Distinguishing Kingella kingae from Pyogenic Acute Septic Arthritis in Young Portuguese Children.

(1) Background : We aim to identify clinical and laboratorial parameters to distinguish Kingella kingae from pyogenic septic arthritis (SA). (2) Methods : A longitudinal, observational, single-centre study of children < 5 years old with microbiological positive SA admitted to a paediatric hospital from 2013-2020 was performed. Clinical and laboratorial data at admission and at 48 h, as well as on treatment and evolution, were obtained. (3) Results : We found a total of 75 children, 44 with K. kingae and 31 with pyogenic infections (mostly MSSA, S. pneumoniae and S. pyogenes) . K. kingae affected younger children with low or absent fever, low inflammatory markers and a favourable prognosis. In the univariate analyses, fever, septic look, CRP and ESR at admission and CRP at 48 h were significantly lower in K. kingae SA. In the multivariate analyses, age > 6 months ≤ 2 years, apyrexy and CRP ≤ 100 mg/L were significative, with an overall predictive positive value of 86.5%, and 88.4% for K. kingae . For this model, ROC curves were capable of differentiating (AUC 0.861, 95% CI 0.767-0.955) K. kingae SA from typical pathogens. (4) Conclusions : Age > 6 months ≤ 2 years, apyrexy and PCR ≤ 100 mg/L were the main predictive factors to distinguish K. kingae from pyogenic SA < 5 years. These data need to be validated in a larger study.