The results of the present study further showed that rater agreement was independent of the raters’ areas of core competence as physical therapists. The raters achieved a high level of agreement in several categories that were not among the physical therapists’ areas of core competence (eg, d330: speaking; d710: basic interpersonal interactions; d850: remunerative employment; and d845: acquiring, keeping, and terminating a job). ICF categories defined as physical therapists’ areas of core competence are aspects of functioning that are treated by physical therapists. However, physical therapists are trained to have comprehensive knowledge of stroke and are experienced in observing and detecting the full scope of patients’ problems, for example, when taking their history. This means that physical therapists are well able to identify patients’ problems, for example, in the category “d330: speaking,” even though these problems are typically treated by speech and language therapists. That is, although the ICF categories that cover the core competencies describe the areas in which physical therapists are usually trained, their experience, skills, and knowledge as health care professionals working in an interdisciplinary team surpass the specified focus of these ICF categories.
At this point, the potential of the ICF Core Set as a basis for multidisciplinary communication and cooperation in rehabilitation practice and management arises. Still, the question of agreement and interrater reliability among health care professionals of different specializations remains open. Future studies involving various health care professions should be conducted to clarify this question.
However, the results also revealed several categories that addressed physical therapists’ areas of core competence but that were rated differently by the 2 physical therapists (eg, b176: mental functions of sequencing complex movements; b755: involuntary movement reaction functions; and b260: proprioceptive functions). These results indicated that for the participants in the present study, the information available on problems was not based on measurements and in-depth examinations but relied on global impressions from the interviews.
The present study has several limitations. Because of the monocentric design, the small sample size, and the small number of participating raters, the generalizability of the results is limited, and the results need to be interpreted with caution. The results suggest next steps for future investigations. In addition, the present study addressed interrater reliability in terms of agreement between 2 raters. It did not address the quality or “truth” of the ratings. Currently, no gold standard exists against which a description of patients’ problems across the categories of the ICF Core Set can be compared. The present study suggests potential improvements to enhance the implementation of the Extended ICF Core Set for Stroke in practice. To enhance interrater reliability, the training of health care professionals with regard to the ICF should be further developed and standardized. Implementing the ICF in rehabilitation practice at an institutional level may enhance the availability and accessibility of information about all aspects of functioning in individual patients, which in turn may enhance the reliability of ICF-based ratings.8 In addition, the metric characteristics of the ICF qualifier scale should be taken into account. In particular, the distance between the steps of the scale may be too narrow and therefore may lead to disagreements. Thus, examining and restructuring the rating scale, for example, with Rasch analyses, may also enhance its interrater reliability.43 However, the results of the present study mainly hint at the importance of operationalization of the categories and standardization of the rating process to control for rater effects and to increase reliability.
In the future, 2 paths toward the operationalization of ICF categories can be followed, namely, the development of ICF-based measures and the development of detailed ICF manuals. Efforts in the latter direction are already being made, for example, by the American Psychological Association51 and the Australian Institute of Health and Welfare.52 Operationalizing the categories of the Extended ICF Core Set for Stroke could be an important next step to ease and to facilitate the application of the ICF in clinical practice and to use its full potential. Physical therapists can make valuable contributions to these developments to enhance professional, scientifically founded, multidisciplinary practice for the benefit of patients.