Randomized controlled trials (RCTs) of PTSD interventions use many different instruments to diagnose PTSD and assess symptom severity, which may limit the ability to compare findings across trials or pool data for meta-analysis. Understanding and comparing these differences can provide guidance on instrument selection for future research and evidence synthesis efforts. We analyzed data from the PTSD-Repository, a database of 318 PTSD RCTs on adults, to describe the instruments used, the assessment types (structured clinical interview, clinical diagnosis, or self-report questionnaire), and the relationship between assessment type and study characteristics (e.g., pharmacologic vs. non-pharmacologic).