Background Serious/life-threatening medical encounters (sltMEs) occur during mass community-based events, with higher incidences in ultra-endurance events. Risk factors associated with sltMEs are under-investigated.

Objective To determine independent risk factors for serious/life-threatening MEs in a 90 km ultramarathon running event.

Design Retrospective clinical audit, cross-sectional analysis

Setting 2014–2018 Comrades ultramarathon (90km, alternating annually between an ‘up’ vs. a ‘down’ run between a coastal and inland city), South Africa

Participants 70328 race starters

Interventions All entrants voluntarily completed two medically-related questions: 1) history of allergies, and 2) history of any medical condition and/or medication use. Race day data (start time, finish time, age, sex, ‘up’ vs. ‘down’ run) was recorded by the race organisers; and sltMEs (as defined in the 2019 consensus statement on mass community-based events) were recorded by race medical staff during and immediately after the event.

Main Outcome Measures Independent risk factors associated with sltMEs (injury- and illness-related) using demographics (age, sex), race day factors (route ‘up’ or ‘down’; race pace), and individual self-reported medical history (allergies, medical conditions/medication use) were investigated using a multivariate model with a Poisson distribution, reporting the prevalence ratio (PR: 95% CI).

Results 159 serious/life-threatening medical encounters were recorded over the 5 years. The following independent risk factors were associated with serious/life-threatening medical encounters: age and sex (males 31–40 yrs vs females >50 yrs, 0.3: 0.1–0.7, p=0.0076; males 41–50 yrs vs females >50 yrs, 0.2: 0.1–0.4, p=0.0002; males >50 yrs vs females >50 yrs, 0.4: 0.2–0.9, p=0.0323), running pace (6-<7 min/km vs 8-<9 min/km, 0.4: 0.2–0.8, p=0.0062; 7-<8 min/km vs 8-<9 min/km, 0.5: 0.3–0.9, p=0.0299) and route (up vs. down, 0.5: 0.3–0.8, p=0.0046).

Conclusion Older females, slower runners and route (down run) were independent factors associated with higher risk of serious/life-threatening MEs in a 90 km ultramarathon. These data will form the basis to design and implement prevention programs to manage risk of sltMEs at these events.