What is a crossover clinical trial?
This post is part of 'Domain | Trial design aspects' series
What is a crossover trial?
- A crossover trial is a clinical trial in which the subjects are randomly allocated to study arms where each arm consists of a sequence of two or more treatments given consecutively. T
- The simplest model is the AB/BA study.
- Subjects allocated to the AB study arm receive treatment A first, followed by treatment B, and vice versa in the BA arm.
- Crossover trials are generally restricted to the study of short term outcomes in chronic diseases or processes because the disease or process needs to persist long enough for the investigator to expose the subject to each of the experimental treatments and measure the response.
- Also, the treatment must be one that does not permanently alter the disease or process under study.
- Crossover trials allow the response of a subject to treatment A to be contrasted with the same subject’s response to treatment B.
- Removing patient variation in this way makes crossover trials potentially more efficient than similar sized, parallel group trials in which each subject is exposed to only one treatment.
- In theory treatment effects can be estimated with greater precision given the same number of subjects.
- The principal drawback of the crossover trial is that the effects of one treatment may “carry over” and alter the response to subsequent treatments.
- The usual approach to preventing this is to introduce a washout (no treatment) period between consecutive treatments which is long enough to allow the effects of a treatment to wear off.