Differential item functioning of the patient-reported outcomes information system (PROMISA®) pain interference item bank by language (Spanish versus English)

About 70% of Latinos, 5 years old or older, in the United States speak Spanish at home. Measurement equivalence of the PROMISA (R) pain interference (PI) item bank by language of administration (English versus Spanish) has not been evaluated. A sample of 527 adult Spanish-speaking Latinos completed the Spanish version of the 41-item PROMISA (R) pain interference item bank. We evaluate dimensionality, monotonicity and local independence of the Spanish-language items. Then we evaluate differential item functioning (DIF) using ordinal logistic regression with item response theory scores estimated from DIF-free "anchor" items. One of the 41 items in the Spanish version of the PROMISA (R) PI item bank was identified as having significant uniform DIF. English- and Spanish-speaking subjects with the same level of pain interference responded differently to 1 of the 41 items in the PROMISA (R) PI item bank. This item was not retained due to proprietary issues. The original English language item parameters can be used when estimating PROMISA (R) PI scores.