Читать книгу Randomised Clinical Trials - David Machin - Страница 33

In certain circumstances, when a new treatment has been proposed for evaluation, all patients are recruited prospectively but allocation to treatment is not made at random. In such cases, the comparisons may well be biased and hence are unreliable. The bias arises because the clinical team choose which patients receive which intervention and in so doing may favour (even subconsciously) giving one treatment to certain patient types and not to others. In addition, the requirement that all patients should be suitable for all options may not be fulfilled – in that if it is known that a certain option is to be given to a particular subject then one may not so rigorously check if the other options are equally appropriate. Similar problems arise if investigators have recruited patients into a single‐arm study, and the results from these patients are then compared with information on similar patients having (usually in the past) received a relevant standard therapy for the condition in question. However, such historical comparisons are likely to be biased also and to an unknown extent so again it will not be reasonable to ascribe the difference (if any) observed entirely to the treatments themselves. Of course, in either case, there will be situations when one of these designs is the only option available. In such cases, a detailed justification for not using the ‘gold standard’ of the randomised controlled trial is required.

Understandably, in this era of EBM, information from non‐randomised comparative studies is categorised as providing weaker evidence than that from randomised trials.

The before‐and‐after design is one in which, for example, patients are treated with the Standard option for a specified period and then, at some fixed point in time, subsequent patients receive the Test treatment. This is the type of design used by Erbel, Di Mario, Bartunek, et al. (2007) to evaluate a bioabsorbable stent for coronary scaffolding. In such examples, the information for the Standard is retrospective in nature and is often obtained from clinical records only and so was not initially collected for trial purposes. If this is the case, the before‐and‐after design is likely to be further compromised as, for example, in the ‘before’ period, the patient selection criterion, clinical assessments and data recorded may not meet the standards required of the ‘after’ component. Such differences are likely to influence the before‐and‐after comparison in unforeseen and unknown ways.