78 Chapter 4 work capacity, including inability to work fulltime, is so unambiguous that further exploration of the medical situation is felt unnecessary. In these cases no additional diagnosis are registered, and therefore not in our dataset, because these have no additional value to the outcome of the work disability assessment. Further research on the impact of multimorbidity, including the effect of the number of diagnosis and specific combinations of diagnoses, on inability to work fulltime is therefore recommended. Our study was a first step towards exploring inability to work fulltime as an outcome of work disability assessment, using register data from work disability assessments according to the UWV. Due to the administrative data source, data was available on diagnosis and certain personal factors. Future studies on inability to work fulltime could be enriched with data from for example assessment reports and questionnaires on therapy, the course of the disease, severity of the disease, and on work and environmental factors to obtain more insight in the position of inability to work fulltime within the biopsychosocial model. Strengths and Limitations In this study, register data of a year cohort of applicants assessed for a long term work disability benefit, covering the entire Dutch population, was used. A strength of this study is the large sample including all assessments, data describing socio-demographics and all diagnoses in a representative sample. Additionally, all comprehensive assessments were carried out by skilled professionals adhering to professional guidelines and assessment methods. A study limitation is that register data was not collected for research purposes and did not contain data on possible determinants such as severity of diseases and treatment, or work and environmental factors. Although the Dutch Social Security System is using a biopsychosocial approach in the work disability assessment, important factors described in this model are lacking in the register data. Unfortunately, we had to exclude 3002 cases because of missing data (only) on educational level, this might have impacted our outcomes, as they had significantly more often a normal ability to work compared to the included sample. Additionally, the cross-sectional design prevents us from drawing conclusions about causal relationships. CONCLUSION The prevalence of inability to work fulltime in work disability benefits assessment is high: 39.4%. Of these applicants with inability to work fulltime, the majority is assessed as not being able to work over 4 h per day. In the total sample, age, gender, education, multimorbidity and specific disease groups were associated with inability to work fulltime. The risk of inability to work fulltime varies between disease groups, with diseases of the blood,
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