Shortened olfactory identification testing to differentiate parkinsonian syndromes

Olfactory testing is an established tool aiding in differentiating Parkinson's disease (PD) from atypical parkinsonian disorders (APD). Several abbreviated tests have been proposed, but none has been validated in an independent cohort. Therefore, we here assessed accuracies of different abbreviated Sniffin Sticks' identification tests (SS-I) in distinguishing PD from APD.

Results from the 16-item SS-I in clinically diagnosed patients with PD and APD from multiple studies at the Medical University Innsbruck were collected. Shortened versions of the SS-I were identified in a literature search. Diagnostic accuracy of each test was determined and areas under the curve (AUC) were compared with the full SS-I using the DeLong test. A non-inferiority margin was defined at 5%. Monte Carlo cross-validation was performed to evaluate generalizability.

A total of 175 PD and 90 APD patients underwent SS-I testing. The full SS-I showed good diagnostic accuracy for discriminating PD from APD (AUC: 0.81 [95%-CI: 0.76-0.87]). Eight abbreviated versions of the SS-I were identified from the literature, of which three were non-inferior to the full SS-I set with AUCs >0.80 (one with 7, one with 8, and one with 12 odors). The other 5 abbreviated versions (4 tests with 3 different odors and one with 5) yielded AUCs <0.78, did not surpass the non-inferiority cut-off margin, and had suboptimal sensitivities and/or specificities. The results remained consistent after cross-validation.

Three of 8 identified abbreviated SS-I tests (including seven odors or more) retained diagnostic value for distinguishing PD from APD in our independent cohort.