Transient synovitis (TS) is a common and benign cause of hip pain in children, but must be distinguished from more serious entities such as septic arthritis, osteomyelitis, and pyomyositis. Our objectives were to determine the risk of missed bacterial musculoskeletal infection and rates of diagnostic testing in children diagnosed with TS.

We performed a cohort study using the Pediatric Heath Information System of children 1–10 years diagnosed with TS in the ED. We determined rates of missed bacterial musculoskeletal infection (defined as a new diagnosis of septic arthritis, osteomyelitis, or pyomyositis within 14 days of initial ED visit). We described the initial diagnostic evaluation and ED management of children diagnosed with TS and variability between sites.

We analyzed 6419 encounters from 37 hospitals. 62 (1.0%, 95%CI: 0.7–1.2%) children were diagnosed with a missed bacterial musculoskeletal infection. Children with missed infection were younger than those without (median age 2.6 vs. 4.6 years, p < 0.01). Serum laboratory testing was performed in 76% of encounters with minimal variation across sites. There was significant variation in the rates of hip ultrasound by site (2 to 92%), which has increased in use over time (from 42% in 2016 to 62% in 2021).

In this large observational study, missed bacterial musculoskeletal infection in children diagnosed with TS was rare but more common in younger children. The optimal combination of bloodwork and radiographic testing, especially ultrasound, to distinguish TS from more serious disease remains unclear.