EMP 194
Therapeutic Index
Definitions:
Therapeutic index: The difference between the toxic level and the minimum therapeutic concentration of a medication in a patient’s bloodstream. This value is usually obtained in the laboratory. For a medication to be effective and safe it must reach a certain concentration at the target tissue.
Minimum therapeutic concentration: The minimal concentration of a drug necessary to cause the desired response. A concentration below the minimum therapeutic concentration will not induce a clinical response. AKA: therapeutic threshold.
Toxic Level: The point at which the drug concentration in the bloodstream gets high enough to be considered toxic or even fatal.
Goal of Medication Administration:
The general goal of drug therapy is to give the minimum concentration of a drug necessary to obtain the desired response.
Examples:
Drugs such as Narcan, a narcotic antagonist, have a significant margin between the effective dose and the toxic dose and are said to have a “high therapeutic index”.
Digitalis, a common medication for A. Fib, is a medication that has a very small difference between the effective dose and the toxic dose. It is said to have a “low therapeutic index”. For this reason, individuals on digitalis must have their blood levels monitored frequently and their medication adjusted by the physician
Acyclovir is an antiviral agent to treat herpes. It is eliminated from the body very quickly and thus patients must take the medication up to six times a day to maintain blood levels within the therapeutic index, otherwise their levels can drop below the therapeutic threshold. Acyclovir is said to have a “fast clearance rate”. This explains why some medications must be taken several times a day to be effective.