Davis RL, et al. - Researchers conducted this systematic review to evaluate serum FGF-21 and GDF-15, together with mitochondrial DNA (mtDNA) copy number levels in peripheral blood cells from Parkinson disease (PD) patients (n = 121) and healthy controls (n = 103), as well as to determine if these measures could act as a biomarker of PD. When comparing patients with PD with healthy controls, no meaningful differences were identified for any of the measures. This illustrates a lack of sensitivity to the diagnosis that is inconsistent with the use of such measures as biomarkers for PD. In patients with PD and healthy controls, serum FGF-21, serum GDF-15, and blood mtDNA levels were comparable and thus unlikely to be satisfactory indicators of mitochondrial dysfunction in PD patients.