immunosorbent assay Positive results should be confirmed by a Western immunoblot for IgG antibodies to B burgdorferi An IgM immunoblot assay is not necessary in late disease and may result in false-positive results Negative serum IgG serology excludes Lyme as the cause of arthritis Treatment for Lyme arthritis consists of a 4-week course of oral antibiotics Doxycycline in a dose of mg/kg/day divided twice daily with a maximum of 100 mg per dose is effective for children older than years, whereas amoxicillin (50 mg/kg/day in three divided doses with a maximum of 1.5 g/day) is sufficient in younger children Cefuroxime (30 mg/kg/day in two divided doses with a maximum of 1,000 mg daily) is an alternative for patients with penicillin allergy Serum antibody titers remain elevated even after adequate antibiotic treatment and should not be used as a measure of success of treatment Persistent or recurrent joint swelling may occur months beyond the initiation of treatment Although this may represent a local autoimmune response, experts recommend retreatment with a second 4week course of oral antibiotics or a 2- to 4-week course of parenteral ceftriaxone FIGURE 121.4 Lyme arthritis of the left knee Transient Synovitis Transient or toxic synovitis is a benign, self-limiting inflammatory condition of the hip It afflicts males more frequently than females and is the most common cause of acute hip pain in children to 10 years of age The underlying cause is unknown, although a postinfectious inflammatory response has been suggested