PURPOSE: The aim of this study is to illustrate an example application of Rach Measurement Theory (RMT) in the evaluation of patient-reported outcome (PRO) measures. RMT diagnostic methods were applied to evaluate the PROMIS® Depression items as part of a series of papers applying different psychometric paradigms in parallel to the same data. METHODS: RMT was used to examine scale-to-sample targeting, scale performance and sample measurement of two PROMIS depression item pools including respectively 28 and 51- items. RESULTS: Sub-optimal but improved targeting was displayed in the 51-item pool which covered 27% of the range of depression measured in the sample compared to only 15% in the 28-item bank, further reducing the sample percentage with lower depression not covered by the scale (28% Vs 34%). Satisfactory scale performance was observed by the 28-item bank with marginal item misfit. However, deviations from the RMT criteria in the 51-itempool were observed including: 9 reversed thresholds; 12 misfitting items and 12 item-pairs displaying dependency. Overall reliability was good for sets of items (Person Separation Index = 0.93 and 0.95), but sub-optimal sample measurement (17% Vs 19% fit residuals outside of the recommended range). CONCLUSIONS: The RMT approach in this exercise provided evidence that compared to the 28-item bank, the extended 51-item version of the PROMIS depression, improved sample-to-scale targeting. However, targeting in the lower end of the concept of interest remained sub-optimal and scale performance deteriorated. There may be a need to improve the conceptual breadth of the construct under investigation to ensure the inclusion of items that capture the full range of the concept of interest for this context of use.