In this study, a number of candidate ICF categories for a Generic ICF Core Set were proposed based on the examination of their explanatory power in relation to general health as defined by question one of the SF-36 using regression modeling. These categories were: b130 energy and drive functions, b152 emotional functions, b230 vestibular functions, b280 sensation of pain, b730 muscle power functions, d450 walking, d620 acquisition of goods and services, d640 doing housework, and d660 assisting others, d850 remunerative employment, d920 recreation and leisure, e450 individual attitudes of health professionals, and e580 health services, systems and policies (see also table ).
Overlap between 13 candidate ICF categories for Generic ICF Core Set and the WHO DAS II 12 item version
With the exception of d235 vestibular functions
, d620 acquisition of goods and services
, d660 assisting others
, and the two categories in the component environmental factors
, the suggested ICF categories were addressed in at least 4 of the 6 most widely used generic health-status measures [29
With the exception of d235 vestibular functions
, which was listed in none, and d660 assisting others
, which was listed in only 6 of the 12 condition-specific Comprehensive ICF Core Sets that have been developed so far, all the other candidate ICF categories were listed in at least 10 condition-specific Comprehensive ICF Core Sets [15
]. Since the Comprehensive ICF Core Sets were developed to guide multidisciplinary assessments [30
] in clinical settings like rehabilitation, they are relatively large. Thus, Brief ICF Core Sets representing a selection of ICF categories of the respective Comprehensive ICF Core Sets and addressing only those aspects of functioning that are essential and can be recorded in clinical studies were also developed. Within this context it is important to mention that b130 energy and drive functions, b152 emotional functions
, b280 sensation of pain
, b730 muscle power functions, d450 walking
, d640 doing housework
, and e580 health services, systems and policies
are included in at least 6 of the 12 Brief ICF Core Sets developed up to now.
In addition, the 13 candidate ICF categories have high face validity, if one considers that a Generic ICF Core Set
will have to include ICF categories which represent problems affecting most patients irrespective of their specific health conditions. As all others ICF Core Sets developed so far [30
], a Generic ICF Core Set
should include as few categories as possible to be practical, but as many as necessary to be sufficiently comprehensive to describe the patients' typical spectrum of problems in functioning across conditions.
The 13 candidate ICF categories already show a considerable overlap with the 12-Item version of the World Health Organization Disability Assessment Schedule II (WHODASII) (see Table ). [31
], which is an instrument with from the same conceptual basis as the ICF. Therefore, it is expected that the results from this and future work on the generic ICF Core Set will contribute to the further development of the WHODAS II, as well as to the development of ICF-based assessment instruments.
Instead of regression modelling, other methods, such as simply rank the strength of the association after using parametric or non-parametric correlation statistics (Sperman or Person Coefficients) could be considered when selecting ICF categories in relation to a broad concept, like health. However, regression modelling enables a better understanding of the relative contribution of individual ICF categories in relation to others [33
In addition, a transparent selection process which acknowledges that within a set of highly-related variables not only the finally-selected variable, but also other variables would have similarly contributed to a model, is necessary to thoroughly study the relationship among possible candidate ICF categories. We developed a selection strategy based on the ICF structure and examined a number of alternative models, considering best possible alternatives from our selection strategy. This process does not lead to a final answer about which ICF category to consider or not, but at least leads to a selection of candidates which needs further consideration and comparison with the results of other selection strategies in developing a Generic ICF Core Set.
The importance of not relying on a purely statistical selection was demonstrated by the example b152 emotional functions. If one had simply followed the statistical modelling, b152 emotional functions would not have been selected in a final model and would not have been considered as a candidate ICF category for the generic ICF Core Set. This would be counterintuitive for most health professionals, since emotional functions in the personal experience of health professionals and patients are relevant domains that always must be taken into account to describe functioning, disability and health. According to Cieza and Stucki (2004; 29) all 6 most widely used generic health-status measures address emotional functions.
As can be seen in Fig. , b130 energy and drive functions are highly related to b152 emotional functions and vice versa. In a model including b152 emotional functions instead of b130 energy and drive functions, the total variance explained is similar, and all the variables entered are the same. A similar example can be seen with d450 walking which, when tracked back in the selection process, was highly associated with d640 doing housework. A model including d450 walking (model V) instead of d640 doing housework (model I) provides virtually a similar answer with regard to the variance explained and the other variables included. D620 acquisition of goods and services also appears to be a suitable substitute for d640 doing housework. However, category b130 energy and drive functions lose some importance when d620 is in the model.
It is also interesting to note that d920 recreation and leisure significantly explains some of the variance in general health when neither d640 doing housework nor d450 walking is in the same model. Taking into account that all the different models explain a similar amount of variance in the end, this indicates that all these variables explain the same variance in general health as measured by item one in the SF-36. The same can be said of d850 remunerative employment. This variable remains in the model when neither d640 doing housework nor d450 walking is in it.
Pain is the most relevant independent variable in all the models. Its importance remains the same, regardless of which variables are additionally included in the model. This result is not surprising, since pain is a leading symptom and one of the key outcomes in many different chronic conditions [34
do not play a significant role in any of the models when explaining general health. This is probably because the organ systems involved in the health conditions affecting the patients in this study are very diverse. The fact that environmental factors are present in only two models is, however, remarkable. Environmental factors are defined as the physical, social and attitudinal environment in which people live and conduct their lives [11
]. They represent factors that have an important influence on patients' health, and one would have expected more of them in the different models. This result should, therefore, be studied in similar studies and reflects the importance of an iterative procedure and the involvement of different criteria and methodological approaches for the development of a final Generic ICF Core Set
Concerning the control variables, only the number of comorbidities has a significant influence on item 1 of the SF-36 in these analyses. This result is consistent through all the models. Individuals with fewer diseases feel healthier.
