Which is superior, the University of Pennsylvania Smell Identification Test or the threshold, Discrimination and Identification Test for testing human olfaction? A systematic review

The awareness and prevalence of olfactory dysfunction (OD) has increased significantly in recent years, with the SARS-CoV-2 pandemic being a major catalyzer. Consequently, demands for reliable OD tests have also risen.
This systematic review compares two commonly used tests for olfactory assessment: the University of Pennsylvania Smell Identification Test (UPSIT) and the Threshold, Discrimination, and Identification test (TDI). The correlation between UPSIT, TDI, and the Visual Analogue Scale (VAS) for olfaction is also examined.
A systematic search of PubMed and EMBASE identified articles validating UPSIT or TDI and comparing these to self-reported OD via VAS. The outcome of interest was test-retest validity and correlation.
The search identified 1536 studies, with nine meeting inclusion criteria. UPSIT showed a ‘very strong’ test-retest correlation, while TDI showed a ‘strong’ correlation. One study addressed VAS in relation to UPSIT and five for TDI. Correlation coefficients varied between VAS and TDI/UPSIT.
UPSIT and TDI exhibited strong to very strong test-retest reliability. Due to the limited number of studies, a meta-analysis was not feasible, and neither test was found to be superior in reliability. VAS did not correlate significantly with either TDI or UPSIT.