Further tests carried out after randomization shows the patient is ineligible

Most trial protocols will list investigations that must be performed before randomization to determine whether a patient is eligible. However, often because of the nature of one of the treatments, further investigations may be performed on one arm (typically the experimental arm) after randomization, the findings of which may mean the patient is 'ineligible.' As an example, in a trial of interstitial versus conventional radiotherapy for patients with brain tumours, it maybe tempting to exclude those patients with multifocal tumours who have a poor prognosis and are unlikely to benefit from either type of radiotherapy. However, such tumours are only readily detectable in those patients having interstitial radiotherapy. To exclude these patients from the analysis would have been inappropriate, because there are probably a similar number of patients with undetected multi-focal tumours in the conventional radiotherapy arm. Exclusion of such patients would therefore introduce systematic differences between the two arms and lead to a biased analysis. This displays the general point that excluding patients on the basis of an investigation performed after randomization is likely to introduce bias, particularly when one treatment group is more likely to undergo the investigation, and such patients should generally be included in the analysis.

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