Sunday, March 4, 2018

PCR tests for HIV

The polymerase chain reaction (PCR) is a method of amplifying a sample of DNA exponentially.

Can be used for:
1. Detection of viral DNA in the patient (for diagnosis.)
2. Detection of HIV RNA (as a measure of viral load - track response to therapy.)
3. Detect mutations in the HIV viral DNA (for determining source of therapy failure.)


1. Detection of viral DNA in the patient (for diagnosis):

Do we routinely test viral DNA for diagnosis?
No! Why? Because there are disadvantages to the polymerase chain reaction:
1. It is too expensive and labor-intensive.
2. It can give false negative results when certain subtypes of HIV-1 are present in the patient. **Read more below.

When do we use HIV DNA  PCR in clinical settings?
Qualitative HIV DNA PCR tests have three main diagnostic applications:
1. Direct detection of viral sequences in the pre-seroconversion window period which may be positive up to 8 days prior to the development of HIV specific antibodies.
2. Resolution of indeterminate HIV serological tests.
3. In the diagnosis of neonates born to seropositive mothers where maternal antibodies may be detectable for up to 15 months postpartum.

2. Detection of HIV RNA (as a measure of viral load - track response to therapy):

Tracking response to therapy:
Plasma HIV RNA should be measured in all patients at baseline and regularly during therapy since it is the most reliable indicator of response to antiretroviral therapy (ART) and is useful in predicting clinical progression. The goal of therapy is viral load suppression below the limits of assay detection.

Why don't we use HIV RNA testing for diagnosis?
1. Viral load tests approved by the US Food and Drug Administration (FDA) only detect HIV-1.
2. It may give false-positive results.

3. Detect mutations in the HIV viral DNA (for determining source of therapy failure):

Some genotypic resistance assays are designed to analyze HIV-1 proviral DNA located in host cells. These tests can detect drug resistance mutations within proviral HIV DNA archived within peripheral blood mononuclear cells or determine viral tropism.

Proviral DNA resistance testing may be useful when switching a patient's ART regimen when the HIV viral load is nondetectable and/or there are no prior resistance results available. As an example, this assay may be helpful when changing a patient with undetectable HIV RNA to a new regimen that contains maraviroc. If only R5 virus is detected, maraviroc could be considered for use in the new regimen. However, the clinical utility of proviral DNA resistance testing or proviral DNA tropism detection has yet to be fully determined.

**About false negative results when certain subtypes of HIV-1 are present in the patient: Although it has high specificity, this only works under certain conditions. The length of the target DNA sequence is a factor in the level of specificity. If the target sequence is too long then it is difficult to ensure that the target sequence will be copied in its entirety and accurately. Also if a primer sequence appears more than once in a DNA molecule then the primer may attach to more than one area, causing the experiment to copy more than one sequence or the wrong sequence completely. This can occur when certain subtypes of HIV-1 are present in the patient, causing false negative results. This is apparent in a documented case of an English tourist who contracted HIV while on holiday in Thailand. The patient displayed the symptoms associated with HIV and so was tested. HIV-1 proviral DNA PCR test where taken approximately 20 days and 26 days after infection and repeatedly gave negative results. Only 33 days after infection did the HIV-1 proviral DNA PCR test give a positive result. Due to this discovery there has been caution about using PCR for detection of HIV and it is recommended that any PCR results should be followed up by a conventional method of diagnosis. (Source here: How the Polymerase Chain Reaction is used in Medicinal Science and the Diagnosis of HIV)

That's all!

I wrote this completely random post just out of curiosity - Ask away questions!

-IkaN

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