Maybe your clinical trial measures cardiovascular deaths or hospitalizations or some other event. Intercurrent events are all the events that affect the measurement of your outcome that happen after T0. In a randomized controlled trial, this is the point at which people are randomized to a treatment or control groups.
Definition: Intercurrent events are post-baseline events (or post-randomisation events in randomised trials) that affect the interpretation or existence of outcome data. These events frequently affect receipt of treatment (eg, treatment switching or treatment discontinuation) or preclude existence of the outcome (eg, death, if it is not defined as part of the outcome).
There are a dizzying number of ways that your clinical trial might chose to handle intercurrent events. Historically these decisions were made implicitly. Clinical trial methodologists everywhere found the failure to specify how intercurrent events are handled to be problematic. So much so that good clinical trial hygiene demands such specification according to guidelines from the de-facto international community for standardizing clinical evidence generation. Intercurrent events are only one component amongst many other aspects of how a trial operationalizes the evaluation of their clinical hypothesis — how a trial is designed, what is measured, the quantitative objective, and the procedure to produce quantitative estimates.
Kahan et. al. 2024 have a nice introduction to 5 different ways one might handle an intercurrent event in their review.
Figure: Different strategies regarding intercurrent events. In this example, a randomized trial compares intervention with control to understand how outcomes differ at month 2. However, one participant stops treatment before month 2 (ie, an intercurrent event). The figure shows what happens to this participant under each intercurrent event strategy. Under a composite strategy, investigators have decided to assign a score of 0 to any participant who experienced an intercurrent event. Under a while-on-treatment strategy, because the participant experienced an intercurrent event before month 2, their month 1 score of 3 is used in place of their month 2 score. Under a hypothetical strategy, the participant’s outcome that would have occurred had they continued treatment at month 2 is used (here, it is a value of 9); but in practice, this value will not be known and so must be estimated. M=month.
Kahan B C, Hindley J, Edwards M, Cro S, Morris T P. The estimands framework: a primer on the ICH E9(R1) addendum BMJ 2024; 384 :e076316 doi:10.1136/bmj-2023-076316
Thanks— do you have a good resource on good clinical trial design hygiene?