Background: Early and accurate MS diagnosis followed by prompt intervention is crucial in modifying the disease's natural history. This study aimed to assess the factors related to diagnostic delay in relapsing-remitting MS (RRMS) and its association with functional status.

Methods: A cross-sectional study at a reference center in Bahia, Brazil, carried out from July/2021 to April/2024. Late MS diagnosis was defined as >3 months since the disease's clinical onset. EDSS assessed functional status.

Results: A total of 265 patients were included. The median time from symptoms' onset until diagnosis was 7 (IQR, 2-29) months, and 159 patients (60 %) had a late diagnosis. Multivariate logistic regression showed that living outside the metropolitan region, comorbidities, prior misdiagnosis, and disease onset before the McDonald 2017 criteria update were associated with late diagnosis. Meanwhile, disease onset with visual symptoms was related to early diagnosis. Finally, late diagnosis was related to worse functional status in RRMS patients.

Conclusions: Diagnostic delays in our population are significant, driven by living in cities outside the metropolitan region, comorbidities, previous misdiagnosis, and the use of McDonald's diagnostic criteria before the 2017 update. Understanding this reality is crucial, as timely diagnosis and treatment are essential for a favorable prognosis in RRMS.