Washington: The inability to identify scents is a strong predictor of death within five years in older adults, a new study led by an Indian-origin researcher has found.
Thirty-nine per cent of study subjects who failed a simple smelling test died during that period, compared to 19 per cent of those with moderate smell loss and just 10 per cent of those with a healthy sense of smell, researchers said.
Olfactory dysfunction was better at predicting mortality than a diagnosis of heart failure, cancer or lung disease. Only severe liver damage was a more powerful predictor of death.
For those already at high risk, lacking a sense of smell more than doubled the probability of death.
"We think loss of the sense of smell is like the canary in the coal mine," said the study's lead author Jayant M Pinto, an associate professor of surgery at the University of Chicago.
"It doesn't directly cause death, but it's a harbinger, an early warning that something has gone badly wrong, that damage has been done. Our findings could provide a useful clinical test, a quick and inexpensive way to identify patients most at risk," Pinto said.
The research was part of the National Social Life, Health and Aging Project (NSHAP) in the US, a study of social relationships and health in a large, nationally representative sample of men and women ages 57 to 85.
In the first wave of NSHAP, conducted in 2005-06, professional survey teams from the National Opinion Research Center at the University of Chicago used a well-validated test - adapted by Martha K McClintock, the study's senior author - for the field survey of 3,005 participants.
It measured their ability to identify five distinct common odours. The modified smell tests used "Sniffin'Sticks," odour-dispensing devices that resemble a felt-tip pen but are loaded with aromas rather than ink.
Subjects had to identify each smell, one at a time, from a set of four choices. The five odours, in order of increasing difficulty, were peppermint, fish, orange, rose and leather.
Almost 78 per cent of those tested were classified as "normosmic," having normal smelling; 45.5 per cent correctly identified five out of five odours and 29 per cent identified four out of five.
Almost 20 per cent were considered "hyposmic." They got two or three out of five correct. The remaining 3.5 per cent were labelled "anosmic." They could identify just one of the five scents (2.4 per cent), or none (1.1 per cent).
Performance on the scent test declined steadily with age; 64 per cent of 57-year-olds correctly identified all five smells. That fell to 25 per cent of 85-year-olds.
In the second wave, during 2010-11, researchers found that those with greater smell loss when first tested were substantially more likely to have died five years later. Even mild smell loss was associated with greater risk.
The study was published in the journal PLOS ONE.