Olfactory Test Accuracy in Distinguishing Tremor-Dominant Parkinson’s Disease from Essential Tremor: A Systematic Review and Meta-Analysis

Background: Hyposmia is a common nonmotor feature of Parkinson’s disease (PD) and is typically absent or mild in essential tremor (ET). Olfactory tests are low-cost, noninvasive tools that may improve diagnostic accuracy in tremor-dominant PD (TD-PD). Methods: PubMed/MEDLINE, Embase, and Cochrane Library were systematically searched through January 2026. Studies evaluating olfactory tests in TD-PD and ET were included. Diagnostic accuracy measures were pooled using MetaDTA, and hierarchical summary receiver operating characteristic (HSROC) curves were generated. PROSPERO: CRD420261437298. Results: Seven studies met inclusion criteria, including 454 TD-PD and 239 ET patients. Two studies used the University of Pennsylvania Smell Identification Test and five used Sniffin’ Sticks. Pooled sensitivity was 78% (95% CI, 67%-86%) and specificity was 91% (95% CI, 83%-95%) for differentiating TD-PD from ET. HSROC analysis demonstrated excellent diagnostic performance (AUC=0.90). The pooled positive likelihood ratio was 8.67 and negative likelihood ratio was 0.24. Most studies reported specificity ≥90%. Sensitivity variability likely reflected differences in disease stage and diagnostic thresholds. Risk of bias was generally high because of nonconsecutive patient selection and limited blinding. Conclusion: Olfactory testing shows high specificity and moderate sensitivity for distinguishing TD-PD from ET and may serve as a practical adjunct when motor findings are equivocal.