Groups are different from individuals

Even if the person in front of you is a German woman with acute atopic eczema, the results of the trial may not translate to real clinical benefit to her for several reasons. First, because the treatment effects reported in clinical trials, whether this is a mean change in severity score or proportion of people cleared, refers to groups of people. In a group of patients with a summary treatment effect such as a mean change in SCORAD (SCORing Atopic Dermatitis) of 5 points, there will be some individuals with score changes of 10 or 15 points, some with changes of 3 or 4, some with no change, and possibly some whose disease worsened. For example, closer inspection of the trial data on sildenafil (Viagra) for the treatment of erectile dysfunction7 suggested that some men had all the fun! It follows that the patient sitting in front of you might benefit a lot or very little from the proposed treatment, and one has little way of knowing, apart from trying the treatment and seeing what happens. Similarly, telling your patient that "on average 60% of patients clear with this treatment" may not be very helpful if the patient then wants to know, "Am I in that 60%, Doctor?".

One way forward is to see whether treatment response varies according to different study subgroups, although it is unusual for dermatology RCTs to be large enough to include such subgroup analysis, and care has to be taken in "data dredging" in such studies. Conducting a series of n of 1 trials on that patient may appear one way forward,8 but such an approach is only suitable for stable chronic diseases and for treatments that do not affect the response to subsequent treatments. Pharmacogenetic testing offers one way of predicting whether an individual will respond to a drug. For example, measurement of thiopurine methyl tranferase levels predicts who will develop serious bone marrow suppression from oral azathioprine.9

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