Diagnosis of HIV infection

What is ELISA test for HIV ?--
Whenever any organism enters the body, the immune system produces antibodies to it. Hence presence of these antibodies is an indirect evidence of the infection.
ELISA test for HIV is a test that detects the antibodies to HIV. A positive ELISA test is therefore an indirect evidence of HIV infection.
However the body takes 6 weeks to 3 months to produce antibodies. Thus even after infection the test becomes positive only after 6 weeks to 3 months. This period is called as the window period. During this period the patient is infective but the test is negative.

Should a single ELISA test positive report be taken as an evidence of HIV infection ?--
In case of ELISA positive; either the test is repeated or a more specific test like Western Blot is done to confirm the diagnosis.

What is Western Blot test?
This is a more specific test for diagnosis of HIV infection. It is generally used to confirm a positive ELISA report.

What is PCR test for HIV ?--
PCR stands for Polymerase Chain reaction. It is an advanced test that detects HIV even if it is present in the blood in very small quantity. Hence it can detect the presence of HIV much earlier as compared to ELISA, thus decreasing the window period considerably.
Similarly testing for p24 antigen is also a test that can decrease the window period.

If a p24 test becomes negative after being positive earlier, does it mean that the person has been cured of HIV infection ?--
NO. p24 is an antigen of HIV. Once HIV enters the body it is found in the blood in high quantity. The test is thus found to be positive at this time. After about 2 to six months the virus goes into hiding in the lymph nodes. At this time the p24 is likely to become negative. However this is the normal progression of HIV infection & does not mean that the patient has been cured of HIV infection

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Diagnosis of HIV ---