An effectiveness trial of a diagnostic test in a busy outpatients department in a developing country: issues around allocation concealment and envelope randomization.

Methods to ensure allocation concealment in a randomized controlled trial in a busy outpatients department may disrupt usual clinical behavior, and thereby modify the effect of a diagnostic test. In a clinical trial of chest radiography in ambulatory children, concealment was maintained by means of sealed sequentially numbered manila envelopes. Baseline information was collected on all potential participants before they were presented to a clinician for a decision on inclusion in the trial. Excluded patients were followed up. Of 59 excluded patients, only 16 allocation envelopes had been opened or were not accounted for, although 12 of these had non-radiograph allocations (P = 0.05). There was no difference between patients excluded from radiograph and non-radiograph groups in baseline characteristics or primary outcome measure. Most of the improper exclusions occurred early in the study; improved monitoring and feedback reduced the problem. Sealed opaque sequentially numbered envelopes may be appropriate for trials of diagnostic tests in settings where centralized randomization is not feasible, and given careful monitoring of the enrollment process.