Comparison between four published definitions of hyposmia in Parkinson's disease

Hyposmia is a common feature of Parkinson's disease (PD), yet there is no standard method to define it. A comparison of four published methods was performed to explore and highlight differences.

Olfactory testing was performed in 2097 cases of early PD in two prospective studies. Olfaction was assessed using various cut-offs, usually corrected by age and/or gender. Control data were simulated based on the age and gender structure of the PD cases and published normal ranges. Association with age, gender, and disease duration was explored by method and study cohort. Prevalence of hyposmia was compared with the age and gender-matched simulated controls. Between method agreement was measured using Cohen's kappa and Gwet's AC1.

Hyposmia was present in between 69.1% and 97.9% of cases in Tracking Parkinson's cases, and between 62.2% and 90.8% of cases in the Parkinson's Progression Marker Initiative, depending on the method. Between-method agreement varied (kappa 0.09-0.80, AC1 0.55-0.86). The absolute difference between PD cases and simulated controls was similar for men and women across methods. Age and male gender were positively associated with hyposmia (p < .001, all methods). Odds of having hyposmia increased with advancing age (OR:1.06, 95% CI:1.03, 1.10, p < .001). Longer disease duration had a negative impact on overall olfactory performance.

Different definitions of hyposmia give different results using the same dataset. A standardized definition of hyposmia in PD is required, adjusting for age and gender, to account for the background decline in olfactory performance with ageing, especially in men.