Diagnosing neurological illness is a lengthy, complex, and expensive process. But one surprising test might soon help to speed things up. A series of recent studies has found that checking patients' sense of smell could help identify Parkinson's and Alzheimer's diseases.

Doctors have known about the link between olfactory (smell-related) dysfunction and neurological diseases for a long time now. Their patients with Parkinson's and Alzheimer's disease often report losing some or all of their ability to smell.

Davangere Devanand is a neurology and psychiatry expert at Columbia University who has spent years investigating this poorly understood connection. In his latest paper on the subject, published last year in the American Journal of Geriatric Psychiatry, Devanand found ample evidence to support doctors' and patients' stories. In many adults, he wrote, anosmia could be seen as a reliable predictor of Alzheimer's disease.

"It's important, not just because it's novel and interesting and simple but because the evidence is strong," Devanand told Scientific American. "In the past, most neurologists thought, 'Maybe there's something there statistically in a paper, but it's a bit flaky.'"

A paper published this month in the journal Lancet Neurology came to a similar conclusion, proposing a single, as-yet-undetermined root cause of anosmia in both illnesses.

Paper author Richard Doty is also the creator of the University of Pennsylvania Smell Identification Test (UPSIT), which asks patients to scratch and sniff 40 different odors. The results are instantaneous, and at $26.95, it's a far cheaper starting point than brain scans.

Neurologist G. Webster Ross of the Veterans Affairs Pacific Islands Health Care System says the test can be a strong negative predictor of neurological issues as well. "If a person scores very well on a smell identification test, then you can be pretty sure they're not going to have Parkinson's, at least within the next four years," he told Scientific American.

It's important to keep in mind that neurological disease is far from the only condition associated with anosmia. Our sense of smell naturally begins to grow duller as we age, and the most common cause of temporary or permanent anosmia is none other than the common cold. So if you can't smell your favorite perfume today, don't panic just yet.

[h/t Scientific American]