Single-group studies in leading physical therapy journals commonly make inappropriate conclusions regarding treatment effect modifiers. A systematic review

BACKGROUND: Characteristics that identify patients who respond differently to certain interventions are called treatment effect modifiers. Some studies inappropriately report the presence of treatment effect modifiers without adequate study designs. OBJECTIVES: To evaluate what proportion of single-group studies published in leading physical therapy journals inappropriately report treatment effect modifiers, and to assess whether the proportion varies over time or between journals. METHODS: A systematic review was conducted of studies published in eight leading physical therapy journals since 2000. Eligible studies were single-group studies (e.g., cohort study or secondary analysis of treatment arm of randomised controlled trial) that investigated any condition, treatment or outcome. Studies that suggested participants with certain baseline characteristics responded better/or worse to the treatment, were considered to have reported inappropriately. Studies reporting that participants with certain baseline characteristics had improved outcomes but did not state it was due to the treatment were considered to have reported appropriately. The proportion of inappropriate reporting was compared over time and between journals. RESULTS: Of the 145 included studies, 73 (50.3%) were categorised as inappropriately reporting treatment effect modifiers. The proportion of inappropriate reporting was highest in the most recent period, 2018 – 2022 (59.6%) and 2006 – 2011 (55.6%). The proportion of inappropriate reporting varied substantially between journals from 0% (Journal of Physiotherapy) to 91.7% (Journal of Neurologic Physical Therapy). CONCLUSIONS: A large proportion (50.3%) of single-arm studies in leading physical therapy journals inappropriately report treatment effect modifiers. This inappropriate reporting risks misleading clinicians when selecting interventions for individual patients.