P24 antigen

Do not let them confuse you with tests that shorten the window period!

Haste and impatience are increasingly present in our daily lives. This urgency is accentuated if you are waiting for is the moment of being able to perform a test to determine whether or not we are infected by the HIV virus. This means that when we are given a test able to drastically reduce the ” window period ” is something tremendously appealing. However, we must be careful and understand what they value and efficiency these tests to be sure that the result is valid.

There are two tests accepted by the FDA (Food and Drug Administration), the regulatory body in the U.S. of all diagnostic methods and drugs, and representing quality standard worldwide reference (www.fda.gov) to diagnose an infection inside the classic window period (3-6 months):

  • PCR.
  • Rating p24 antigen.

Before dealing with the study and comparison of the two tests, we see how the virus once it has managed to infect:

At the time when the virus enters the body, it multiplies, appreciable amounts appearing in blood. This is the feature that can detect the infection now , which as seen in the graph, the immune response is not yet active . Once the immune response is given, it is able to reduce the levels of virus in the blood , when it has passed the “window period” (initial red phase), and can detect the virus by assessing the production of antibodies by the immune response.

– PCR detects the presence of virus genetic material itself in the early stages of infection (blue graph) .

– Measurement of p24 antigen detects the presence of a protein of the virus , also soon after becoming infected (yellow graph).

But… are both equally reliable evidence? Likewise does it reduce the window period?

Studying the literature, we find sufficient evidence to make us think that the best technique to detect the virus in the early stages of infection is the PCR. In fact, in the graph shown above, the share valuing the PCR (blue) is always higher than values ​​p24 antigen (yellow) line.

What basis do we assert this?

  • FDA recognizes that PCR-based techniques, the window period is reduced much more than valuation of p24 antigen (www.fda.gov).
  • The exclusive detection of this antigen is not used as a routine method in U.S.  (see approved assays) or in Europe (Euro Surveill 2005, 10 (2): 3 -4), but in combination with antibody detection. In fact, if there is a possibility of a PCR test type, it is recommended to make an assessment of p24.
  • There are case reports of patients negative for p24 antigen assessment results, however, using the PCR test POSITIVE ( Transfusion Medicine 17 (3), 200-204, 2007, Journal of Clinical.
  • It has been reported that PCR can reduce the window period between 2 and 14 days, while the value of p24 can reach 28 days.
  • Finally, detection of virus by PCR, you can get to perform there from 20 to 200 viral particles per milliliter blood (Journal of Clinical Virology, 30 (2), 205-206 , 2004), while the assessment of p24 may need up to 1000 particles ((Transfusion Medicine 17 (3), 200-204, 2007) ie assessing p24 has a sensitivity.

These data are not intended to discredit the test based on the detection of p24, we seek to influence that in a situation of urgency, it is preferable to a PCR test done. For thy tranquility.

And if you have doubts … come see us!!