To evaluate the reliability and validity of the Spanish version of the Medical Outcomes Study Pain Severity Scale (MOS-PSS) in patients with rheumatic diseases.
A cross-sectional study was carried out from January 2019 to March 2020 using a nonprobabilistic sampling of consecutive cases. The internal consistency of the scale was evaluated through the Cronbach α coefficient and by the item response theory reliability coefficient. Construct validity was evaluated by factor analysis; relationships between MOS-PSS and visual numeric scale for pain, numeric rating scale for pain, and verbal rating scale for pain; and differences between relevant groups. Item response theory-based methods were used to assess item performance.
A clinical sample of 796 outpatients was recruited. Most patients presented moderate to severe pain. Two subscale solutions showed a good model fit in confirmatory factor analysis. The overall model fit of multidimensional generalized partial credit model showed to be adequate. The most discriminating item was "average pain intensity." Evidence revealed disordered thresholds in 2 items. Collapsing categories resulted in ordered thresholds for all items and significantly improving the overall model fit. The MOS-PSS and modified MOS-PSS yielded high reliability. Both scales were very strongly correlated with numeric rating scale for pain, visual numeric scale for pain, and verbal rating scale for pain (ρ ≥ 0.85). All hypotheses related to subgroups comparison were fulfilled.
Overall, the Spanish version of the MOS-PSS showed good reliability and construct validity. Nevertheless, the statistical evidence from this study would suggest the modified MOS-PSS should be the version of choice for measuring pain in Mexican patients with rheumatic diseases.