Intercurrent

Intercurrent is an adjective derived from Latin intercurrere, meaning to run between or to occur between. In English, it denotes something that occurs during the course of something else or that intervenes in the sequence of events. The term is used across several domains to indicate an event or condition that arises within an ongoing process.

In medical usage, intercurrent illness or intercurrent infection refers to a new illness that arises while a patient has an existing disease or is undergoing treatment. Such illnesses can affect the course of the primary condition, require adjustments to management, and influence outcomes. For example, a patient with a chronic respiratory condition who develops influenza faces an intercurrent illness that may worsen respiratory status and complicate care.

In statistics and clinical research, intercurrent events are events that occur after a treatment is assigned and affect the interpretation or existence of endpoint data. The concept is central to the estimands framework, which guides how results should be interpreted when such events occur. Examples include discontinuation of the assigned treatment, use of rescue medication, or other unplanned changes to therapy. The framework distinguishes how to handle these events, with strategies such as a treatment-policy approach (analyzing data regardless of the event), a hypothetical approach (imagining the event did not occur), and other methods like composite or principal-stratum strategies. Understanding intercurrent events helps align trial analysis with the specific scientific question being asked.

Outside medicine and statistics, intercurrent is a relatively uncommon term used to describe processes that occur within or interrupt an ongoing sequence, but its primary technical usage remains in clinical and research contexts. See also intercurrent illness and estimands.