A simple scratch-and-sniff test could predict Parkinson’s disease even earlier than scientists previously thought possible.
The test could potentially identify certain people who are at an increased risk of developing the disease up to 10 years before they are actually diagnosed. Previous research has shown an association between sense of smell and disease progression of up to four to five years.
“One of the key differences in our study was we followed older white and black participants for an average of about 10 years, much longer than any other previous study,” says Honglei Chen, professor of epidemiology at Michigan State University. “We found that there was a strong link between smell and disease risk for up to six years. After that, the link remained, but just wasn’t as strong.”
The study—one of the first to follow black people—suggests the relationship between smell and Parkinson’s risk in black participants was not as strong as in white participants.
“Previous studies have shown that black people are more likely to have a poor sense of smell than whites and yet may be less likely to develop Parkinson’s disease,” Chen says.
“We found no statistical significance for a link between poor sense of smell and Parkinson’s disease in blacks, but that may have been due to the small sample size and more research is needed.”
Further, the study found that older men with a poor sense of smell are more likely to develop the disease than women.
For the study, published in Neurology, 1,510 white and 952 black participants with an average age of 75 were asked to smell 12 common odors including cinnamon, lemon, gasoline, soap, and onion, and then select the correct answer from four choices.
Based on their scores, participants were divided into three groups—poor sense of smell, medium, and good. Researchers then monitored their health through clinical visits and phone interviews for more than a decade.
Overall, 42 people developed Parkinson’s during the study including 30 white people and 12 black people.
People with a poor sense of smell were nearly five times more likely to develop the disease than people with a good sense of smell. Of the 764 people with a poor sense of smell, 26 people developed the disease, compared to just seven of the 835 people whose sense of smell was good and nine of the 863 people whose sense of smell was categorized as medium.
The results stayed the same after adjusting for other factors that could affect risk including smoking, coffee intake, and history of head injury.
“It’s important to note that not everyone with low scores on the smell test will develop Parkinson’s disease,” Chen says. “More research is needed before the smell test can be used as a screening tool for Parkinson’s, but we are definitely on to something and our goal now is to better characterize populations that are at higher risk for the disease and to identify other factors involved.”
The National Institute on Aging, National Institute of Nursing Research and National Institute of Environmental Health Sciences funded the study.