Development of a multicentre cohort study to understand the role of MRI and ultrasound in the diagnosis of acute haematogenous bone and joint infection in children (the PIC Bone study): a study protocol

Aims Bone and joint infections (BJI) in children are rare but can be serious. Differentiating BJI from other conditions with similar symptoms is critical. Advanced imaging (ultrasound scans (USS) and MRI) is often required to confirm the diagnosis. The differing merits of imaging type and regional variation in access to advanced imaging can lead to diagnostic uncertainty and treatment variation. The aim of this study is to evaluate the diagnostic accuracy of MRI and USS for the investigation of BJI in children, and develop and validate prediction models to aid the diagnosis of BJI in children. A nested qualitative sub-study will explore acceptability of the imaging to children, parents, and health practitioners. Methods A multicentre retrospective cohort of children (aged < 16 years) with suspected diagnosis of BJI will be used to estimate the diagnostic accuracy of the two imaging methods and develop the prediction models. The models will be evaluated in a second cohort of prospectively recruited children. Diagnostic test accuracy will be estimated overall, and separately for children aged under and over five years. The prediction models will be fit using logistic regression, with candidate predictors chosen based on clinical plausibility and from a review of the literature. Continuous predictors will be examined for non-linearity with confirmed BJI using fractional polynomials. Multiple imputation will be used to replace missing values. Internal validation will be carried out using bootstrapping. Model performance will be assessed with discrimination and calibration. Discussion Ethical approval for this study (registration: ISRCTN15471635) was granted (REC reference 23/WM/0027). Informed consent is being obtained from participants in the prospective cohort and the qualitative sub-study. Study findings will be published in an open access journal and presented at relevant national and international conferences. Relevant charities and associations are being engaged to promote awareness of the project.