Inflammatory bowel disease Systemic lupus erythematosus Henoch–Schönlein purpura Other Transient synovitis of the hip Malignancy Leukemia Neuroblastoma Bone tumor Hemophilia A complete blood count (CBC) and differential, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) are indicated for the febrile child with signs of joint inflammation, especially in the absence of trauma Blood cultures will have a low yield, but should be obtained in the febrile patient or when there is concern for bone or joint infection Additional laboratory studies, such as an antistreptolysin-O titer or antinuclear antibody (ANA) test should be guided by the history and physical examination Radiographs of the affected joint are particularly useful in the setting of trauma or acute monoarthritis without an obvious cause to evaluate for fractures, dislocations, or occult tumors or other bony abnormalities The Ottawa knee rules can be used to guide the decision to obtain radiographs of the knee following injury In a recent validation study in children aged to 16 years of age, the rules were found to be 100% sensitive in detecting fractures while eliminating the need for about one-third of the radiographs, however there were a limited number of children below years of age and caution must be used when applying these rules in younger children Ultrasound is more sensitive than plain radiographs in detecting an effusion In most febrile children with monoarthritis and a joint effusion, an arthrocentesis, usually ultrasound guided if involving the hip, is needed to assist in determining if septic arthritis is the etiology Magnetic resonance imaging is most useful to detect subtle fractures not visualized on plain films and to help establish a diagnosis of osteomyelitis