Risk classification
We used a risk classification table and computed likelihood ratios (LRs)30 with associated 95% CI to assess the main model’s ability to stratify the population as low or high-risk. We divided the population into five groups of predicted probability: very low risk (<1.5 per 1000), low risk (1.5 to 3 per 1,000), intermediate risk (3 to 5 per 1,000), high risk (5 to 15 per 1,000) and very high risk (>15 per 1,000). These cut-offs were chosen based on the overall incidence of our primary outcome of 3.1 per 1,000 which we assumed to reflect the risk among the majority of the cohort. Positive LRs of >5 and >10 were interpreted as moderately or very useful “rule-in” tests, while values between 0.2 and 0.5, and <0.1 were considered moderately and very useful “rule-out” tests.31