Minimally important differences and severity thresholds are estimated for the PROMIS depression scales from three randomized clinical trials

•A minimally important difference (MID) for the PROMIS depression T-score is 3 to 4 points.•PROMIS depression scales of varying lengths have a similar MID.•A PHQ-9 change of 1 point equates with a PROMIS T-score change of 1.25 points.•PROMIS T-scores of 55, 60, 65, and 70 represent mild, moderate, moderately severe, and severe depression. Patient Reported Outcomes Measurement Information Systems (PROMIS) scales are increasingly being used to measure symptoms in research and practice. The purpose of this study was to determine the minimally important difference (MID) and severity thresholds (cut-points) for the four fixed-length PROMIS depression scales. The study sample was adult participants in three randomized clinical trials (N=651). MID was estimated using multiple distribution- and anchor-based approaches including assessing correspondence between Patient Health Questionnaire (PHQ-9) and PROMIS depression scores. The best MID estimate was a PROMIS depression T-score of 3.5 points with most methods producing an MID in the 3 to 4 point range across all three samples. MID estimates were similar for all four PROMIS scales. A PHQ-9 1-point change equated to a PROMIS 1.25-point T-score change. PROMIS T-scores of 55, 60, 65, and 70 appeared to be reasonable thresholds for mild, moderate, moderately severe, and severe depression, respectively. The study sample was predominantly male veterans with either chronic pain (2 trials) or previous stroke (1 trial). The severity of depression was mild to moderate. A T-score of 3 to 4 points is a reasonable MID for PROMIS depression scales and can be used to assess treatment effects in both practice and research as well to calculate sample sizes for clinical trials. Severity cut-points can help interpret the meaning of scores and action thresholds for treatment decisions.