Predictive value of abbreviated olfactory tests in prodromal Parkinson disease

Abstract

There is disagreement in the literature whether olfaction may show specific impairments in Parkinson Disease (PD) and if olfactory tests comprised of selected odors could be more specific for diagnosis. We sought to validate previously proposed subsets of the University of Pennsylvania Smell Identification Test (UPSIT) odors for predicting conversion to PD in an independent, prodromal cohort. Conversion to PD was assessed in 229 participants in the Parkinson At Risk Study who completed baseline olfactory testing with the UPSIT and up to 12 years of clinical and imaging evaluations. No commercially available or proposed subset performed better than the full 40-item UPSIT. The proposed "PD-specific" subsets also did not perform better than expected by chance. We did not find evidence for selective olfactory impairment in Parkinson disease. Shorter odor identification tests, including commercially available 10-12 item tests, may have utility for ease of use and cost, but not for superior predictive value.

Fig. 1. Performance of proposed and commercially available odor subsets.

A The UPSIT consists of four 10-item books (top), which can be administered individually. Four commercial subsets of odors (middle) have been produced—two versions of the 12-item Brief Smell Identification Test (B-SIT), and two versions of the 4-item Pocket Smell Test (PST). Several publications (bottom) have evaluated other combinations of odors and, in some cases, proposed that they may have particular value in discriminating PD from healthy controls. In some cases, multiple combinations were proposed, denoted by −1, −2, or −3. Filled squares indicate odors included in the subset. B Discriminatory power of commercially available and proposed odor subsets. No subset outperformed the full 40-item UPSIT in this independent prodromal cohort.

Fig. 2. Expected performance of odor combinations selected by chance, based on length.

The mean and 95% confidence interval for the expected AUC of a randomly selected combination of odors of a given length (1 odor to 40 odors) was computed (black) or estimated (gray). The performance of each proposed or commercially available subset is shown relative to these intervals (squares). Only the Pocket Smell Test Version B (PST-B, 4 items) was outside of the 95% confidence interval for length; but was within the 99% confidence interval (not shown).