Establishing a Common Metric for Physical Function: Linking the HAQ-DI and SF-36 PF Subscale to PROMISA® Physical Function

Physical function (PF) is a common health concept measured in clinical trials and clinical care. It is measured with different instruments that are not directly comparable, making comparative effectiveness research (CER) challenging when PF is the outcome of interest. Our goal was to establish a common reporting metric, so that scores on commonly used physical function measures can be converted into PROMIS scores. Following a single-sample linking design, all participants completed items from the NIH Patient Reported Outcomes Measurement Information System (PROMISA (R)) Physical Function (PROMIS PF) item bank and at least one other commonly used "legacy" measure: the Health Assessment Questionnaire (HAQ) or the Short Form-36 physical function ten-item PF scale (SF-36 PF). A common metric was created using analyses based on item response theory (IRT), producing score cross-walk tables. Participants (N = 733) were part of an internet panel, many of whom reported one or more chronic health conditions. PROMIS PF, SF-36 PF, and the HAQ-Disability Index (HAQ-DI). Our results supported the hypothesis that all three scales measure essentially the same concept. Cross-walk tables for use in CER are therefore justified. HAQ-DI and SF-36 PF results can be expressed on the PROMIS PF metric for the purposes of CER and other efforts to compare PF results across studies that utilize any one of these three measures. Clinicians seeking to incorporate PROs into their clinics can collect patient data on any one of these three instruments and estimate the equivalent on the other two.