Identifying the prevalence of Parkinson's disease in Denmark using healthcare registries and self-reported survey data

Introduction: Existing estimates of PD prevalence in Denmark are lower than those in the rest of Europe and are based on identification via single registries. Hence, are aim was to use a combined registry/self-report survey approach to identify people with PD and also investigate whether using different registry methods led to differences in the accuracy, completeness and characteristics of the identified cohorts. Methods: This study had a cross-sectional design using routinely collected health registry data to identify adults, >= 18 years of age and resident in Denmark, with PD from either the Danish National Patient (DNP) registry or Danish Prescription Medicines (DPM) registry. Those identified were asked to confirm their PD diagnosis using a national self -report survey. Results: 13,433 people were identified potentially as having PD via the DNP or DPM registry and sent a survey. Of these, 9094 responded (68 %) of which 85 % confirmed they had PD (n = 7763; 194/100,000; 95%CI:7650-7876). When adjusting for non -respondents, assuming an equal rate of confirmation in respondents and non -respondents, estimated Danish PD population was 11,467 (198.4/100,000; 95 % CI:197.2-199.6). Identification of people using those found in both registries led to 98 % confirming they had PD versus using one registry: DNP 93 % and DPM 88 %. No clear differences in sociodemographic characteristics were found between different registry identification methods. Conclusions: Estimated PD population in Denmark was significantly higher than previous Danish estimates and close to existing estimates in other European countries. The most accurate PD population was identified when including those found in both the DNP and DPM registries.