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HIV Test after 21 days : which test to do

HIV Test after 21 days : which test to do

Learn which test has the best chance of diagnosing an HIV infection , 21 days after exposure. In depth analysis of 3rd generation, 4th generation and RNA PCR tests as well as their likelihoods of showing positive.
Updated Date : 2023-09-09T20:54:05.696+00:00

How does the HIV virus multiply inside the body

In response to exposure to the HIV virus, replication ensues within submucosal cells. CD4 helper T cells, integral to immune function, become involved as the virus establishes contact. This interaction prompts the infiltration of viral RNA, the virus's genetic material, into these CD4 cells. The viral RNA is subsequently converted into viral DNA through the function of reverse transcriptase. The viral DNA takes command over the cellular mechanisms, leading to the production of an augmented number of viral particles. Ultimately, the infected cell reaches its limit, culminating in its rupture and the release of newly formed virions into the bloodstream.

What viral particles are in the body around 21 days after exposure?

At 21 a large percentage of patients will show the presence of the viral RNA using molecular assays, as well as the p24 antigen by means of lab assays. A large group of patients may also show the IgM antibodies.

The HIV Virus comprises of four detectable markers : HIV RNA, HIV P24 antigen, HIV IGM Antibodies and HIV IGG ANTIBODIES

Marker Description Day at First Detectable Day at Peak
HIV RNA A sign that the virus is active and multiplying in your body. Around 12 days Between 20 and 30 days
p24 Protein A protein produced by the virus, showing that its growing. About 15 days Between 25 and 30 days
IgM Antibodies Early soldiers your body sends to fight the virus. Around 20 days Between 30 and 35 days
IgG Antibodies Experienced soldiers your body creates for long-term defense. Between 30 and 35 days Not applicable

What is a window period?

In HIV diagnosis, the window period corresponds to the duration required for the immune system to generate detectable antibodies that can be identified through testing.

Which are the different tests that can be done to detect HIV?

Testing for HIV involves diverse methods that seek out various components of the virus. These methods include tests that pinpoint the p24 antigen, antibodies like IgM and IgG, and the utilization of RNA PCR for enhanced sensitivity.

Third Generation Tests

3rd generation tests are so called because they detect antibodies to the HIV 1 and HIV 2 strains. These tests are available in two formats - lab based automated assays, which automate the processes of pipetting and dispensing and point of care assays that can be done using standard manual pipetting.

Test Type Description Test Names
3rd Generation 3rd generation tests detect antibodies against HIV.
  • ADVIA Centaur HIV 1/O/2 Enhanced (Siemens): A lab-based automated test that detects antibodies to HIV-1 and HIV-2.
  • GS HIV-1/2 PLUS O EIA (Bio-Rad): This lab-based test identifies antibodies against HIV-1 and HIV-2.
  • VITROS Anti-HIV 1+2 (Ortho): An automated test for detecting antibodies to both HIV-1 and HIV-2.
  • INSTI HIV-1/HIV-2 Rapid (BioLytical, Point of Care): A point-of-care test that rapidly detects antibodies to HIV-1 and HIV-2.
Fourth Generation Tests

4th generation tests are so called because they detect both the HIV P24 antigen as well as antibodies to the HIV 1 and HIV 2 strains. These tests are available in two formats - lab based automated assays, which automate the processes of pipetting and dispensing and point of care assays that can be done using standard manual pipetting.

Test Type Description Test Names
4th Generation 4th generation tests detect both antigen and antibodies.
  • ADVIA Centaur HIV Ag/Ab Combo: A lab-based automated test that combines antigen and antibody detection for accurate HIV diagnosis.
  • ARCHITECT HIV Ag/Ab Combo (Abbott): Another lab-based automated test known for its reliability in detecting HIV antigen and antibodies.
  • BioPlex 2200 HIV Ag-Ab (Bio-Rad): This lab-based test utilizes a multiplex technology to simultaneously identify HIV antigen and antibodies.
  • Alere Determine (Point of Care): A point-of-care test that provides quick results, detecting both HIV antigen and antibodies.
Will a 4th generation lab based test detect HIV at 21 days?

Will a 4th generation lab based test detect HIV at 21 days?

There is a moderate chance of a 4th generation lab based test being able to detect the virus in your body on day 21 after exposure to the HIV virus. This is because the analytes measured by this test, namely - p24 antigen, IgM and IgG antibodies are not yet present in adequate concentration in your blood by day 21 of the infection.

The 4th generation HIV test is a modern screening method that simultaneously detects the HIV p24 antigen and both IgM and IgG antibodies in a single test..Lab based assays as described above are fully automated and are considered to be one of the most sensitive tests for early diagnosis of HIV. If you were exposed to HIV, there is a 48.8% chance that the 4th generation lab based assay will be able to detect the virus at 21 days. This is based on the varying levels of the p24 antigen as well as the time it takes for the antibodies to develop in the body agains the virus. It is recommended that you retest after some days since the chances of detecting HIV increase with time.

