will exclude many children with K. kingae disease and spuriously increase the bacteriologic detection rate in the remaining patients. In summary, children with K. kingae infections involving the skeletal system present with an unimpressive clinical picture, requiring a high index of clinical suspicion and sensitive molecular diagnostic testing. Unless physicians are aware of the peculiar and benign characteristics of K. kingae skeletal system infections and employ sensitive detection assays, many cases would be misdiagnosed as noninfectious conditions, and the organism would be overlooked.