young children This infection is frequently characterized by mild clinical symptoms, lack of leukocytosis and relatively low concentrations of acute-phase serum reactants and would be missed on such algorithms A blood culture should be obtained in patients with concern for osteomyelitis, abscess, or septic arthritis Laboratory studies are also indicated in the absence of fever if the child has been limping for several days without evidence of trauma on plain films While rheumatoid factor is typically negative in children with JIA, antinuclear antibody testing is frequently positive in patients with the oligoarticular and polyarticular forms of JIA Acute lymphocytic leukemia (ALL) may present with musculoskeletal complaints; bone and joint pain often begin before the appearance of blasts in the peripheral blood Children with evidence of infection or inflammation with a joint effusion may require arthrocentesis for definitive diagnosis In areas of endemic Lyme disease, Lyme testing should be performed in a patient with monoarticular arthritis A creatine phosphokinase level may be helpful if muscle inflammation is suspected When the initial history, physical examination, imaging, and laboratory evaluation indicate the cause of the limp, specific treatment can be initiated Abnormalities in the initial workup without a definitive diagnosis should prompt further imaging or laboratory studies Computed tomography is an excellent imaging modality for cortical bone It serves as a useful diagnostic adjunct in certain fractures, bony coalitions, and bone tumors Ultrasound is the preferred modality for diagnosing hip effusions; it is also useful for guiding needle aspirations of the hip joint While historically ultrasonography was only performed by radiologists, point-of-care ultrasound (POCUS) to quickly identify hip joint effusions is becoming increasingly common in pediatric emergency medicine Magnetic resonance imaging has become the gold standard for the diagnosis of musculoskeletal infections and Perthes disease It can additionally be useful in the workup of malformative conditions, rheumatologic disease, oncological tumors, and hematologic malignancies Historically, MRI was best utilized with well-localized symptoms rather than as a screening tool However, technological advances in whole-body MRI and lack of radiation exposure increase the appeal of MRI over bone scintigraphy in many situations The latter remains an option where MRI is not readily available In a prospective study of limping children with ESR >10 mm/hr or CRP >10 mg/L, 75% had positive MRI findings leading to a definitive diagnosis If the initial workup in a limping child is completely normal, including screening radiographs and laboratory studies, the child may be followed closely as an outpatient The child should be examined frequently (e.g., every few days)