Beyond performance: the role of illness perception and internal comparison standards in patient-reported physical function assessments

Abstract Background Previous research has yielded inconsistent findings regarding the relationship between patient-reported physical function (PF) and physical performance. Patient-reported PF has been discussed to be more strongly associated with psychological variables, which could explain discrepancies with performance assessments. The aim of this study was to investigate differential association patterns of illness perception and internal comparison standards with patient-reported versus performance-based PF. Methods A total of N = 106 patients from three medical specialties (Cardiology, n = 22; Rheumatology, n = 21; Psychosomatic medicine, n = 63) were included in this cross-sectional study. Patient-reported PF was assessed using a 20-item short form of the Patient-Reported Outcomes Measurement Information System (PROMIS), the PROMIS-PF SF-20a, while performance-based PF was measured using the Physical Performance Test (PPT), comprising 9 physical performance tasks. Illness perception was assessed using the Brief Illness Perception Questionnaire (BIPQ), while social and temporal comparisons were assessed using single-item scales. Multiple linear regression analyses were conducted to examine the relationships of illness perception and internal comparison standards with both patient-reported and performance-based PF. Results Physical performance was strongly associated with patient-reported PF (Pearson’s r = .74), accounting for more than 50% of the variance in the regression model with patient-reported PF as dependent variable. Illness perception and internal comparison standards also contributed significantly to the prediction of patient-reported PF beyond actual performance. Specifically, more negative illness perceptions, as well as poorer self-assessments compared to peers (social comparisons) and compared to one’s own best functioning (temporal comparisons), were associated with lower patient-reported PF. Illness perception was also significantly associated with performance-based PF, however, this association was very small. Social and temporal comparisons were not associated with performance-based PF. Consistent with this pattern, the additional explained variance by psychological variables was substantially larger for patient-reported PF than for performance-based PF. Conclusions While patient-reported functioning was highly associated with actual performance, illness perception as well as social and temporal comparisons appeared to play an additional role in the self-assessment of PF. In contrast, psychological variables showed only negligible effects when performance-based PF was used as the outcome. These findings highlight the importance of considering psychological variables when interpreting patient-reported PF. Future research is needed to explore the role of further psychological factors not considered in this study to further improve our understanding of self-reported PF.