Measurement properties of the eight-item abbreviated functional assessment of cancer therapy--breast symptom index and comparison with its 37-item parent measure

Context: The Functional Assessment of Cancer Therapy--Breast Symptom Index (FBSI) is an eight-item instrument extracted from the Functional Assessment of Cancer Therapy--Breast (FACT-B). There has been no formal assessment of this eight-item version.

Objectives: This study aimed to examine the measurement properties of and comparability between the English and the Chinese versions of the FBSI and to compare it with its parent instrument, the FACT-B, in breast cancer patients in Singapore.

Methods: This was an observational study of 271 breast cancer patients. Known-group validity of FBSI scores was assessed using four health indicators. Convergent and divergent validity was examined by correlation coefficients between the FBSI and the FACT-B. Responsiveness was assessed in relation to longitudinal changes in performance status. Test-retest reliability was evaluated by the intraclass correlation coefficient. Multiple regression analyses were performed to compare the scores on the two language versions. Receiver operating characteristic curve analyses were used for comparison between the FBSI and the FACT-B.

Results: For both language versions, the FBSI demonstrated known-group validity, convergent and divergent validity, and sufficient test-retest reliability (intraclass correlation coefficient = 0.75-0.77). The English version was responsive to changes in performance status. The Chinese version was responsive to decline in performance status, but there was no conclusive evidence about its responsiveness to improvement in performance status. No practical significant difference was found in the outcomes between the two language versions despite minor difference in one item. The FBSI performed comparably with the FACT-B.

Conclusion: The English and Chinese versions of the FBSI are valid and reliable and provide comparable FBSI scores. The English version is responsive to change, whereas the responsiveness of the Chinese version warrants further study.