This study also presents several limitations that require special annotation.
It has to be recognized that the significance threshold of <0.01 that was needed to deal with the vast number of variables and that was set in the second step of the selection process is low. This might lead to the exclusion of potentially valuable ICF categories from the outset. This limitation is also in line with other limitations of the study, i.e., that our data were based in the ICF categories included in the ICF Checklist. As we now know from the development of the Comprehensive ICF Core Sets, a number of other categories not included in the ICF checklist, such as b455 Exercise tolerance functions, are relevant for 10 of the 12 health conditions considered. Regarding the Brief ICF Core Sets, the categories b455 Exercise tolerance functions, d240 Handling stress and other psychological demands and d230 Carrying out daily routine are included in seven, seven and five of the Brief ICF Core Sets, respectively, but are not included in the ICF checklist. Whether such categories will be included in the definitive Generic ICF Core Set will be investigated in the future.
A major limitation of our study is the relatively low variance explained by the ICF categories. There are a number of explanations to be considered which are relevant in the process of developing a Generic ICF Core Set. First of all, our selection was based only on the categories included in the ICF Checklist. Second, personal factors are still lacking within the scope of the ICF. However, personal factors refer to variables, such as fitness, lifestyle, social background and coping styles that definitely influence and determine general health. It has to be taken into account that, in this analysis, only the variables sex, age and number of concomitant diseases were included. Future studies should also include further personal factors as relevant independent variables. Third, no interaction terms were included in the models since the purpose of the study was simply to propose a method to select ICF categories and to identify candidate ICF categories for a Generic ICF Core Set. However, it is possible that interaction terms including, for example, b130 energy and drive functions and b280 sensation of pain, contribute to general health as measured by the item 1 of the SF-36. The interaction terms should be included in future studies. Four, and probably most important, in our patient sample there was not much variance in most of the categories, i.e., in many categories most people had no limitations. According to the guidelines that establish what kind of patients have access to rehabilitation after discharge from an acute hospital in Germany, all the patients have to be able to eat and wash themselves without external support, as well as be able to move independently on the ward. Therefore, one can assume that the limitations in functioning suffered by the patients included in this study do not represent the whole spectrum of severity of limitation in functioning of patients suffering from chronic conditions.
This limitation points out an additional limitation of the study – the generalization of the results. Only patients treated in rehabilitation centers in Germany were included in this study. Thus, patients treated in acute hospitals, day clinics and outpatients and inpatients in countries other than Germany are not represented. This again emphasizes the importance of performing similar analyses with patients in different countries who are being treated in different settings. This will be possible with the data collected in the international validation study of the ICF Core Sets involving over 50 countries and 270 study centres that is being performing until the end of 2006.
It has also to be taken into account that no further operationalisation of the qualifier scale besides the broad ranges of percentages provided by WHO were used in this study. The ICF checklist in its current form contains a more detail description of the qualifiers. For example, in the component body functions 1 (mild impairment) is defined as "a problem that is present less than 25% of the time, with an intensity a person can tolerate and which happens rarely over the last 30 days". The descriptions of the ICF qualifiers were not available at the time when the data collection of the study presented here was planed and carried out. Future studies should include the actual descriptions of the qualifier scale, since they may improve the reliability and validity of the data. Within this context it is important to mention that reliability studies are being currently performed at the ICF Research Branch at the Ludwig-Maximilian University to study the psychometric properties of the qualifier scale.
The SF-36 summary scores of the persons participating in this study averaged approximately 13 and 7 points less than the German normal population on physical and mental health, respectively. There are no substantial differences when compared to the German reference population with chronic conditions [44
The fact that patients reported greater limitations in the PHI score than in the MHI score of the SF-36 is not surprising, as only 65 of the 1039 patients in our sample had a health condition that is traditionally considered as a mental-health related condition, namely depression. It is also important to mention that the German reference population with chronic conditions does not include mental-health related conditions.
In our study, we controlled for the presence/absence of each single health condition, including depression, in all performed regression models. However, the disproportionate higher number of patients with physical-health conditions makes it difficult to generalize our results to patients with mental-health conditions.
It is still an open question whether to consider the ICF qualifiers "8 – not specified" and "9 – not applicable" missing values represents the best strategy to cope with these response alternatives. The qualifier "8 – not specified" is used when the available information does not suffice to quantify the severity of the problem and "9 – not applicable" when a determine category is not applicable to a patient. These qualifiers are very useful from a clinical point of view but they represent a 'barrier' which is difficult to overcome with parametric statistical methods from a research or statistical point of view. Therefore, alternatives to how the information currently recorded with the qualifiers 8 and 9 could be recorded without using response alternatives that are part of the qualifiers' scale that ranges from 0 to 4 should be considered.
The question whether item one of the SF-36 is the appropriate external standard for the study presented here has to be posed within this context. As mentioned in the introduction, this item has the advantage of being straight, simple and intuitive. However, it also presents different challenges because it is subjective and very broad. One could even argue that, since the concept of health is subjective and represents something different for each different person, one does not know at the end what is assessed with an item like item one of the SF-36. Thus, one does not know at the end what is explained when a regression analysis is performed using such an item. Similar analyses have to be performed with alternative Items and/or indices in future studies.