This question is related to an area of active research in the PROMIS network, namely the determination of the “minimally important difference” or “MID” for a PROMIS instrument. A manuscript in the Journal of Clinical Epidemiology outlines the process for MIDs for PROMIS measures and estimates the MIDs for six PROMIS-Cancer scales:

Yost, K. J., Eton, D. T., Garcia, S. F., & Cella, D. (2011). Minimally important differences were estimated for six PROMIS-Cancer scales in advanced-stage cancer patients. Journal of Clinical Epidemiology, 64(5), 507-16.

As described in that manuscript, the MID is a tool to enhance the interpretability of patient-reported outcomes and is often defined as the “the smallest difference in score in the domain of interest which patients perceive as beneficial and which would mandate, in the absence of troublesome side effects and excessive cost, a change in the patient’s management” (Jaeschke R, Singer J, Guyatt GH. Measurement of health status. Ascertaining the minimal clinically important difference. Controlled Clinical Trials 1989; 10(4):407-415). However, for patients with advanced stage disease where palliation is the intent of treatment, the MID is patient-centered and may have no specific reference to the clinical aspect of the patient’s change (Wyrwich KW, Bullinger M, Aaronson N, Hays RD, Patrick DL, Symonds T. Estimating clinically significant differences in quality of life outcomes. Qual Life Res 2005; 14(2):285-95).

The recommended MID ranges for the PROMIS-Cancer T-Scores reported in that paper ranged from 2.5 to 6.0 points, depending on the specific short form of interest. Additional studies are currently being conducted to confirm these MIDs in other patient populations and to estimate MIDs for other short forms and CATs.