In the past few months, several studies have come out showing the value of three different tests of people's sense of smell for improving the accuracy of MCI and Alzheimer's diagnosis, or pointing to increased risk. The studies also add to growing evidence that a decline in sense of smell is an early marker for mild cognitive impairment and Alzheimer’s. Indeed, it appears that this sensory loss is a very early symptom, preceding even the shrinking of the entorhinal cortex (the first brain region to show signs of atrophy).
Smell test improves accuracy of MCI & Alzheimer's diagnosis
A simple, commercially available test known as the Sniffin' Sticks Odor Identification Test, in which subjects must try to identify 16 different odors, was given to 728 older adults, as well as a standard cognitive test (the Montreal Cognitive Assessment).
The participants had already been evaluated by doctors and classified as being healthy (292 subjects), having MCI (174: 150 aMCI, 24 naMCI), or having Alzheimer's (262).
It was found that, while the cognitive test alone correctly classified 75% of people with MCI, the number rose to 87% when the sniff test results were added. Diagnosis of Alzheimer's, and of subtypes within MCI, was also improved.
The smell test normally takes 5 to 8 minutes to administer; the researchers are trying to get it down to 3 minutes, to encourage greater use.
A new smell test
Another recent study validates a new smell test which is rather more complicated. The test was developed because the standard University of Pennsylvania Smell Identification Test doesn’t take into account the great variation in olfactory ability among healthy individuals. The ability of normal individuals to recognize and discriminate between odors can vary by as much as 40 times!
The new test is actually four tests:
- In the OPID (Odor Percept IDentification)-10 test, participants are presented with 10 odors (menthol, clove, leather, strawberry, lilac, pineapple, smoke, soap, grape, lemon) for two seconds each. They are then asked whether the scent is familiar and given a choice of four of the 10 words from which are asked to pick the best one that describes the odor.
- The Odor Awareness Scale (OAS) assesses their overall attention to environmental odors and how they are affected emotionally and behaviorally by scents.
- The OPID-20 test includes an additional 10 odors (banana, garlic, cherry, baby powder, grass, fruit punch, peach, chocolate, dirt, orange). Participants are first asked whether a presented odor was included in the OPID-10 test and then asked which word best describes the odor. Their ability to remember odors from the first test determines their POEM (Percepts of Odor Episodic Memory) score.
- In the Odor Discrimination (OD) test, participants are presented with two consecutive odors and asked whether they were different or the same, a process that is repeated 12 times with different paired scents.
The study involved 183 older adults, of whom 70 were cognitively normal, 74 tested normal but were concerned about their cognitive abilities, 29 had MCI and 10 had been diagnosed with possible or probable Alzheimer's disease.
Results of the OPID-20 test significantly differentiated among the four groups of participants, and those results correlated with the thinning of the hippocampus and the entorhinal cortex. Participants' ability to remember a previously presented aroma, as reflected in the POEM score, was also significant, with participants with Alzheimer's disease performing at no better than chance.
POEM scores of the two cognitively normal groups were compared with what would have been predicted based on their ability to identify and differentiate between odors, as reflected in the OAS and OD tests. Poor POEM performers were more likely to have the ‘Alzheimer's gene’ (APOEe4), showed thinning of the entorhinal cortex, and poorer cognitive performance over time.
Validation of UPSIT
However, two 2016 studies support the use of the University of Pennsylvania Smell Identification Test (UPSIT), and suggest it may offer a practical, low-cost alternative to other tests.
In one study, UPSIT was administered to 397 older adults (average age 80) without dementia, who were also given an MRI scan to measure the thickness of the entorhinal cortex (the first brain region to be affected by Alzheimer's disease). After four years, 50 participants (12.6%) had developed dementia, and nearly 20% had signs of cognitive decline.
Low UPSIT scores, but not entorhinal cortical thickness, were significantly associated with dementia and Alzheimer's disease, and with cognitive impairment. Entorhinal cortical thickness was significantly associated with UPSIT score in those who transitioned from MCI to dementia.
In other words, it looks like impairment in odor identification precedes thinning in the entorhinal cortex.
In another study, UPSITwas administered to 84 older adults, of whom 58 had MCI, as well as either beta amyloid PET scanning or analysis of cerebrospinal fluid. After six months, 67% had signs of memory decline, and this was predicted by amyloid-beta levels (assessed by either method), but not UPSIT score. However, participants with a score of less than 35 were more than three times as likely to have memory decline as those with higher UPSIT scores.
The researchers suggest the association wasn’t as strong in this study because of the younger age of participants (median age 71), their higher education, and the short follow-up.
 . Odor Identification Screening Improves Diagnostic Classification in Incipient Alzheimer’s Disease. Journal of Alzheimer's Disease [Internet]. 2017 ;55(4):1497 - 1507. Available from: http://content.iospress.com/articles/journal-of-alzheimers-disease/jad160842
 Episodic memory of odors stratifies Alzheimer biomarkers in normal elderly. Annals of Neurology [Internet]. 2016 ;80(6):846 - 857. Available from: http://onlinelibrary.wiley.com/doi/10.1002/ana.24792/abstract
Lee, Seonjoo et al. 2016. Predictive Utility of Entorhinal Cortex Thinning and Odor Identification Test for Transition to Dementia and Cognitive Decline in an Urban Community Population. Presented at the Alzheimer's Association's International Conference in Toronto.
Kreisl, William et al. 2016. Both Odor Identification and Amyloid Status Predict Memory Decline in Older Adults. Presented at the Alzheimer's Association's International Conference in Toronto.