Читать книгу Evidence in Medicine - Iain K. Crombie - Страница 20

The assignment of patients to treatment should correspond exactly to the randomisation sequence. However the process could be distorted, if the clinician recruiting the patient knew in advance which treatment the next patient would be given. For example, concerns about possible side effects might lead a clinician to decide not to recruit more severely ill patients if they were to be randomised to receive the new treatment. Thus fewer severely ill patients would be allocated to the active treatment. The resulting differences between the groups at baseline would bias the estimate of treatment benefit.

The solution is to ensure those involved in the trial have no access to the randomisation sequence, a process called allocation concealment. To achieve this, the randomisation is commonly handled by a remote site, such as a clinical trials unit, which could, for example, provide the treatments in separate containers labelled A or B. The methods used to conceal treatment allocation should be clearly described in the trial report, to give reassurance that bias is unlikely to have occurred.

Overviews of trials have shown that inadequate or poorly reported methods of allocation concealment are common. A major review of over 20,000 trials found that allocation concealment was adequate in only 35% of trials [5]. Other overviews in different clinical specialties found that the process was adequately described in 53% of neurological trials [8], 43% of surgical trials [3] and 27% of trials in multiple sclerosis [9]. An extreme example comes from the field of oral health in which only 15% of trials had low risk of bias for allocation concealment [10]. These studies suggest that as many as two third of trials are at risk of producing biased estimates of treatment.