• What is their prior belief about the proposed intervention?
• Do they prefer a topical or systemic medication?
• Would they prefer no pharmacological treatment at all?
• What treatments have they had before?
ask an evidence-based question in obvious biological ways such as age, sex and clinical disease type.4 In most circumstances, these differences do not prevent you from making some useful generalisations from the literature. For example, I would be quite happy to generalise from a randomised controlled trial (RCT) of topical steroids for atopic eczema in the absence of strict diagnostic criteria, provided that the description of that disease made sense to me, for example phrases such as "itchy red flexural eczema".
Occasionally, the description of the clinical trial participants may render it difficult to extrapolate study results to the patient in front of you. For example, it may be unrealistic to generalise the results of an RCT dealing with high-dose UVA in the treatment of German women with an acute flare of atopic eczema to a South African man with chronic lichenified atopic eczema. In such circumstances, perhaps more weight is then given to one trial of chronic eczema in adult men than to several trials conducted in younger women.
Perhaps one of the most frequent problems encountered here is that of having to generalise trials of adult therapy to children, in whom RCTs are rarely performed. Yet, children can suffer almost all of the "adult" skin diseases, and practitioners frequently have no choice but to use adult-based data as a guide.
Another difficulty is that treatments that appear promising when tested in enthusiastic and healthy "atypical" clinical trial volunteers often turn out to be less effective when applied to a wider group of patients with other co-morbidities and levels of compliance. Such a divergence between study participants and real patients has been termed the difference between efficacy and effectiveness.5 A regimen that involves fiddling around with creams three times a day may be just maintained under the special conditions of a clinical trial with continuous encouragement by the study assessors (and sometimes financial inducements), but when it comes to trying it in everyday life it may simply be too much trouble. These effects can often be explored by looking at the dropout rate for different interventions and reasons for such dropouts. Finding such effects should not deter the reader from using the evidence, but rather make him/her more aware of the factors that need to be taken into account when describing the potential efficacy of a treatment to the patient. One type of trial design, namely the pragmatic clinical trial, overcomes the "ideal patient" effect by recruiting as wide a mix of patients as possible and by capturing the effects of poor compliance in the final analysis.6
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