## TABLE 12 Computation of Relative Risk Reduction RRRAbsolute Risk Reduction ARRand Number Needed to Treat NNT

Presence of Rebleeding by Day 30 Yes No

RRR = (expected rate - experimental rate/expected rate = (c - a)/c = (27/120 - 8/120)/(27/120) = (22.5% - 6.6%)/22.5% = 15.9%/22.5% = 70.6%

ARR = (expected rate - experimental rate) = (c - a) = (27/120) - (8/120) = 22.5% x 6.6% = 15.9% NNT = 1/(ARR)

Adapted from Lau et al, 2000. PPI = proton pump inhibitor.

describes the additional portion of the experimental group who would benefit as a result of using the experimental therapy rather than the control therapy. The reciprocal of the ARR is the number needed to treat (NNT), which is the number of patients who must be treated with the experimental therapy to achieve one additional outcome of interest. To illustrate this, consider a trial where the outcome is death. Assume that the baseline risk of death is 1 out of 1,000 (0.1%) with the control therapy. If the risk of death with the experimental therapy is 0.05%, then the RRR is 50%, the ARR is 0.05%, and the NNT is 2,000. It is up to the clinician to decide, based upon the cost, potential harms, feasibility and patient acceptance, whether or not pursuing the experimental therapy is reasonable. Additionally, it is important to realize that the NNT only applies to the period of time over which the study was conducted. For the trial by Lau and colleagues (2000), the RRR of 70.6% with IV PPI indicates that these patients are 70.6% less likely to rebleed than patients treated with placebo. The ARR of 15.9% indicates that rebleeding episodes will be decreased to 6.6% with use of an intravenous PPI, down from the baseline risk of 22.5%. The NNT of 6.3 indicates that between 6 and 7 patients need to be treated with IV PPI to prevent one rebleeding episode within 30 days.

The decision whether or not to reject the null hypothesis (that two different interventions do not affect the outcome) is commonly reported as the p value. By convention, we reject the null hypothesis and conclude that different outcomes exist as a result of true differences in interventions if the p value is "statistically significant" or < .05. That is, there is a 99.5% chance that the observed outcome differences are due to differences in intervention rather than due to chance alone. Alternatively, there is a less than 0.5% chance that the outcome differences are due to chance alone. Although statistical significance is an important result, the accuracy of the results and the clinical significance of treatment options are also important considerations. One of the most common ways to report the accuracy of the results is 95% CIs. For the trial in question, the relative risk (RR) of rebleeding with IV PPI relative to placebo was 30%. The 95% CI associated with this RR is 14.5% to 64.7%. This simply means that if the same trial were conducted 100 times, the true RR of rebleeding with IV PPI relative to placebo would be between 14.5% to 64.7%, 95 times out of 100. The 95% CI tells us, within the range of plausibility, how much greater or smaller the "true" effect is relative to the measured effect. The narrower the 95% CI, the closer to the truth the measured effect is.

Although this study offers a concise answer to our question, several other issues must be considered. This study was not adequately powered to demonstrate differences in mortality between treatment groups, was performed in Asian patients, and used a different IV PPI than is available in the United States. It is therefore up to the clinician to determine whether or not these issues are important enough to prevent him or her from using a different, albeit similar, medication in his non-Asian patient. This highlights one of the limitations of evidence-based medicine. Individual patients and scenarios rarely match those of a randomized controlled trial. There is a separate chapter on upper gastrointestinal bleeding (see Chapter 28, "Upper Gastrointestinal Bleeding"). A cost effectiveness analysis is presented in Chapter 2, "Decision Analysis in the Management of Digestive Diseases."

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