A project of the New Mexico AIDS Education and Training Center. Partially funded by the National Library of Medicine
Fact Sheets can be downloaded from the Internet at http://www.aidsinfonet.org
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Fact Sheet Number 127
DRUG LEVEL TESTING
It can be helpful to test a patient’s blood
to check the levels of a medication they
are taking. Drug levels that are too high
sometimes cause serious side effects.
Levels that are too low might allow HIV to
multiply and develop resistance. Testing
drug levels is also called therapeutic
drug monitoring, or TDM.
TDM is not generally used or available yet
in the United States. Doctors do not
agree on its benefits.
HOW CAN TDM HELP?
Even when people take the same dose of
a drug, blood levels can be very different,
If the viral load isn’t going down far
enough, it might be because drug levels
are too low. A doctor might be able to
increase the dose and bring HIV under
If a patient is having serious side effects,
it might be because drug levels are too
high. If they are, a smaller dose might still
control HIV but relieve some side effects.
Several factors can affect drug levels:
• Food effects: more or less drug can
be absorbed depending on the amount
and kind of food in the stomach
• Body weight: very low weight can
increase drug levels. Very high body
weight may reduce drug levels.
• Metabolism: Different people break
drugs down faster or slower. This is
partly due to genetic factors.
• Age: children and adolescents process
drugs differently than adults.
• Drug interactions: some drugs affect
the metabolism of other drugs, and can
raise or lower their levels.
• Smoking and drinking habits
• Herbal and other supplements. For
example, St. John’s Wort reduces
blood levels of protease inhibitors.
• Kidney or liver problems, including
hepatitis, can cause higher drug levels
• Pregnancy: as body size changes,