Internal construct validity of the Swiss Spinal Stenosis questionnaire: Rasch analysis of a disease-specific outcome measure for lumbar spinal stenosis.

STUDY
DESIGN: Rasch analysis of an outcome tool using data from questionnaires completed by patients with lumbar spinal stenosis.
OBJECTIVE: To evaluate the psychometric properties of the Swiss Spinal Stenosis questionnaire using the Rasch measurement model. SUMMARY OF BACKGROUND DATA: Lumbar spinal stenosis (LSS) is a common cause of low back pain and leg symptoms in older patients. The SSS questionnaire was devised as a condition-specific outcome measure for patients with LSS. It comprises two subscales; one to measure symptom severity and another to measure physical functioning. The SSS is commonly used in trials for patients with LSS but has not, to our knowledge, been subjected to rigorous modern psychometric analysis methods.
METHODS: Data from a total of 190 SSS assessments in 98 people with LSS were fitted to the Rasch measurement model. This method was used to examine the validity of the item scoring functions, the presence of item bias or differential item functioning and the fit of data to model expectations. In addition, the analysis was used to assess whether or not the item set in each scale formed a unidimensional scale to provide a valid summed score.
RESULTS: The symptom severity scale and the physical function scale of the SSS were found to be reliable and well targeted to the study population. However, the symptom severity scale was found to be multidimensional; three items focused on pain and four items on neuroischemic symptoms. The physical function scale performed well as a unidimensional scale, but needed to be modified by excluding one item in order for it to fit the Rasch model.
CONCLUSION: Development of a more robust measurement tool for this patient group may be warranted. A new tool may need to include three scales to measure the separate domains of pain severity, neuroischemic symptom severity, and physical function.