A significantly lower level of Cp in the serum was observed in PD patients using I-FP-CIT-SPECT in 2017277; however, no significant change of Cp in the serum was reported in PD patients in 2018.278 Interestingly, a study conducted in 2020 found higher serum Cp levels specifically among females with PD, which were associated with increased impulsivity.279 Therefore, it may be necessary to consider using more sensitive detection methods and increasing the number of samples from PD patients to confirm changes in Cp levels. Here, CP is linked to Parkinson disease.