Diagnosing HIV – Concepts and tests

This video is an instructive video to understand the diagnosis process of HIV. In the case of most diseases, the diagnosis is symptom-based. However in the case of HIV, the diagnosis is rather tricky, as its symptoms are too general. HIV infection is marked by extremely general unspecific symptoms like headaches, body aches, flu etc. Thus, the diagnosis of HIV cannot be symptom-based.

HIV is diagnosed primarily by blood tests. Blood tests are performed to check for antibodies against HIV or viral particles belonging to HIV. At about 1-month post-infection, antibodies against HIV are formed by the body of an infected individual. The levels of antibodies increases rapidly and then there is a slight dip in the number, again followed by a rise in the number.


Until a few years ago, reliable HIV detection by blood tests was possible only after about 2 months of infection. However with the advent of more specific test methodologies and more efficient techniques, this period has now been reduced to about 1 month. Such blood tests not only detect antibodies against HIV but also certain viral proteins like P24. But in order to detect P24 in the blood of the infected individual, the test period is rather limited. With the rise in the level of antibodies, P24 levels go down, so it can only be detected within 2 weeks post infection. Detection of Antibodies or the P24 protein in the blood is done by a laboratory technique called ELISA or Enzyme-Linked Immunosorbent Assay. If ELISA is positive, then a more specific test called Western Blot is done in order to confirm the diagnosis. It is important to do so as ELISA may also be positive if the person is affected by certain other diseases like Lupus or Syphilis. A negative ELISA test usually denotes that there is no HIV Infection.

Western Blots

A positive ELISA test leads to confirmation of HIV diagnosis through western blots. They are specialized tests which check specifically for viral proteins belonging to the HIV virus. When both ELISA and western blot are positive, then HIV diagnosis is confirmed. However if ELISA is positive but western blot turns out to be negative, then it means that the ELISA reacted due to some reason other than HIV infection.

HIV1 and HIV2

HIV has two subtypes, namely HIV1 and HIV2. HIV1 is the most commonly occurring subtype It is essential to know which subtype has infected an individual as certain drugs targeted towards HIV1 are not effective for treating HIV2. Both subtypes are rather similar, with only some differences in proteins and enzymes. The HIV subtype is determined by analysing the RNA by Nucleic Acid Testing (NAT). This also facilitates early detection at about 12 days, which is much earlier than regular ELISA.


RAPID HIV Testing has come to the forefront only in the recent years. It is a fast and easy technique to conduct an initial screening for HIV infection at STD clinic. A mere mouth swab or finger prick suffices for conducting this test. It is also highly time efficient, as the results are obtained in 20 minutes. As it is very convenient and cheap it has been instrumental in making HIV screening widely available. The only drawback of RAPID testing is that its speed and cost-effectiveness have compromised its accuracy. Thus it is mainly used as a screening test and a positive RAPID test needs to be confirmed by other more specific diagnostics test.