A project of the New Mexico AIDS Education and Training Center. Partially funded by the National Library of Medicine
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Fact Sheet Number 126
HIV RESISTANCE TESTING
WHAT IS RESISTANCE?
HIV is “resistant” to a drug if it keeps
multiplying rapidly while you are taking the
drug. Changes (mutations) in the virus
HIV mutates almost every time a new copy
is made. Not every mutation causes
resistance. The “wild type” virus is the most
common form of HIV. Anything different
from the wild type is considered a mutation.
An antiretroviral drug (ARV) won’t control a
virus that is resistant to it. It can “escape”
from the drug. If you keep taking the drug,
the resistant virus will multiply the fastest.
This is called “selective pressure.”
If you stop taking medications, there is no
selective pressure. The wild type virus will
multiply the fastest. Although tests may not
detect any drug resistance, it might come
back if you re-start the same drugs.
testing helps health care
providers make better treatment decisions
for their patients.
HIV usually becomes resistant when it is not
totally controlled by drugs someone is
taking. However, more people are getting
infected with HIV that is already resistant to
one or more ARVs.
The more that HIV multiplies, the more
mutations show up. These mutations
happen by accident. The virus doesn’t
“figure out” which mutations will resist
Just one mutation can make HIV resistant to
some drugs. This is true for 3TC (Epivir) and
the non-nucleoside reverse transcriptase
inhibitors (NNRTIs). However, HIV has to go
through a series of mutations to develop
resistance to other drugs, including most
The best way to prevent resistance is to
control HIV by taking strong ARVs. If you
miss doses of your medications,