Culture:It directly detects the virus & hence is highly specific. It is a costly test. It can take 2-4 weeks for the report. It is also positive during window period & during terminal phase when the viral load is high & when the ELISA may be negative. With availability of PCR, HIV culture is no more routinely done.

PCR:Polymerized chain reaction has revolutionized the diagnosis & treatment of HIV infection. Two types of PCR are available - PCR for DNA of provirus present in the infected host cells & PCR for HIV RNA from plasma, which detect the free virus present in plasma. PCR can detect as low as 20 copies/ml of plasma & is highly sensitive & specific test (both = 99.9%). It is very useful for early diagnosis of HIV infection in newborn. It is also positive early in window period & during last phase of HIV infection when ELISA is likely to be negative. Results of PCR are available in 24-48 hours unlike viral culture that takes 2-4 weeks. Quantitative PCR can be done to define the viral load in copies per ml of plasma. This is useful before starting therapy & monitoring response & relapse following anti- retroviral drugs. The only problem with PCR is its high cost & need for sophisticated laboratory equipment.

P24 antigen detection:It is detectable during window period & during late phase of infections when the virus is replicating fast. It is often not detectable at other times as the viral replication is low and it gets attached to the antibody. Hence it is not sensitive enough to be of use for routine diagnosis. It is useful in blood banks where P24 antigen along with ELISA can shorten the window period to less than 2 weeks.

ELISA/ Immunofluoroscence:ELISA is more popular & widely done test. ELISA tests have nearly 99% sensitivity & specificity especially on repeated testing. These tests are also much cheaper than PCR & culture. It can be done at any center. In West, it is mandatory to do a Western Blot if ELISA is positive before labeling a person HIV positive. In out country and as per WHO, it is not necessary to do a Western Blot if ELISA is positive. Instead it should be confirmed by one or two more ELISA done using different kits on different blood samples especially one of them being a rapid test. So if such 2 or preferably 3 ELISA tests are positive one can label a person as HIV infected.

One will have to wait till 18 months to diagnose HIV infected in newborn if one depends on ELISA. Hence ELISA cannot help in early diagnosis of HIV infection in newborn. Similarly ELISA is negative during window period i.e. 6-12 week following HIV exposure & sometimes during last phase of HIV infection due to severe immunodeficiency leading to severe hypogammaglobulinemia. PCR is useful in such cases.

Being cheap & easy to do, ELISA is the mainstay of HIV diagnosis in older children & adults.

Western Blot analysis:

Western blot detects specific antibodies and shows them as separate bands. Hence it is more specific than ELISA. W.B. is interpreted as positive if at least 2 of 3 bands i.e. p24, gp41, gp120/160 bands are positive. If none are positive it is labeled as negative. If only one band is positive, it is labeled as indeterminate. When the test is indeterminate, one needs to repeat both ELISA & W.B. once or twice after 4-6 weeks until it becomes definitely positive or negative.

Like ELISA, W.B. is also negative during window period & sometimes in late phase of HIV infection. It cannot be relied upon for early diagnosis of HIV in newborn as it will detect maternal antibodies for as long as 18 months.

Immunological tests:
CD4 + T cell count, CD4 + T cell % & CD4/CD8 ratio are tests done to determine immune status. Gradual decline in CD4 + T cell count suggests disease progression. CD4+T cell % is more constant whereas absolute CD4 is age dependent. It helps to prognosticate, monitor disease progression & helps to determine response & relapse following anti - retroviral drug therapy.

Last created on 23-02-2001
Last updated on 26-05-2007