A Blinding Success?: The Debate over Reporting the Success of Blinding

While the use of blinding is a hallmark of placebo-controlled trials, whether the blinding was successful - i.e., whether or not participants were able to figure out the treatment condition to which they have been assigned - isn't always tested, nor are the results of these tests always reported. The measurement of the success of blinding in trials is controversial and not uniformly used, and the item has been dropped from subsequent versions of the CONSORT reporting items for trials. According to a recent discussion of the pros and cons to measuring the success of blinding, only between 2-24% of trials perform or report these types of tests.

As Webster and colleagues explain, the benefits to measuring the success of blinding are as follows:

• the success (or failure) of blinding in a placebo-controlled trial can introduce a source of bias that affects the results. 
• while the effect of blinding itself may be small, these small effects could still result in changes to policy or practice
• there are documented instances in which the failure to properly blind (for instance, providing participants with a sour-tasting Vitamin C condition versus a sweet lactose "placebo") led to an observed effect (for instance, on preventing or treating the common cold) whereas there was no effect in the subgroup of participants who were successfully blinded.

Reasons commonly given against the testing of successful blinding include the following:

• At times, a break in blinding can lead to conclusions in the opposite direction. For instance, physicians who are unblinded may assume that the patients with better outcomes received a drug widely supposed to be "superior," when in fact, the opposite occurred.
• In some cases, a treatment with dramatically superior results can result in unblinding, even when the treatment conditions were identical - but that doesn't necessarily mean the blinding was a failure or could have been prevented, given the dramatic differences in outcomes.
• If the measurement of blinding is performed at the wrong time - such as before the completion of the trial - participants may become suspicious and this in itself could potentially confound treatment effects.

Webster RK, Bishop F, Collins GS, et al. (2021). Measuring the success of blinding in placebo-controlled trials: Should we be so quick to dismiss it? J Clin Epidemiol, pre-print.

Manuscript available from publisher's website here.