Linking Oswestry Disability Index to the PROMIS pain interference CAT with equipercentile methods

BACKGROUND CONTEXT: When different health care providers use different patient-reported outcome (PRO) instruments, it is challenging to integrate findings that describe particular patient groups or to establish treatment effectiveness across studies. It is therefore critical to develop accurate ways to convert scores between various instruments for clinicians and researchers to make comparisons across health outcomes.PURPOSE: To develop a common metric so that scores on the Oswestry Disability Index (ODI) and scores on the PROMIS Physical Function can be converted interchangeably.STUDY DESIGN/SETTING: Data were collected from a prospective study. A single-group linking design was used.PATIENT SAMPLE: The study population included 9020 patients presented to an orthopedic spine clinic from November 2013 to March 2019.OUTCOME MEASURES: Patients completed the ODI and the PROMIS Pain Interference scale delivered by Computerized Adaptive Testing (CAT) at the same time prior to their visit with a spine clinician.METHODS: Equipercentile linking methods based on log-linear smoothing approach and non-smoothing approach were used to establish a common metric across the two measures.RESULTS: The two measures assess the similar contruct of pain. The correlation between the scores of the ODI and the PROMIS PI was 0.81. The standardized Root Expected Mean Square Difference (REMSD) values for gender, ethnic, and racial groups ranged from 3.55% to 4.81%. Hence, the assumptions for the equipercentile linking method were met. The crosswalk derived linked scores based on the log-linear smoothing method yielded small deviations (Δ = 0.09) from the observed scores. We then identified linked PROMIS-PI scores corresponding to the benchmark ODI scores for the five disability levels and for various categories of patients.CONCLUSIONS: This study is the first to create crosswalks to interchangeably convert scores between the ODI to the PROMIS-PI in a large population of spine patients using the equipercentile linking method. The results of this study provide confidence in the validity and usefulness of the derived crosswalks based on the equipercentile linking approach. The crosswalks are helpful for comparing new and old studies on the two measures and identifying benchmark scores for various diseases and disability levels.