Measurement Equivalence of the Patient Reported Outcomes Measurement Information System^® (PROMIS^®) Pain Interference...

Reducing the response burden of standardized pain measures is desirable, particularly for individuals who are frail or live with chronic illness, e.g., those suffering from cancer and those in palliative care. The Patient Reported Outcome Measurement Information System^® (PROMIS^®) project addressed this issue with the provision of computerized adaptive tests (CAT) and short form measures that can be used clinically and in research. Although there has been substantial evaluation of PROMIS item banks, little is known about the performance of PROMIS short forms, particularly in ethnically diverse groups. Reviewed in this article are findings related to the differential item functioning (DIF) and reliability of the PROMIS pain interference short forms across diverse sociodemographic groups.

DIF hypotheses were generated for the PROMIS short form pain interference items. Initial analyses tested item response theory (IRT) model assumptions of unidimensionality and local independence. Dimensionality was evaluated using factor analytic methods; local dependence (LD) was tested using IRT-based LD indices. Wald tests were used to examine group differences in IRT parameters, and to test DIF hypotheses. A second DIF-detection method used in sensitivity analyses was based on ordinal logistic regression with a latent IRT-derived conditioning variable. Magnitude and impact of DIF were investigated, and reliability and item and scale information statistics were estimated.

The reliability of the short form item set was excellent. However, there were a few items with high local dependency, which affected the estimation of the final discrimination parameters. As a result, the item, "How much did pain interfere with enjoyment of social activities?" was excluded in the DIF analyses for all subgroup comparisons. No items were hypothesized to show DIF for race and ethnicity; however, five items showed DIF after adjustment for multiple comparisons in both primary and sensitivity analyses: ability to concentrate, enjoyment of recreational activities, tasks away from home, participation in social activities, and socializing with others. The magnitude of DIF was small and the impact negligible. Three items were consistently identified with DIF for education: enjoyment of life, ability to concentrate, and enjoyment of recreational activities. No item showed DIF above the magnitude threshold and the impact of DIF on the overall measure was minimal. No item showed gender DIF after correction for multiple comparisons in the primary analyses. Four items showed consistent age DIF: enjoyment of life, ability to concentrate, day to day activities, and enjoyment of recreational activities, none with primary magnitude values above threshold. Conditional on the pain state, Spanish speakers were hypothesized to report less pain interference on one item, enjoyment of life. The DIF findings confirmed the hypothesis; however, the magnitude was small. Using an arbitrary cutoff point of theta (θ) ≥ 1.0 to classify respondents with acute pain interference, the highest number of changes were for the education groups analyses. There were 231 respondents (4% of the total sample) who changed from the designation of no acute pain interference to acute interference after the DIF adjustment. There was no change in the designations for race/ethnic subgroups, and a small number of changes for respondents aged 65 to 84.

Although significant DIF was observed after correction for multiple comparisons, all DIF was of low magnitude and impact. However, some individual-level impact was observed for low education groups. Reliability estimates were high. Thus, the PROMIS short form pain items examined in this ethnically diverse sample performed relatively well; although one item was problematic and removed from the analyses. It is concluded that the majority of the PROMIS pain interference short form items can be recommended for use among ethnically diverse groups, including those in palliative care and with cancer and chronic illness.