BackgroundMitochondrial dysfunction, particularly complex I (CI) deficiency, is considered an integral feature of Parkinson's disease (PD). However, recent findings indicate that widespread neuronal CI deficiency in the brain is only present in a subpopulation of 20-30% of cases. This stratification may be relevant for selecting participants for clinical trials, emphasizing the need for clinically applicable biomarkers. We previously reported CI deficiency in skeletal muscle biopsies of a subpopulation of persons with PD (PwPs), suggesting potential for mitochondrial stratification using extra-neural tissues. Platelets are another tissue previously reported to exhibit mitochondrial respiratory defects in PD. However, studies have generally involved small sample sizes and reported variable results.ObjectiveTo determine whether platelets exhibit impaired mitochondrial respiratory chain complex activity in PwPs, or in a subpopulation of PwPs.MethodsUsing spectrophotometric activity assays, we assessed CI and complex IV (CIV) activities in platelet samples from 61 PwPs and 31 neurologically healthy controls from a well-characterized prospective cohort. The correlation between activities measured in platelets and skeletal muscle was also explored in 51 of the same individuals.ResultsPlatelet CI and CIV activities showed no difference between PwPs and controls at the group level, nor evidence of a subgroup with deficiency of either complex. There was no correlation between complex activities in platelet samples and skeletal muscle biopsies from the same individuals.ConclusionsBased on these results, we propose that platelet CI or CIV activities are not sensitive markers of mitochondrial dysfunction in PD.