Despite various possibilities to assess odor memory, currently none of the available tests can be performed by individuals themselves in an unassisted manner. Hence, this study aimed to develop an objective, self-administered assessment based on an odor memory task. To make it easy to use and comprehend, we conducted a memory match game, with 12 pairs needed to be matched from 24 odors.

A relatively stable test performance was observed after 14 days with a significant correlation coefficient of 0.45. However, the retest reliability is not as ideal as expected, which may result from the limited number of items (12 pairs of odorants). It has been reported that the coefficients of correlations between test and retest scores decreased from 0.93 over 0.73 to 0.60 when reducing the number of olfactory items from 32 over 16 to 12 (e.g., [16, 17]). In addition, it has to be kept in mind that the subgroup invited for the test-retest analysis were relatively homogeneous in terms of their olfactory function. If this variance between tested individuals had been larger it is conceivable that the test-retest reliability would have increased. The self-ratings of NOST and SST showed that, compared with the SST, participants could properly finish the NOST, and were satisfied with their own test performance.

The present NOST showed a good validity relating to the SST, a standardized clinical test. It is well-established that olfactory impairments in neurodegenerative diseases can be detected through olfactory identification, odor sensitivity and other dimensions of olfactory function [18]. Cognitive deficits in memory loss, dementia, are important early signs of prodromal neurodegenerative disease [19]. Therefore, an olfactory memory test, a combined index of olfactory and cognitive function, could be a promising method for the diagnosis for neurodegenerative disease [20].

As a home test, the NOST presents some advantages that are not satisfied by traditional olfactory tests. First, it can be conducted by individuals themselves so that patients can establish their olfactory disorders quantitatively without the immediate need to present themselves repetitively in specialized centers – these may be of specific significance in rural areas with some distance to larger cities, or also in clinics with no established smell and taste dysfunction department. Having said that, however, the test is not meant to replace professional diagnosis and counseling.

Home tests are often available for online purchase or pharmacies, making them accessible to a wide range of people. It removes the necessity of making appointments and waiting in clinics, thereby saving patients’ time. Besides, it seems to be useful for the rehabilitation of patients with olfactory dysfunction. Due to the widespread olfactory dysfunction caused by COVID-19, the NOST can serve as a quick, convenient home test that allows the patient to track olfactory function (which is different form olfactory screening tests based on odor identification where the correct odors are quickly learned and memorized which is a strong bias for consecutive tests). Furthermore, the frequent odor exposure that the NOST may provide, appears to make it a perfect companion for olfactory training [21]. With the present test, individuals could be able to take control of their health of olfactory function by providing them with tools to monitor and manage their well-being independently. This could be another promising direction for the future research and application.

What is more, the NOST revealed a poorer performance of olfactory function in severely hyposmic patients compared to the normosmic group, further suggesting its effect to differentiate between severe hyposmia and normosmia. Hyposmia can be predicted by the present test when using a score of 5.5 with a sensitivity of 76.2% and specificity of 77.6%.

In accordance with the previous results, the present results suggest that older people have worse performance in NOST, suggesting a significant age-related olfactory and cognitive decline [22, 23]. As for the gender effect of olfactory functions, many studies reported that women outperform men [11, 12, 14], while some found no significant differences [24]. These somewhat controversial results can be explained by the weak effect size for the factor “gender” ranging from 0.08 to 0.30 [25]. In fact, for the current study, no gender effect was found.

Despite of these promising results, several limitations remain. First, the current test was performed with small glass containers, which are very practical but bulky. Because of that, the test may not be very convenient to use when space is limited. The NOST could be improved for self-administration if it was presented in a more portable and convenient form, like smaller bottles or other delivery devices. Furthermore, the NOST should also be applied in anosmic patients in future studies to evaluate whether it can distinguish anosmia from hyposmia/normosmia. In addition, as an auxiliary diagnostic tool, the selection of norms and cutoff values for the NOST is crucial, which is expecting a more accurate result by the future studies with larger sample sizes.

In conclusion, the present study showed the good reliability, validity and possible clinical usage of the NOST. Compared with existing tools, it can be comprehended and conducted easily, and without any help from others, which may provide a quick and simple approach to get a global estimation of one’s olfactory and cognitive condition. Among others it may help not only to facilitate the early diagnosis of neurodegenerative diseases, but also to recognize olfactory dysfunction as well as recovery from olfactory loss.