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14-September-2008 10:43:23 - 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 in order to prevent one additional bad outcome i.e. to reduce the expected number of cases of a defined endpoint by one. It is defined as the inverse of the absolute risk reduction. It was described in 1988.1 Contents 1 Derivation 2 Relevance 3 Example: statins for primary prevention 4 Worked example 5 See also 6 References 7 External links Derivation In general, NNT is computed with respect to two treatments A and B, with A typically a drug and B a placebo e.g., A might be a 5-year treatment with a drug, while B is no treatment. A defined endpoint has to be specified e.g., the appearance of colon cancer in a five-year period. If the probabilities pA and pB of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/pB - pA. Relevance The NNT is an important measure in pharmacoeconomics. If a clinical endpoint is devastating enough e.g. death, heart attack, drugs with a high NNT may still be indicated in particular situations. If the endpoint is minor, health insurers may decline to reimburse drugs with a high NNT. Example: statins for primary prevention For example, the ASCOT-LLA manufacturer-sponsored study addressed the benefit of atorvastatin 10 mg a cholesterol-lowering drug in patients with hypertension high blood pressure but no previous cardiovascular disease primary prevention. The trial ran for 3.3 years, and during this period the relative risk of a primary event heart attack was reduced by 36%. The absolute risk reduction ARR, however, was much smaller, because the study group did not have a very high rate of cardiovascular events over the study period: 2.67% in the control group, compared to 1.65% in the treatment group.2 Taking atorvastatin for 3.3 years, therefore, would lead to an ARR of only 1.02% 2.67% minus 1.65%. The number needed to treat to prevent one cardiovascular event would then be 99.7 for 3.3 years.3 4 Worked example Abbreviation Variable Equation Value - subjects in control group - 250 - subjects in experimental group - 150 - events in control group - 100 - events in experimental group - 15 CER control event rate = events / subjects in control group 0.4, or 40% EER experimental event rate = events / subjects in experimental group 0.1, or 10% ARR absolute risk reduction or increase = CER - EER 0.3, or 30% RRR relative risk reduction or increase = CER - EER / CER 0.75 NNT number needed to treat/number needed to harm = 1 / ARR 3.33 OR, RR odds ratio, relative risk not really identical, but similar -- see articles for details = CER / EER 4 The relative risk odds ratio is .25 in the example above. It is always 1-relative risk reduction, or vice versa. See also Number needed to harm - the converse for side-effects References ^ Laupacis A, Sackett DL, Roberts RS 1988. An assessment of clinically useful measures of the consequences of treatment. N. Engl. J. Med. 318 26: 1728-33. PMID 3374545. ^ Sever PS, Dahlöf B, Poulter NR, et al 2003. Prevention of coronary and stroke events with atorvastatin in hypertensive patients who have average or lower-than-average cholesterol concentrations, in the Anglo-Scandinavian Cardiac Outcomes Trial--Lipid Lowering Arm ASCOT-LLA: a multicentre randomised controlled trial. Lancet 361 9364: 1149-58. doi:10.1016/S0140-67360312948-0. PMID 12686036. ^ Bandolier - Statin effectiveness: ASCOT update. Retrieved on 2008-03-31. ^ Do Cholesterol Drugs Do Any Good?, Business Week. Retrieved on 2008-03-31. 'That means in a large clinical study, 3% of patients taking a sugar pill or placebo had a heart attack compared to 2% of patients taking Lipitor.' ... The numbers in that sentence mean that for every 100 people in the trial, which lasted 3 1/3 years, three people on placebos and two people on Lipitor had heart attacks.... One fewer heart attack per 100 people. So to spare one person a heart attack, 100 people had to take Lipitor for more than three years. The other 99 got no measurable benefit. Or to put it in terms of a little-known but useful statistic, the number needed to treat or NNT for one person to benefit is 100. External links EBEM's Calculator for NNT Bandolier article on NNT Number needed to treat Slate What is an NNT? Hayward Medical Communications Number Needed to Treat Centre for Evidence Based Medicine Online Calculator for NNT GraphPad Software Retrieved from http://en..org/wiki/Number_needed_to_treat Categories: Pharmacology | Epidemiology Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Nederlands 日本語 This page was last modified on 18 August 2008, at 19:32
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