Efficacy is the capacity to produce an effect. It is these conditions that distinguish efficacy from the related concept of effectiveness, which relates to change under real-life conditions.

Context

Healthcare

In a healthcare context, efficacy indicates the capacity for beneficial change (or therapeutic effect) of a given intervention (e.g. a medicine, medical device, surgical procedure, or a public health intervention).

If efficacy is established, an intervention is likely to be at least as good as other available interventions, to which it will have been compared. Comparisons of this type are typically made in 'explanatory' randomized controlled trials, whereas 'pragmatic' trials are used to establish the effectiveness of an intervention.

The concept of 'self-efficacy' is an important one in the self-management of chronic diseases because doctors and patients often do not follow best practice in using a treatment. For instance, a patient using combined oral contraceptive pills to prevent pregnancy may sometimes forget to take a pill at the prescribed time; thus, while the perfect-use failure rate for this form of contraception in the first year of use is just 0.3%, the typical-use failure rate is 8%.[1]

Pharmacology

Semi-log plots of two agonists with different K_d.

In pharmacology, intrinsic activity or efficacy refers to the ability of a drug-receptor complex to produce a functional response. This must be distinguished from the affinity, which is a measure of the ability of the drug to bind to its molecular target, and the EC50, which is a measure of the potency of the drug and which is proportional to both efficacy and affinity. This use of the word "efficacy" was introduced by Stephenson (1956)^[1] to describe the way in which agonists vary in the response they produce, even when they occupy the same number of receptors. High efficacy agonists can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors in that system. Agonists of lower efficacy are not as efficient at producing a response from the drug-bound receptor, by stabilising the active form of the drug-bound receptor. Therefore, they may not be able to produce the same maximal response, even when they occupy the entire receptor population, as the efficiency of transformation of the inactive form of the drug-receptor complex to the active drug-receptor complex may not be high enough to evoke a maximal response. Since the observed response may be less than maximal in systems with no spare receptor reserve, some low efficacy agonists are referred to as partial agonists^[2]. However, it is worth bearing in mind that even partial agonists may appear full agonists in a different system/experimental setup, as when the number of receptors increases, there may be enough drug-receptor complexes for a maximum response to be produced, even with individually low efficacy of transducing the response.

• agonist: affinity and high efficacy
• antagonist : affinity without efficacy
• partial agonist: affinity and low efficacy, in a system with a small number of receptors

Lighting

Main article: Luminous efficacy

In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of energy consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible energy as a ratio of the energy consumed in watts. The visible energy can be approximated by the area under the Planck curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total energy under the blackbody curve. Efficiency values for light from a heat source are typically less than two percent.

Difference amplifiers

The efficacy of a differential amplifier is measured by the degree of its rejection of common-mode signals in preference to differential signals. Referred to as common-mode rejection ratio (CMRR); typically specified in decibels.

See also

• Placebo (origins of technical term)
• Vaccine efficacy
• Efficiency
• Figure of merit
• Self-efficacy
• Effectiveness

References

1. Stephenson RP 1965 A modification of receptor theory. Br J Pharmacol 11:379-393
2. Glaxo Wellcome pharmacology guide

Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Efficacy". Image Credit.

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