Intention-to-treat analysis — MRCPsych Paper A MCQ

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ModerateEpidemiology and research methodsIntention-to-treat analysisMRCPsych Paper A

In a placebo-controlled randomised trial of an antidepressant, 18% of participants discontinue their allocated treatment and 6% switch treatment arms. The primary analysis retains every participant in the group to which they were originally randomised, with missing outcome data handled using a prespecified imputation method. Which analysis strategy is being used?

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Correct answer: E — Intention-to-treat analysis

The correct answer is **D, intention-to-treat analysis**. Participants are analysed according to their original randomised allocation regardless of treatment discontinuation, non-adherence or crossover. This preserves the prognostic balance created by randomisation and reduces selection bias. Appropriate methods are still required for missing outcome data. In an as-treated analysis, participants are analysed according to the treatment actually received. A complete-case analysis excludes participants whose required outcome data are missing. A per-protocol analysis is restricted to participants who adhered sufficiently to the protocol. Modified intention-to-treat has inconsistent definitions but generally excludes some randomised participants according to additional criteria; it does not describe an analysis explicitly retaining every randomised participant.

Reference: Siemieniuk RAC et al. Nutrition users’ guides: RCTs part 1 – structured guide for assessing risk of bias in randomised controlled trials that address therapy or prevention questions, section 3b, 2024. https://nutrition.bmj.com/content/7/2/402