There is life with HIV

Is P24 Antigen in Fourth Generation HIV Tests Reliable?

Testes de quarta geração para HIV com Antígeno P24. São Precisos?

Is P24 Antigen in Fourth Generation HIV Tests Reliable? Many people ask me this and the answer is always the same. Yes. But people don't seem to accept the logic, the facts, the everyday truth proved by science. I believe that the same stubbornness generates “influencers”Anti-vaccines, who do not believe in global warming, and even those who, crazy maximum, among all the follies, claim that the land is flat and supported by four elephants. Jesus, help them!

So, and you can continue to search for me on WhatsApp, I ended up researching and translating this text. To make your life easier. May the answer to this question be good for you:


Is P24 Antigen in Fourth Generation HIV Tests Reliable?


Are fourth-generation tests that use the P₂₄ antigen in HIV tests reliable? Is it accurate, is it accurate?  Os Combined, fourth-generation rapid HIV tests offer greater accuracy?

Well, these are the recommended tests for routine use in the UK, a wealthy nation!

How accurate are the 4th generation combined tests with P24 Antigen for HIV diagnosis? 


Important things to know about P24 Antigen in Fourth Generation Tests

  • These are the tests recommended for routine use in the UK.
  • They test the p24 antigen as well as the antibodies, so they are more suitable for detecting recent infections.
  • Their sensitivities and specificities are excellent. In theory, and I reinforce this, this is the most accurate and reliable HIV diagnostic test used routinely. After all, it is a laboratory test that can detect anti-HIV antibodies e p24 antigen.
  • Understand that they are called combined tests, because they detect both substances. These are “fourth generation” tests, with modern technology and better than previous generations.

The fourth generation HIV test looks for immune responses to HIV infection. 

Well, antibodies to HIV are produced by the human body in response to HIV infection. However, weeks after exposure to HIV, the immune system recognizes some components of the virus. Therefore, the P24 antigen in fourth-generation HIV tests can be trusted.

You look for this information, in general, because of things like this link exposes, fear after unprotected sex!

Antígeno P24 Em Testes De Quarta Geração And so, it starts to generate HIV antibodies, in order to damage it, neutralize it or kill it.

Therefore, this period is known as the 'seroconversion' period, and such antibodies persist throughout life. And this way, as much as I wanted to say something different to you, and that’s why, even if you become undetectable, and that was never a problem for me or Mara, your HIV test will always be reactive

A p24 antigen and the 4th generation test.

It is a structural protein that makes up most of the HIV viral core. Keep in mind that high levels of p24 are present in the blood during the short period between HIV infection and seroconversion, before disappearing.

So, after a short window of time, they can become inaccurate! In this way, as the antigen p24 is usually detectable a few days before antibodies to HIV, a test that can detect p24 has a slightly shorter window period.

Smaller, in fact than a test window that detects “only” antibodies!

Fourth Generation HIV Tests.

Combined fourth-generation tests are usually performed on plasma or serum. And this is where the property of expression is born seropositive.

Two blood components, separated from whole blood using laboratory equipment. A blood sample is collected through a needle from a vein in the arm.

Samples from many individuals are analyzed simultaneously, on a laboratory machine.

"Like any screening test, a reactive result - reagent - a preliminary positive result must be verified with confirmatory tests."

These tests can also be called “fourth generation” or ELISA (enzyme immunoabsorbent assay). The first and second generation laboratory tests are no longer in use. The third generation laboratory test detects only antibodies and is no longer recommended for routine use in the UK.

Types and Brands of Tests.

Tests of this type include: Abbott Architect HIV Ag / Ab combo, ADVIA Centaur HIV Ag / Ab Combo, Genscreen ULTRA HIV Ag-Ab, VIDAS HIV panel e Vitros HIV combo. In product names, 'Ag / Ab' indicates that the test looks for antigen and antibody.

Some tests can also be described as the fifth generation.

They detect the same biomarkers.

But they can also distinguish between samples positive for the p24 antigen and antibodies. As well as between HIV-1 and HIV-2.

