Defining standard of practice: pros and cons of the usual care arm

The aim of this review is to describe the use of usual care arms in randomized trials.

Randomization of patients to an experimental or a control arm remains paramount for the estimation of average causal effects. Selection of the control arm is as important as the definition of the intervention, and it might include a placebo control, specific standards of care, protocolized usual care, or unrestricted clinical practice. Usual care control arms may enhance generalizability, clinician acceptability of the protocol, patient recruitment, and ensure community equipoise, while at the same time introducing significant variability in the care delivered in the control group. This effect may reduce the difference in treatments delivered between the two groups and lead to a negative result or the requirement for a larger sample size. Moreover, usual care control groups can be subject to changes in clinician behavior induced by the trial itself, or by secular trends in time.

Usual care control arms may enhance generalizability while introducing significant limitations. Potential solutions include the use of pretrial surveys to evaluate the extent to which a protocolized control arm reflects the current standard of care and the implementation of adaptive trials.