Quantitative Olfactory Dysfunction and Parosmia After COVID-19: An Observational Cohort Study

Olfactory dysfunction (OD) is a common post-COVID-19 condition; however its prognosis remains unclear. This study aims to describe patient characteristics across different types of post-COVID-19 OD (C19OD), identify factors influencing the persistence of parosmia, and investigate the impact of parosmia on quantitative olfactory function.

In this longitudinal cohort study, 190 patients with C19OD were recruited from an otorhinolaryngology department in Denmark. Patients had a baseline visit and four follow-up visits at six-month intervals. Quantitative OD was measured using the "Sniffin' Sticks" threshold, discrimination and identification test (TDI). Parosmia was assessed from medical history. Subjective assessments of olfactory and gustatory function, and quality of life (QoL) were evaluated on numerical rating scales.

The median time since infection was 20 months (IQR: 16-24). Patients had a median TDI score of 27.50 (IQR: 21.50-31.31) at the first visit and 28.63 (IQR: 22.75-31.50) at the final visit. Parosmia was present in 73.7%. Patients with parosmia had significantly higher TDI scores (p = 0.001). Patients with quantitative OD rated their QoL worse than those with parosmia alone (p = 0.008). No significant differences in baseline characteristics were observed between patients with persistent parosmia and those recovered. Patients with persistent parosmia improved TDI scores by 2.25 points at the last visit, whereas patients who recovered from parosmia showed no change.

Most patients experienced parosmia, which was associated with better quantitative olfactory function. Patients with quantitative OD reported the lowest QoL. No baseline characteristics predicted the persistence of parosmia. Patients with persistent parosmia showed a tendency for improvements in TDI scores.