A biomarker can be defined as “a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic process or pharmacological response to a therapeutic intervention.”1 Parkinson disease (PD) is in desperate need for accurate biomarkers: patients with PD typically become symptomatic when more than 50% of dopaminergic neurons are lost, suggesting a prolonged presymptomatic period. Early in the disease course, it can be difficult to differentiate PD from other conditions such as essential tremor (ET), multisystem atrophy, and progressive supranuclear palsy. Even with the correct diagnosis, PD has wide variability in disease course, rate of progression, and response to treatment, making an objective marker extremely useful.
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McKeown et al. (2015) studied this question.
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