Because many interventions in medicine are of only modest effect, their apparent benefit may not be that noticeable after one has tried the intervention on a few patients. One way to understand the magnitude of benefit in relation to baseline risk is to use the concept of "number needed to treat" (NNT).17 This refers to the number of patients that on average you would need to treat in order to see one additional success in the new treatment when compared with standard treatment. NNT is calculated simply as the reciprocal of the difference in success rates between the treatments being compared. Thus, a new treatment that results in clearing of psoriasis in 40% of patients compared with 30% for the conventional treatment translates to a risk difference of 10% (40 - 30) and an NNT of 1/0-10 which equals 10. In other words, one needs to treat 10 patients on average in order to see on extra gain in terms of clearance for the new treatment.
Patients' versus physicians' views regarding the threshold for what might constitute a useful NNT may differ significantly. Thus, in a study of perspectives of physicians and patients on anticoagulation for atrial fibrillation, patients placed significantly more value on the avoidance of bleeding than did doctors.18 Again, the message here is not to think of NNT as belonging exclusively to doctors - patients too need to be incorporated into the decision-making process of determining what is useful and important.
It is also important that the dermatologist and patient decide for themselves as to what might constitute a useful NNT, rather than blindly accepting the sort of conventions that have been derived from acute medicine where the stakes are perhaps higher. So, although it may be perfectly justifiable to treat 200 patients with a low dose, aspiring to prevent one stroke, I would certainly not be willing to work with such an NNT for a new antibiotic if the gain was just one extra short-term remission of acne. In a pressurised health service I might even question the value of a new treatment for plaque psoriasis with an NNT of 20. Perhaps the opportunity costs associated with seeing the extra 20 patients needed in order to achieve one extra response from the new treatment could be better spent discussing other treatment options with them or assessing other new patients. Despite these caveats, the NNT is a more useful tool than measures of relative risk such as odds ratios to translate the evidence back to the patient.
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Do You Suffer From the Itching and Scaling of Psoriasis? Or the Chronic Agony of Psoriatic Arthritis? If so you are not ALONE! A whopping three percent of the world’s populations suffer from either condition! An incredible 56 million working hours are lost every year by psoriasis sufferers according to the National Psoriasis Foundation.