Olfactory threshold predicts treatment response in relapsing multiple sclerosis

Background: Olfactory threshold (OT) is associated with short-term inflammatory activity in relapsing multiple sclerosis (RMS). Objective: We aimed to investigate OT for prediction of treatment response in RMS. Methods: In this 5-year prospective study on 123 RMS patients, OT was measured at disease-modifying treatment (DMT) initiation (M0), after 3 months (M3), and 12 months (M12) by Sniffin' Sticks test. Primary endpoint was defined as an absence of relapse during the observation period, with Expanded Disability Status Scale (EDSS) progression and magnetic resonance imaging (MRI) activity being the secondary endpoints. Optimal cutoff values were determined by receiver operating characteristic analyses and their predictive value assessed by multivariable Cox regression models. Results: Higher OT scores at M0, M3, and M12 were independently associated with decreased relapse probability with the strongest risk reduction at M3 (hazard ratio (HR) = 0.44, p < 0.001). Improvement of OT scores from M0 to M3 (Delta OTM3) was also associated with reduced relapse risk (HR = 0.12, p < 0.001). OT score > 6.5 at M3 was the strongest predictor of relapse freedom (HR = 0.10, p < 0.001) with high diagnostic accuracy (positive predictive value (PPV) = 87%), closely followed by Delta OTM3 > 0.5 (HR = 0.12, p < 0.001, PPV = 86%). Conclusions: OT is an independent predictor of freedom of disease activity upon DMT initiation within 5 years and may be a useful biomarker of treatment response.