Validation of hospital register-based diagnosis of Parkinson's disease.
INTRODUCTION
Denmark has a long-standing tradition of maintaining one of the world's largest health science specialized register data bases as the National Hospital Register (NHR). To estimate the prevalence and incidence of diseases, the correctness of the diagnoses recorded is critical. Parkinson's disease (PD) is a neurodegenerative disorder and only 75-80% of patients with parkinsonism will have idiopathic PD (iPD). It is necessary to follow patients in order to determine if some of them will develop other neurodegenerative diseases and a one-time-only diagnostic code for iPD reported in the register may be incorrect.


MATERIAL AND METHODS
This was a large nationwide population-based study of risk factors for iPD, called Parkinson's disease in Denmark (PASIDA). We evaluated the iPD diagnosis reported in the NHR. Medical records with primary diagnoses of iPD from six neurological departments were collected and abstracted using a standardized system to review the diagnostic accuracy of the ICD codes.


RESULTS
Among the 1,040 medical records abstracted, 857 (82.4%) patients met our criteria for iPD. 183(17.6%) of the patients suffered from other diagnoses such as atypical PD (66 patients), secondary PD (60 patients) and other diagnoses (46 patients).


CONCLUSION
Possibly only about 82% of the patients with the primary diagnosis of iPD in the Danish NHR actually suffered from iPD. To improve diagnostic validity, we appeal to update the ICD code and to identify the correct parkinsonian phenotype to reduce biased case sampling in register-based studies and appropriate treatment for these rare diseases.


FUNDING
This study was supported by a grant from the National Institutes of Environmental Health Sciences, USA (grant No R01 ES013717). The funding source had no role in the design or analysis of the study or in the decision to submit the manuscript for publication.


TRIAL REGISTRATION
Ethical approval: The study protocol was approved by the Danish Data Protection Agency (No 2006-41-7323) and by UCLA-IRB.