Psychometric properties of the Danish SDM-Q-9 questionnaire for shared decision-making in patients with pelvic floor disorders and low back pain: item response theory modelling.

Background: Worldwide, involving patients in healthcare has become a focus point. Shared decision-making (SDM) is one element of patient involvement and, in many countries, including Denmark, requires culturally adapted and validated questionnaires to measure diverse patient populations' perceptions of this concept. SDM-Q-9, a widely used nine-item generic questionnaire, assesses patients' perception of nine elements during decision-making in consultations. The primary aim of this study is to assess the psychometric performance of the Danish version of the SDM-Q-9 through item response theory (IRT). Additionally, to assess the questionnaire's generic applicability among patients with pelvic floor disorders or low back pain.

Methods: After treatment decisions, Danish patients with pelvic floor disorders or low back pain rated the level of SDM by completing the SDM-Q-9 questionnaire. Iitem response theory (the Graded Response Model by Samejima) was applied to assess each item's psychometric performance and the questionnaire's generic applicability (among others discriminative ability, precision and item differential functioning).

Results: The study invited 825 patients for participation and comprised 758 patients for analysis;73% were women, with a mean age of 52 years and a mean SDM score of 3.87. Discrimination parameters (a-scores) for the model ranged from 2.39 (item 1) to 4.48 (item 8). Analysis of the item-information function curves reflected that item 8 demonstrated the highest maximum, indicating higher precision, while items 1, 2 and 9 showed the lowest maxima. Chi²-test statistics showed no significant differential item functioning at the 0.01-significance level for any item between the two patient groups. A ceiling effect was observed as most patients selected the highest score, while a low information load was identified in the SDM's upper load for each item and the overall instrument.

Conclusions: The Danish SDM-Q-9 demonstrates strong overall performance, with the ability to differentiate between the distinct levels of the underlying construct of SDM. However, the high ceiling effect is a critical limitation. While the SDM-Q-9 could serve as a generic questionnaire across samples with varying demographic composition, further exploration of these findings is warranted, particularly across patient samples encompassing more diverse decisions, e.g. patients with life-threatening diseases.