However, in some cases the nature of the treatments under investigation may not permit blinding. It is therefore recommended that trials be blinded (or masked) as far as possible to limit bias. For example, enthusiasm for a newer treatment could lead to better outcomes being reported in this group of patients, regardless of any treatment efficacy. If patients, clinicians, or assessors are aware of treatment assignments, this may influence reporting or measurement of the outcome and introduce bias. However, randomisation alone does not preclude the possibility of systematic differences. The main goal of a randomised controlled trial (RCT) is to ensure that, apart from the intervention, there are no systematic differences between treatment groups under study, thereby ensuring an unbiased estimate of treatment effect. When blinded outcome assessment is not possible, it may be useful to modify the outcome definition or method of assessment to reduce the risk of bias. This process allowed the unblinded clinician to be involved in the patient’s care, while reducing the potential for bias. If a chest x-ray indicated that more than a third of the pleural space filled with fluid, the patient could be referred for a procedure otherwise, the unblinded clinician was required to reach a consensus on referral with a blinded assessor. Therefore, the decision pathway for procedure referral was modified. Assessment by an adjudication committee was not possible, as the outcome either occurred or did not. Allowing a blinded assessor to decide whether to refer the patient for a procedure was not feasible as many clinicians may be reluctant to enrol patients into the trial if they cannot be involved in their care during follow-up. TAPPS was an open-label trial whose primary outcome was whether the patient was referred for a pleural drainage procedure. Therefore, the definition of further bleeding was modified to exclude subjective aspects (such as whether symptoms like vomiting blood were severe enough to indicate the outcome had been met), leaving only objective aspects (the presence versus absence of active bleeding in the upper gastrointestinal tract confirmed by an internal examination). A blinded adjudication committee was also not feasible as it was impossible to compile relevant information to send to the committee in a blinded manner. Because of the cluster randomisation, all researchers in a hospital were aware of treatment allocation and so could not perform a blinded assessment. TRIGGER was an open-label cluster randomised trial whose primary outcome was further bleeding. We describe two randomised trials where blinded outcome assessment was not possible, and discuss the strategies used to reduce the possibility of bias. It is therefore important to find other means of reducing bias in these scenarios. Blinded outcome assessment is recommended in open-label trials to reduce bias, however it is not always feasible.
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