Validity of the Brief Illness Perception Questionnaire in people with knee pain: a Rasch analysis

Abstract

Objective

The Brief Illness Perception Questionnaire (B-IPQ) is a frequently used measure of illness perception (IP). The aim of this study was to explore the psychometric properties of the questionnaire when used in elderly people with knee pain.

Method

Based on data from the Frederiksberg Cohort on elderly people reporting knee pain (N = 836), the psychometric properties of the eight B-IPQ items (1 ‘Consequences’, 2 ‘Timeline’, 3 ‘Personal control’, 4 ‘Treatment control’, 5 ‘Identity’, 6 ‘Concern’, 7 ‘Coherence’, and 8 ‘Emotions’) were analysed using Rasch analysis to establish whether the questionnaire provides reliable and valid measures of IP.

Results

Threshold disordering was found on the 1–10 rating scale in all items. When rescaling to a 0–2 rating scale, disordering was resolved in six items. Item goodness-of-fit analyses revealed that two items displayed underfit misfit and four items overfit misfit; hence, the B-IPQ does not present unidimensionality. The person separation index indicated that items separate respondents into only two IP levels. Finally, floor and ceiling effects were found, indicating that the B-IPQ may not fully describe the extent of IP in this population.

Conclusion

The 1–10 rating scale used in the Danish B-IPQ targeting knee pain is suboptimal, while a 0–2 scale improves the psychometric properties of the scale. The B-IPQ does not present unidimensionality and the use of a sum score is therefore not recommended when assessing IP. The B-IPQ may not cover the extent of IP in elderly people with knee pain.

Disclosure statement

No potential conflict of interest was reported by the authors.

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Elisabeth Ginnerup-Nielsen, Eva Ejlersen Wæhrens, Cecilie Bartholdy, and Elisabeth Bandak. The first draft of the manuscript was written by Elisabeth Ginnerup-Nielsen, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Ethics approval

The cohort ‘Frederiksbergundersøgelsen’ was reviewed by the local health research ethics committee, which deemed that the study exempt from approval (j.no. 17024697) as this study was only based on questionnaires. Such studies can be implemented without permission from the Health Research Ethics Committee according to Danish legislation (Committee Act §1, paragraph 1). Nevertheless, all procedures performed in this study were in accordance with the ethical standards of national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. All participants consented to participate by responding to the survey.

Data availability statement

The data generated and analysed during the current study are not publicly available owing to Danish legislation, but are available from the corresponding author upon reasonable request.

Additional information

Funding

This research received no specific funding, but the Cohort ‘Frederiksbergundersøgelsen’ was funded by Ellen Mørchs Foundation [grant number 324962019] and Minister Erna Hamiltons Legat for Videnskab og Kunst [grant number 013-2021]. The Parker Institute, Bispebjerg and Frederiksberg Hospital is supported by a core grant from the Oak Foundation [OCAY-18-774-OFIL].