Blindsided: challenging the dogma of masking in clinical trials

Evidence based medicine compels us to challenge assumptions and dogma. Health interventions should be subject to rigorous assessment—ideally in well conducted randomised trials. But what of the dogma of evidence based medicine itself? Blinding, also called masking, of participants, study staff, and outcome assessors is considered crucial to avoid performance and detection biases, and it is a key element of the Cochrane risk of bias tool for randomised trials.1 However, the linked comprehensive meta-epidemiological study by Moustgaard and colleagues (doi:10.1136/bmj.l6802) challenges the dogma that blinding is always necessary to protect against bias in trials.2

The MetaBLIND study compared the results of trials that assessed the same treatments and outcomes but differed with regard to blinding of patients, healthcare providers, or outcome assessors. Surprisingly, lack of blinding was not associated with an exaggeration of treatment effects compared with use of blinding. These findings were consistent across patient and investigator reported outcomes of varying degrees of subjectivity and were robust to sensitivity analyses …