septic arthritis with 100,000 to 300,000 white blood cells (WBCs) per cubic millimeter (see Chapter 101 Rheumatologic Emergencies ) In the absence of a clear history of a tick bite, arthralgia secondary to early, localized Lyme disease may be a challenging diagnosis to establish because only about 40% to 70% of children have the characteristic erythema migrans rash, constitutional symptoms may be mild, and serologic tests will be normal in the early stages of disease However, serologic testing will be confirmatory in patients with Lyme arthritis secondary to disseminated disease Among those with monoarticular arthritis, it may be difficult to distinguish septic arthritis from arthritis associated with Lyme disease on clinical grounds alone, with or without analysis of synovial fluid There are clinical features that favor the diagnosis of Lyme disease such as knee involvement, absence of recent fever, and lower inflammatory markers, but these characteristics may be shared with a subset of patients who have septic arthritis Complicating this further is that synovial fluid analysis may not be definitive There is a wide range of synovial WBC counts in children with Lyme disease and although median values are lower than those typically seen in patients with septic arthritis, there is considerable overlap Given the difficulties of distinguishing Lyme disease from septic arthritis among children presenting with monoarticular arthritis, management of cases where suspicion for septic joint is significant may require an approach that includes analyzing synovial fluid (including Lyme PCR) and performing Lyme serology testing, especially if one practices in a region where Lyme disease is endemic Transient (also called toxic) synovitis is a poorly understood inflammation of large joints, afflicting children to years of age The diagnosis is typically made on clinical grounds, and this self-limited disease does not result in joint destruction When the hip is involved, the challenge for clinicians is to distinguish transient synovitis from Lyme (in endemic areas) or septic arthritis Reactive, or postinfectious, arthritis is probably more common than septic arthritis Arthritis following various enteric infections is not rare in children, and joint complaints after parvovirus B19 infection are seen among adolescents Chlamydia trachomatis infection of the genitourinary tract should be considered in any sexually active adolescent with new-onset arthritis With postinfectious arthritis, antimicrobial treatment does not modify the disease course Traumatic injuries to a joint may cause periarticular swelling or an effusion indicative of a hemarthrosis In addition, ligamentous or tendon injuries will result in joint pain and impaired range of motion Serum sickness and Henoch– Schönlein purpura (HSP) are marked by characteristic rashes EVALUATION AND DECISION