And include the Bio-Rad BioPlex 2200 HIV Ag / Ab e Elecsys HIV Duo.

A quick test of combination of service points is also available, the Alere HIV Combo. There is information about this test on another page.

Immune Window periods and accuracy

Antígeno P24 Em Testes De Quarta Geração The period of the immunological window comprises the time immediately after infection, during which the tests are not always able to detect antibodies or p24 antigen, therefore, they can give a result false negative.

It is difficult to say exactly how long the immunological window period of any tests lasts, as there are variations between individuals and it is a difficult topic to research, as recently infected people would need know exactly when they were exposed to HIV and then they will give blood multiple times.

And there will be multiple samples over the next few days and weeks.

However, it is estimated that the average window period for fourth generation tests is 18 days when the P24 Antigen is used in Fourth Generation Tests 

With half of all infections being detected between 13 and 24 days after exposure.

Occasionally this period is a little longer, 99% HIV-infected individuals would have detectable responseswithin 44 days after exposure!

And that, readers, clarifies, and well, the reasons for the 60-day follow-up prescribed by the Brazilian Health System!

My Apology

And although I do not feel entitled to point fingers, it is up to me to apologize. Misplaced information ended up giving me opportunity and unfortunate opportunities to misinform. The only thing I can say about it is that I am very sorry and ashamed of it.

However, this text deals with a situation in a different, much richer country.

Therefore, the UK guidelines claim that a laboratory test for antibodies / antigens will detect the majority of HIV-infected individuals four weeks after specific exposure.

The fourth generation laboratory tests are extremely accurate.

But not infallible!

The main precision measures are a sensitivity, the percentage of results that are correctly positive when HIV is actually present.

And in specificity, the percentage of results that are correctly negative when HIV is not present.

Rationalizing in sensitivity, One review of the Center for Disease Control and Prevention (CDC) identified four studies from two different trials, with sensitivity always above 99,7% for established infection.

99% efficiency for testing with P24 Antigen in Fourth Generation Tests 

 An evaluation by the Health Protection Agency found that nine of the ten tests evaluated had a sensitivity of 100%, while a french assessment found that ten of the twelve tests had a sensitivity of 100%. The lowest sensitivity was 99,8%.

Similar results were found for specificity!

Both tests verified by the CDC had a specificity of 99,5% or more.

All tests in the evaluation from the Health Protection Agency had a specificity of 99,7% or more, and the french study found that everyone the tests produced after the year 2000 had a specificity of 99,8% or more.

Like any screening test, a reactive result, or reagent, a preliminary positive result, must be verified with confirmatory tests.

Confirmation tests are always necessary.

The laboratory must retest the sample with a different test. Even though it was done with the P24 Antigen in Fourth Generation Tests; then, a new sample must be collected and tested, in order to rule out an administrative error, such as two patient samples being mixed.

Test results should be available in busy clinics and in urgent cases within a day or two.

But other patients may be asked to wait longer. The main delay in obtaining results is due to the fact that the samples are analyzed in batches.

And it can take a laboratory several days to accumulate enough samples to be analyzed.

The British HIV Association says the test results should generally be available to the doctor who ordered the test in five days and to the person who takes the test in ten days. THESE ARE MINUTES OF THE UNITED KINGDOM

Would Fourth Generation tests be more reliable?

The fourth generation tests are extremely sensitive and specific. Are more reliable in the early detection of infections, than quick tests.

And they are suitable for use when providing immediate results.

It is not necessary and when people are likely to return to get their results. As the tests are carried out by a central laboratory, quality control programs are easier to implement. 

Maybe you should go to this text, about immunological window. He can be good for you! It opens in this same tab and is a frequently updated text. 

Translated by Original Claudio Souza in How accurate are fourth-generation combination tests for HIV diagnosis?

Anyway, the answer to the question Tests with the P24 Antigen In Fourth Generation Tests Are Precise The answer is yes!

This page was last revised in June 2019. It should be revised in June 2022.


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