Efficacy

Efficacy is the extent to which a particular health technology (medical device, drug, procedure, health program, or health service, including interventions) does what it is intended to do (i.e., a beneficial health outcome or result) under ideal conditions. For instance, clinical trials, which are conducted with a selected population and during which subjects may be monitored more closely than they would be in a clinical practice, can establish the efficacy of a health technology.

In fact, these ideal conditions are similar to those sought in conducting experimental research. Therefore, efficacy can be determined or estimated on the basis of the design, the analysis, the conduct, and the results of randomized controlled trials. The characteristics of properly designed and conducted randomized controlled trials make it possible to attain the highest possible control of variables, and to minimize biases as well as other threads to their internal validity. For these reasons, this type of study design can reproduce the closest conditions to the ideal. Of course, the results obtained under the artificial conditions of a clinical trial may not be replicated in ordinary clinical practice, which explains why the efficacy of a treatment is often higher than its efficiency.

Efficacy is calculated using defined endpoints that are primary or secondary events observed in a patient during the course of a treatment or in a health program during its implementation. Examples of endpoints used to estimate efficacy in randomized trials include 1-, 3-, or 5-year survival after the administration of chemotherapy; the number of people immunized after the administration of a vaccine to a group of individuals at risk of becoming infected; the death rate at the end of a randomized trial in patients receiving a drug for lowering hypertension (compared with the death rate of those receiving placebo or other drugs in the control arm of the trial); reduction of the mortality rate from myocardial infarction at the end of the Multiple Risk Factor Intervention Trial; the viral load and the adherence rate among nonadherent HIV-infected patient groups receiving different prescribed regimes of antiretrovirals; and the 24-hr pain relief response to sumatriptan and naproxen in a double-blind, two-arm controlled trial to determine the efficacy of sumatriptan for the acute treatment of migraine.

It is important to clearly distinguish among efficacy, effectiveness (results obtained under realistic conditions, such as in regular medical practice), and efficiency (the relationship between the cost or the resources used to provide a specific treatment, intervention, program, or procedure and the end results obtained), since they are often confused. The basic difference between efficacy and effectiveness of a given medical technology lies in the conditions under which it is provided and estimated: Efficacy refers to benefits when it is deployed under ideal conditions and effectiveness when it is provided under field conditions—that is, those encountered in clinical practice.