PROMIS Physical Function short forms display item- and scale-level characteristics at least as good as the Roland Morris Disability Questionnaire in patients with chronic low back pain

OBJECTIVE: To compare dimensionality, item-level characteristics, scale-level reliability and construct validity of PROMIS® Physical Function short forms (PROMIS-PF) and 24-item Roland Morris Disability Questionnaire (RMDQ-24) in patients with chronic low back pain (LBP).

DESIGN: Cross-sectional study.

SETTING: Secondary care center for rehabilitation and rheumatology.

PARTICIPANTS: Patients with non-specific LBP ≥ 3 months (n = 768).

INTERVENTIONS: Not applicable.

MAIN OUTCOME MEASURES: Dutch versions of the 4-, 6-, 8-, 10- and 20-item PROMIS-PF, and of the RMDQ-24.

RESULTS: Mean age was 49 years (±13), 77% female, 54% displayed pain for more than 5 years. PROMIS-PF-6, PROMIS-PF-8 and RMDQ-24 exhibited sufficient unidimensionality (confirmatory factor analysis: CFI >0.950, TLI >0.950 and RMSEA <0.060) whereas the other instruments did not. All instruments were free of local dependence except PROMIS-PF-20 with four item pairs with clear residual correlations. Mokken scale analysis found one non-monotone item for PROMIS-PF-20 and eight for RMDQ-24 (i.e. the probability of endorsing these items was not increasing with increasing level on the underlying construct). PROMIS-PF-20 displayed two misfitting items (S-X2 p-value > 0.001). Two-parameter item response theory models found two items with low discrimination for RMDQ-24. All other instruments had adequate fit statistics and item parameters. PROMIS-PF-20 displayed the best scale-level reliability. Construct validity was sufficient for all instruments as all hypotheses on expected correlations with other instruments, and differences between relevant subgroups, were met.

CONCLUSIONS: PROMIS-PF-6, PROMIS-PF-8 and RMDQ-24 exhibited better unidimensionality, whereas PROMIS-PF-4, PROMIS-PF-6 PROMIS-PF-8 and PROMIS-PF-10 showed superior item-level characteristics. PROMIS-PF-20 was the instrument with the best scale-level reliability. This study warrants assessment of other measurement properties of PROMIS-PF short forms in comparison to disease-specific physical functioning instruments in LBP.