

CURRICULUM IN CARDIOLOGY  STATISTICAL PAGES 

Year : 2017  Volume
: 3
 Issue : 2  Page : 106108 

Number needed to treat
Amitabh Biswas
Division of Forensic Science, School of Basic and Applied Sciences, Galgotias University, Greater Noida, Uttar Pradesh, India
Date of Web Publication  20Nov2017 
Correspondence Address: Amitabh Biswas Division of Forensic Science, School of Basic and Applied Sciences, Galgotias University, Greater Noida, Uttar Pradesh India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/jpcs.jpcs_31_17
The Number Needed To Treat (NNT) is a measure used in epidemiology to convey the effectiveness of an intervention. It is the average number of patients who need to be treated to prevent one bad outcome. It is the reverse of the Absolute Risk Reduction. The lower the NNT, the more effective the intervention. In this article we discuss the concept and limitations of this measure.
Keywords: Number needed to treat, odd's ratio, randomized clinical trials, relative risk reduction
How to cite this article: Biswas A. Number needed to treat. J Pract Cardiovasc Sci 2017;3:1068 
Introduction   
Number needed to treat (NNT) is a simple measure depicting the effectiveness of a healthcare intervention. The NNT presents a measure of the effect of a medicine or therapy by estimating the number of patients who need to be treated to benefit one person. This is a statistical concept, but intuitive, for we know that everyone is not benefited by a medicine or intervention – some benefit, some are harmed, and some are unaffected. The NNT tells us how many of each.^{[1],[2]}
In any comparison of a previous treatment with a new drug treatment, the clinical question is whether this new treatment benefit the patients, has no effect or is harming patients. The researchers are focused on whether the difference is small or big. For example, if the new drug improved the left ventricular ejection fraction by 15% with respect to placebo, is that 15% improvement meaningful? To answer these questions, there are calculations which measure the effect size. These measures include the standardized mean difference, relative risk (RR), odd's ratio (OR), NNT, and number needed to harm.
Definition of Terms   
Absolute risk (AR) = The number of events in treated or control groups, divided by the number of people in that group.
 ARC = the AR of events in the control group
 ART = the AR of events in the treatment group
 AR reduction (ARR) = ARC − ART
 RR = ART/ARC. RR is the ratio of the probability of an event occurring in an exposed/treatment group to the probability of the event occurring in a control group
 The NNT is the inverse of the ARR
 An OR is a measure of association between an exposure and an outcome. It represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.
Odds ratio = (Rate group 1/[1 − Rate group 1]) ÷ (Rate group 2/[1 − Rate group 2]).
Understanding the Calculation for Number Needed to Treat   
Let us suppose, two treatments are administered in a randomized control trial. Rate or proportions of events for both drugs are found to be p1 and p2, respectively. Difference in the probability/rate (p1p2) of both drugs is defined as ARR which expresses in absolute terms the benefit or harm of the drug tested over the control treatment. RR, OD and RR reduction are other measures to stipulate the benefit or harm of the new drug treatment. At the same, NNT is suggested as the measure to express the benefit of the new treatment.^{[3]} The inverse of AR reduction is defined as NNT, that is, NNT = 1/ARR.
An Example   
Let us suppose, in a doubleblind, randomized, placebocontrolled clinical trial, Valsartan 160 mg/day treatment was randomly given to 35 hypertensive patients and placebo treatment was given to 37 patients. The effect of the drug was measured through ambulatory blood pressure (BP) monitoring. It is found that three patients in the placebo arm showed lowering of BP as compared to 11 patients on Valsartan (P< 0.05, Fischer's exact test).
If we calculate the proportion of patients with lowering BP: Proportions are p1 = 11/35 = 0.314 for Valsartan and p2 = 3/37 = 0.081 for placebo. The AR reduction, ARR = p1p2 = 0.233, which measures the absolute gain due to the new drug. We can calculate OR (0.314/0.686)/(0.081/0.919) = 5.19.^{[4]} With a RR of 3.87 (p1/p2 = 0.31/0.80), probability of lowering of BP is about four times more than that of control treatment. NNT calculated as 1/0.233 = 4.29, means that four patients must be treated with Valsartan to benefit one extra patient as compared to control treatment.
Another Example   
Let us assume that in a cohort of patients with heart failure, a new drug has to be tested called TEST DRUG. We do a study where all the patients get the best medical care, including a PLACEBO in group 1 and get the best medical care and TEST DRUG in group 2. After 6 months, the mortality in the PLACEBO arm is 5.9% and in the TEST DRUG arm is 2.6%. Hence, the TEST DRUG reduced Deaths by 3.3%. The numbers needed to treat (ARR = 3.3% =3.3/100 = 1/30> NNT = 1/ARR = 30) is 30. So you have to treat 30 additional patients to prevent one death.
Drawbacks of Number Needed to Treat   
 It is described as a single number; and therefore, some investigators have started describing a confidence interval (e. g., NNT 5 (95% confidence interval 3–9)
 It does not account for a patient's baseline risk
 The NNT of a given treatment will be different when it is compared to different drugs or placebo and so it cannot be used as a single value
 The time frame of a given study should also be mentioned. For example, if a treatment was conducted over a mean of 4 years, its NNT should be expressed with the same time component.^{[5],[6]}
Online Calculator for Number Needed to Treat   
There are many online calculators available in the market, GraphPad software is one of them providing all sorts of statistical calculators for the investigators. Simply insert the number of patients benefited for control and new treatment with their total numbers of trials for each.^{[7]} For the abovementioned example, you may see the result in [Figure 1].  Figure 1: GraphPad NNT calculator overview for the example used in this article.
Click here to view 
Conclusion   
The “NNT” is a simple concept which is a measure of the impact of an intervention or a medicine. It shows the impact by calculating the number of patients, which need to be treated to affect the outcome of one patient. The advantage is that it is simple to understand, can be easily calculated from the ARR, and gives a clear idea of the impact of an intervention. NNT is the inverse of the ARR. ARR is the Control Event Rate – Experimental Arm Event Rate.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References   
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3.  Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 1988;318:172833. [ PUBMED] 
4.  Hutton J. Number needed to treat: Properties and problems. J R Stat Soc Ser A 2000;163:40319. 
5.  McQuay HJ, Moore RA. Using numerical results from systematic reviews in clinical practice. Ann Intern Med 1997;126:71220. [ PUBMED] 
6.  Altman DG. Confidence intervals for the number needed to treat. BMJ 1998;317:130912. [ PUBMED] 
7.  
[Figure 1]