Will a 4th generation Point of Care based test detect HIV at 21 days?

Will a 4th generation Point of Care based test detect HIV at 21 days?

There is a moderate chance of a 4th generation point of care based test being able to detect the virus in your body on day 21 after exposure to the HIV virus. This is because the analytes measured by this test, namely - IgM and IgG antibodies are not yet present in adequate concentration in your blood by day 21 of the infection.

.Point of care assays as described above are generally performed with methods like lateral flow immunochromatography and are well suited for low resource and bedside settings. These assays have now achieved comparable results to lab based assays. If you were exposed to HIV, there is a 47.3% chance that a 4th generation point of care based assay will be able to detect the virus at 21. This is based on the varying levels of the p24 antigen as well as the time it takes for the antibodies to develop in the body agains the virus. It is recommended that you retest after some days since the chances of detecting HIV increase with time.

Will a third generation lab based assay detect HIV at 21 days?

Will a third generation lab based assay detect HIV at 21 days?

There is a moderate chance of a 3rd generation lab based test being able to detect the virus in your body on day 21 after exposure to the HIV virus. This is because the analytes measured by this test, namely - IgM and IgG antibodies are not yet present in adequate concentration in your blood by day 21 of the infection.

An established approach to HIV testing, the 3rd generation test identifies both IgM and IgG antibodies against the virus, aiding in its diagnosis..Third generation assay only test for the IgM and IgG antibodies, as such the latency for positivity with these assays is greater. Third generation lab based assays use automation in various steps of the test and as such are considered to be more accurate than the point of care tests, however they require more expensive equipment. If you were exposed to HIV, there is a 43.8% chance that a 3rd generation lab based assay will be able to detect the virus at 21 days. This is based on the time it takes for the antibodies to develop in the body agains the virus. It is recommended that you retest after some days since the chances of detecting HIV increase with time.

Will a third generation point of care based assay detect HIV at 21 days?

Will a third generation point of care based assay detect HIV at 21 days?

There is a moderate chance of a 3rd generation point of care based test being able to detect the virus in your body on day 21 after exposure to the HIV virus. This is because the analytes measured by this test, namely - IgM and IgG antibodies are not yet present in adequate concentration in your blood by day 21 of the infection.

.Third generation assay only test for the IgM and IgG antibodies, as such the latency for positivity with these assays is greater. Third generation point of care based assays use lateral flow chromatography or similar techniques and are suited for low resource settings. If you were exposed to HIV, there is a 34.9% chance, that a 3rd generation point of care based assay will be able to detect the virus at 21 days. This is based on the time it takes for the antibodies to develop in the body agains the virus. It is recommended that you retest after some days since the chances of detecting HIV increase with time.

Will a RNA PCR test detect HIV at 21 days?

Will a RNA PCR test detect HIV at 21 days?

There is a good chance of a RNA PCR Molecular Test being able to detect the virus in your body on day 21 after exposure to the HIV virus.

RNA PCR assays can detect viral loads as low as 20 copies of viral RNA/ml. RNA PCR assays can detect the virus as early as 10-12 days, and almost all patients will show positivity between 20-30 days.Based on the fact that you have been exposed 21 days ago, there is a 87.4% chance of the RNA PCR test being able to detect the virus.You should retest after some days, for confirmation.

What is the confirmatory test for HIV at 21?

There is no 100% confirmatory test for HIV. Any negative test is recommended to be repeated after 45-90 days. Positive tests are usually reconfirmed on repeated sampling in accordance with the guidelines of your country's health organizations. While the RNA PCR test usually has the highest chance of detecting the virus, it is never recommended as a first line test, owing to its cost and complexity. A first line screening test is usually done (4th generation) and if it is positive, additional testing is recommeded.

References

  • Hurt CB, Nelson JAE, Hightow-Weidman LB, Miller WC. Selecting an HIV Test: A Narrative Review for Clinicians and Researchers. Sex Transm Dis. 2017 Dec;44(12):739-746. doi: 10.1097/OLQ.0000000000000719. PMID: 29140890; PMCID: PMC5718364.Hurt et,al
  • Kevin P. Delaney and others, Time Until Emergence of HIV Test Reactivity Following Infection With HIV-1: Implications for Interpreting Test Results and Retesting After Exposure, Clinical Infectious Diseases, Volume 64, Issue 1, 1 January 2017, Pages 53–59,Delaney et.al

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Dr.Bhargav Raut is a qualified Pathologist, with over 5 years of experience in the field
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