Number Needed to Treat – Important Concept, Easy Calculation

Do your eyes glaze over when you start hearing or reading statistics about a new drug? Perhaps a quick breakdown of a few key concepts like “Number Needed to Treat” (NNT) will help you sort out the true clinical impact.

Studies usually report a difference between a control group and the treatment group. The difference is known as the Absolute Risk Reduction (ARR) (ARR = Incidence in Control Group minus Incidence in Intervention Group). For example, in the 2007 HORIZON study, osteoporotic women were randomized to treatment with yearly zoledronic acid (Reclast). After 3 years, 2.5% of women in the placebo group had a hip fracture, but only 1.4% in the treatment group had a hip fracture. The ARR is: 2.5% minus 1.4% = 1.1%.

The “Number Needed to Treat” (NNT) is determined when you divide 100 by the ARR: 100/1.1 = 91. Thus, 91 osteoporotic women need to take this medication for 3 years to prevent a hip fracture.  When thought about from that perspective, the other 90 women will incur the expense and be exposed to the medication side-effects- yet gain no benefit. Is it worth it?

NNT isn’t the only tool to help in interpreting a study’s results, but it is easy to calculate (especially with practice!) and gives a number that patients can understand. Your patient needs to decide whether to take a medication, but now you can give her some numbers to think about as she ponders her decision.

—Jeremy Golding, MD