Definition of Trough Level

trough level of drugsTrough Level is the amount of a drug in the blood circulation at the drug’s lowest therapeutic concentration.

Generally the trough level occurs immediately before the person is due to take the next dose of the drug.

The trough level helps the doctor determine if the dosage is appropriate to achieve the desired therapeutic effect and is useful information primarily at the onset of treatment. The goal of most medication therapy is a steady state of the drug’s concentration in the body, at which there is little difference between the drug’s peak level (highest concentration in the blood circulation) and trough level.

It is an especially important measure for narrow therapeutic index (nti) drugs (drugs for which the margin between therapeutic and toxic is very close) such as theophylline (to treat asthma), certain antibiotic medications, cyclosporine for immunosuppressive therapy, some antiseizure medications, and many antiarrhythmia medications.


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