Number Needed to Treat

Here’s a little something to get you in the right frame of mind for some upcoming posts.

From Wikipedia, number needed to treat:

The number needed to treat (NNT) is an epidemiological measure used in assessing the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the number of patients who need to be treated to prevent one additional bad outcome (i.e. the number of patients that need to be treated for one to benefit compared with a control in a clinical trial). . . . The ideal NNT is 1, where everyone improves with treatment and no one improves with control. The higher the NNT, the less effective is the treatment.

This goes hand-in-hand with its fraternal twin, number needed to harm:

The number needed to harm (NNH) is an epidemiological measure that indicates how many patients need to be exposed to a risk-factor over a specific period to cause harm in one patient that would not otherwise have been harmed. . . . Intuitively, the lower the number needed to harm, the worse the risk-factor.

Basically these are the number of people you would have to treat if you were to fix the medical problem or make it worse. If you have your own data set, you can easily calculate NNT and NNH. (Compute them with software, of course! What are we savages?)

How about a diabetes-oriented example, care of the Centre for Evidence-Based Medicine?

“The results of the Diabetes Control and Complications Trial (DCCT) into the effect of intensive diabetes therapy on the development and progression of neuropathy indicated that” the NNT was 15. “We would need to treat 15 diabetic patients with intensive therapy to prevent one from developing neuropathy.”

Compare that to another study that looked at the role of “regular telephone contact with a diabetes nurse educator for advice about adjustment of insulin therapy vs. regular clinic visits and usual contact with the endocrinologist for insulin adjustment.” In this study a positive outcome was defined by examining the “mean HbAlc level and proportion of patients who achieved a reduction in HbAlc level ≥ 10%.” It found that the NNT was 2. Two! For every two patients who get called by a nurse educator, one of them will likely see an improvement in self-management abilities.

These studies aren’t mutually exclusive. In fact, you might say that having nurses call patients is an effective part of “intensive therapy” that contributes to lower incidence of neuropathy.

The main point I want you to take away from this small digression is that whenever you hear medical reporting about the value of new or existing therapies, you should think in terms of NNT and NNH. If we gave everyone this test (or drug or medical intervention) how many would benefit? How many would suffer as a result? If the cost of the treatment is large but it only saves one life in 1,000, is it worth the cost? How many of those 1,000 are harmed? Are there other therapies with better NNT and NNH numbers that could be used instead?

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