Optimizing odor identification testing as quick and accurate diagnostic tool for Parkinson's disease

Abstract

Introduction: The aim of this study was to evaluate odor identification testing as a quick, cheap, and reliable tool to identify PD.

Methods: Odor identification with the 16-item Sniffin' Sticks test (SS-16) was assessed in a total of 646 PD patients and 606 controls from three European centers (A, B, and C), as well as 75 patients with atypical parkinsonism or essential tremor and in a prospective cohort of 24 patients with idiopathic rapid eye movement sleep behavior disorder (center A). Reduced odor sets most discriminative for PD were determined in a discovery cohort derived from a random split of PD patients and controls from center A using L1-regularized logistic regression. Diagnostic accuracy was assessed in the rest of the patients/controls as validation cohorts.

Results: Olfactory performance was lower in PD patients compared with controls and non-PD patients in all cohorts (each P < 0.001). Both the full SS-16 and a subscore of the top eight discriminating odors (SS-8) were associated with an excellent discrimination of PD from controls (areas under the curve ≥0.90; sensitivities ≥83.3%; specificities ≥82.0%) and from non-PD patients (areas under the curve ≥0.91; sensitivities ≥84.1%; specificities ≥84.0%) in all cohorts. This remained unchanged when patients with >3 years of disease duration were excluded from analysis. All 8 incident PD cases among patients with idiopathic rapid eye movement sleep behavior disorder were predicted with the SS-16 and the SS-8 (sensitivity, 100%; positive predictive value, 61.5%).

Conclusions: Odor identification testing provides excellent diagnostic accuracy in the distinction of PD patients from controls and diagnostic mimics. A reduced set of eight odors could be used as a quick tool in the workup of patients presenting with parkinsonism and for PD risk indication. © 2016 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Keywords: Parkinson's disease; diagnosis; olfactory dysfunction; parkinsonism; tremor.

Figure 1

Identification of individual odors in the study groups in the three different centers (A). Gray horizontal line indicates the probability of correctly guessing an odor in the employed forced choice test. AUCs with 95% CIs as a function of combination of odors best predicting PD according to the LASSO analysis in the various cohorts (B). The three horizontal lines in each graph represent the AUCs with 95% CIs of the full test used (SS‐16 for the upper row and SS‐12 for the lower row). The best eight discriminating odors derived from the full SS‐16 were used for the SS‐8 subscore (licorice, anise, mint, cinnamon, banana, pineapple, rose, and coffee).