In assessing the efficacy of natalizumab, a human monoclonal antibody against alpha-4 integrin, in the treatment of multiple sclerosis (MS), a randomized double-blinded placebo-controlled study was performed with a total of 1000 people divided equally between the treatment and placebo groups. The study lasted for three (3) years and during this period, the reduction in relative risk for primary events (relapse of current MS or progression to a more chronic form of MS) was 8%. The absolute risk reduction was smaller because the study group had a low incidence of MS-associated events., 1% in the control group and 3% in the treatment group. How many patients must be treated for three years with natalizumab to prevent a primary event in one person?
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- 2
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- 1
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- 50
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- 100
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- 20
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EXP:
The question asks you to determine the number needed to treat (NNT) which is the reciprocal of the absolute risk reduction (ARR). That is, NNT = 1/ARR. The absolute risk reduction is the decrease in risk of an event in relation to the control. That is, ARR = Incidence in test group - incidence in control group. Here ARR = 3%-1% ⇒ 2%. So, NNT = 1/ARR = 1/0.02 = 50. (Don’t forget that you are inverting percentages not numbers!)