Reliability and validity of the Bowel Function Index for evaluating opioid-induced constipation: translation, cultural adaptation and validation of the Portuguese version (BFI-P)

Abstract

Objective: The Bowel Function Index (BFI) is a simple and sound bowel function and opioid-induced constipation (OIC) screening tool. We aimed to develop the translation and cultural adaptation of this measure (BFI-P) and to assess its reliability and validity for the Portuguese language and a chronic pain population.

Methods: The BFI-P was created after a process including translation, back translation and cultural adaptation. Participants (n = 226) were recruited in a chronic pain clinic and were assessed at baseline and after one week. Internal consistency, test–retest reliability, responsiveness, construct (convergent and known groups) and factorial validity were assessed.

Results: Test–retest reliability had an intra-class correlation of 0.605 for BFI mean score. Internal consistency of BFI had Cronbach’s alpha of 0.865. The construct validity of BFI-P was shown to be excellent and the exploratory factor analysis confirmed its unidimensional structure. The responsiveness of BFI-P was excellent, with a suggested 17–19 point and 8–12 point change in score constituting a clinically relevant change in constipation for patients with and without previous constipation, respectively.

Conclusions: This study had some limitations, namely, the criterion validity of BFI-P was not directly assessed; and the absence of a direct criterion for OIC precluded the assessment of the criterion based responsiveness of BFI-P. Nevertheless, BFI may importantly contribute to better OIC screening and its Portuguese version (BFI-P) has been shown to have excellent reliability, internal consistency, validity and responsiveness. Further suggestions regarding statistically and clinically important change cut-offs for this instrument are presented.

Keywords:

• Bowel function
• chronic pain
• factor analysis
• opioid-induced constipation
• reliability
• responsiveness
• validity

Transparency

Declaration of funding

This work was supported by intramural funds from the Chair on Pain Medicine of the Faculty of Medicine, University of Porto; and by unrestricted research grants from CPH Pharma, Ferrer Internacional SA and Mundipharma Pharmaceuticals SL. The participation of M.D. in this project was also supported by a grant from the Spanish Pain Foundation. The sponsors did not participate in the design or conduct of this study; in the collection, management, analysis, or interpretation of data; or in the preparation, review and approval of the manuscripts or decisions to submit for publication.

Author’s contributions: M.D.: statistical data analysis, manuscript writing and final revision. L.M.: coordination of the data collection and final revision of the manuscript. R.S.: data collection and final revision of the manuscript. C.G.: data collection and final revision of the manuscript. D.O.: data collection and final revision of the manuscript. J.M.C.-L.: coordination, manuscript writing and final revision. L.F.A.: coordination, study design, data collection, statistical data analysis, manuscript writing and final revision. All authors approved the final version of the manuscript and agree to be accountable for all aspects of the work.

Declaration of financial/other relationships

M.D., L.M., R.S., C.G., D.O., J.M.C.-L., and L.F.A. have disclosed that they have no significant relationships with or financial interests in any commercial companies related to this study or article.

CMRO peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Acknowledgments

We are most grateful to all the patients that agreed to participate in this study for their support, time and patience. We are also deeply grateful to all members of the Chronic Pain Clinic that worked with us all the way through this project – Unidade de Dor do Centro Hospitalar do Porto (coordinator: Dr. José Romão).