Post-mortem studies of people with Parkinson's also reveal that the same protein deposits which occur in the brain with this condition also accumulate in the skin.
To test if the same was true in life as after death, the researchers recruited 65 volunteers - 12 who were healthy controls and the remaining 53 who had either Parkinson's disease, Alzheimer's or another type of dementia.
Specifically, they looked for the presence of two proteins - tau and alpha-synuclein.
The 20 people with Alzheimer's and the 16 with Parkinson's had raised levels of both these proteins in their skin compared to the healthy controls and the patients with other types of dementia.
The people with Parkinson's also had higher levels of alpha-synuclein protein.
"This new test offers a potential biomarker that may allow doctors to identify and diagnose these diseases earlier on." It could also guide research into new treatments, he said.