The use of olfactory testing when diagnosing Parkinson's disease - a systematic review.

Introduction: The diagnosis of Parkinson's disease (PD) is typically based on the presence of motor symptoms, but in the early phase of the disease, the diagnostic process can be challenging. Examination of non-motor symptoms in patients suspected of PD has gained growing attention. Olfactory tests have shown promising results as ancillary diagnostic tests. The aim of this study was to investigate how olfactory tests may be used clinically in diagnostic process in PD.

Methods: A systematic search was conducted in PubMed for relevant literature on 8 March 2017. A total of 358 articles were found. Our screening process left 27 articles, which were included for further analysis.

Results: In all, 20 of the included studies analysed the diagnostic value of olfactory testing by comparing patients with PD to healthy controls. Sensitivities varied from 61% to 95% and specificities from 66% to 99%. Ten studies used olfactory tests to distinguish between PD and diseases that mimic PD. The sensitivities varied from 62% to 92% and the specificities from 65% and 96%.

Conclusions: Olfactory test can be a valuable ancillary tool in the diagnostic process in PD. In a clinical setting, the identification part from Sniffin' Sticks 16 is the most usable because it may be conducted quickly and independently of disease duration and severity. Before using an olfactory test in a clinical setting, it is necessary to adjust the odours to the patient population, and to establish the optimal specificity-adjusted cut-off.