Hustad E, et al. - Researchers analyzed how accurate the clinical diagnosis of Parkinson disease (PD) is for enrollees of The Nord-Trøndelag Health Study (HUNT), a health survey containing data from about 126,000 people and biological material from 80,000 people. Nine hundred eighty participants from the HUNT study diagnosed with PD or secondary parkinsonism/related parkinsonian disorders were included. Results revealed that PD was verified in 65% (358/550) and excluded in 35% (192/550) of the participants. The overall quality of the clinical diagnosis of PD for participants in the HUNT study was not ideal. Findings suggested that the ICD codes entered into health registers must be quality-assured before biological material obtained from these populations can be used in the search for new biomarkers for PD. Incorrect diagnosis of cases and controls might give biased effect size estimates and misleading association outcomes.