Validation of the Spine Oncology Study Group-Outcomes Questionnaire to assess quality of life in patients with metastatic spine disease

Background context: General questionnaires are often used to assess quality of life in patients with spine metastases, although a disease-specific survey did not exist until recently. The Spine Oncology Study Group has developed an outcomes questionnaire (SOSG-OQ) to measure quality of life in these patients. However, a scoring system was not developed, and the questionnaire was not validated in a group of patients, nor was it compared with other general quality of life questionnaires such as the EuroQol 5 Dimensions (EQ-5D) questionnaire.

Purpose: Our primary null hypothesis is that there is no association between the SOSG-OQ and EQ-5D. Our secondary null hypothesis is that there is no difference in coverage and internal consistency between the SOSG-OQ and EQ-5D. We also assess coverage, consistency, and validity of the domains within the SOSG-OQ.

Study design/setting: A survey study from a tertiary care spine referral center was used for this study.

Patient sample: The patient sample consisted of 82 patients with spine metastases, myeloma, or lymphoma.

Outcome measures: The SOSG-OQ (27 questions, 6 domains) score ranges from 0 to 80, with a higher score indicating worse quality of life. The EQ-5D (5 questions, 5 domains) index score ranges from 0 to 1, with a higher score indicating better quality of life.

Methods: The association between the SOSG-OQ and EQ-5D index score was assessed using the Spearman rank correlation. Instrument coverage and precision were assessed by determining item completion rate, median score with range, and floor and ceiling effect. Internal consistency was assessed using Cronbach alpha. Multitrait analysis and exploratory factor analysis were used to analyze properties of the individual domains in the SOSG-OQ.

Results: The Spearman rank correlation between the SOSG-OQ and EQ-5D questionnaire was high (r=-0.83, p<.001). Internal consistency of the SOSG-OQ (0.92, 95% CI: 0.89-0.94) was higher as compared to the internal consistency of the EQ-5D (0.73, 95% CI: 0.63-0.84; p<.001). The SOSG-OQ score had no floor or ceiling effect indicating good coverage (median 30, range 3-64), whereas the EQ-5D had a ceiling effect of 10% (median 0.71, range 0.05-1).

Conclusions: In conclusion, our study proposes a scoring methodology-after reversing four inversely scored items-for the SOSG-OQ and shows that the questionnaire is a valid tool for the assessment of quality of life in patients with metastatic spine disease. The SOSG-OQ is superior to the EQ-5D in terms of coverage and internal consistency but consists of more questions.