More Details on Number Needed to Treat

Definition

The NNT is the number of patients who need to be treated in order to prevent one additional bad outcome. It is the inverse of the Absolute Risk Reduction (ARR).

How to Calculate NNTs

NNT = 1/ARR
ARR = |CER – EER|

where

CER = control group event rate
EER = experimental group event rate

Sample Calculation

The results of the Diabetes Control and Complications Trial (Ann Intern Med 1995; 122:561-8) into the effect of intensive diabetes therapy on the development and progression of neuropathy indicated that neuropathy occurred in 9.6% of patients randomised to usual care and 2.8% of patients randomised to intensive therapy. The number of patients we need to treat with the intensive diabetes therapy to prevent one additional occurrence of neuropathy can be determined by calculating the absolute risk reduction as follows:

ARR = |CER – EER| = |9.6% – 2.8%| = 6.8%
NNT = 1/ARR = 1/6.8% = 14.7 or 15

We therefore need to treat 15 diabetic patients with intensive therapy to prevent one from developing neuropathy.

Practice Calculation

In a randomised controlled trial (Stroke 1997; 28:1861-6) looking into the long-term outcome for stroke patients treated in stroke units (SU) compared with patients treated in general wards (GW), the mortality rate 5 years after the onset of stroke was 59.1% in the patients treated in SU and 70.9% in those treated in the GW. How many patients need to be treated in stroke units to prevent one additional death?

NNT = _______

See our answer and compare

ARR = |CER – EER| = |0.709 – 0.591| = 0.118
NNT = 1/ARR = 1/0.118 = 9

Nine patients would need to be treated in stroke units to prevent one additional death.

Evidence-based medicine (4th ed.)