Wife smelled husband's Parkinson's disease 12 years before clinicians diagnosed it

As reported by the American Parkinson Disease Association:

Joy Milne was the care partner for her husband with PD. For many years, she noticed that he emitted a musky odor, but assumed that this scent was unique to him. In 2012 however, she smelled the same odor on a fellow support group member with PD, which led her to question whether this was a wider phenomenon. Her curiosity led her to a collaboration with Dr. Tilo Kunath at the University of Edinburgh in Scotland, who hypothesized that it was possible that PD produces a particular metabolite that gives off a specific odor. Dr. Kunath tested Milne and she was able to correctly identify with incredible accuracy whether a person had PD or not by smelling clothing that that person had worn. This early effort was chronicled in an article in 2016 in Lancet Neurology...

The source of the smell appeared to be the back of the neck, where there are many sebaceous glands that produce sebum, an oily, waxy substance produced by the skin. It is well known that people with PD have increased rates of seborrheic dermatitis which causes patches of scaly, red skin due to over-secretion of oils from the sebaceous glands. One hypothesis for why people with PD have seborrheic dermatitis at higher rates than the general population is that in PD there is dysfunction of the autonomic nervous system that controls the oil glands on the face...

A related news story is about the existence of programs which train dogs, well known to have much better senses of smell than humans, to smell PD. One such program, the first of its kind established in the US, is PADs (which stands for Parkinson’s Alert Dogs) for Parkinson’s and operates in the Pacific Northwest. This program was established directly as a result of Joy Milne’s story.  Accounts from PADs for Parkinson’s and Medical Detection Dogs certainly support the idea that dogs can be trained to identify an odor in people who have been diagnosed with PD. For both these programs, the ultimate objective is not for trained dogs to diagnose PD by smelling bio-samples, but rather to identify the chemicals that the dogs are detecting so that an early diagnostic test can be developed.

Note Joy Milne has hyperosmia - a markedly heightened sense of smell.  Most spouses (and most people) cannot detect an altered odor in Parkinson's patients.