Laboratory Tests for HIV Infection

• Screening tests
• Supplemental tests
• Confirmatory tests

Screening tests - are designed to detect Antibodies against HIV. they are of three types.
ELISA - enzyme linked immunosorbent assay test
Rapid Tests
Simple Tests

ELISA technology is based on antigen-antibody and enzyme substrate reactions.

Rapid Tests - Dot Blot and Latex Agglutination Tests

Simple Tests - Particle Agglutination tests.

Both Simple and Rapid Tests do not need costly equipments.

Strategies employed for HIV screening.

Strategy I
Used for blood transfusion safety. The serum of the donor is tested by one of the techniques E/R/S (ELISA, Rapid, Simple Tests) If reactive it is taken as Positive and if non reactive its taken as negative.

Stategy II
If the serum is reactive with one of the E/R/S it is tested with a second E/R/S based on a different antigen or on different test principle. If the second test is reactive it is reported as positive. If it turns out to be non reactive it is reported as negative.

Strategy III
If the serum sample is found to be reactive with two E/R/S tests, it is retested with a third E/R/S, again with a different antigen or a different test principle.

Supplemental Tests - Western Blot test, Immunofluorescent tests. These are used to validate results obtained by the screening tests. Western Blot test is highly sensitive test. Like ELISA it may not be positive in the initial 3-4 wks. of infection.  

Confirmatory Tests - aim at demonstration of Viral Antigen(P24), isolation of HIV and detection of viral nucleic acid. These are done in the reference centres and are time consuming and very costly. The confirmatory tests can diagnose HIV infection even during the initial two to three weeks - the window period, in which both the screening and the supplemental tests fail to diagnose the infection.

Elisa (Enzyme Linked Immunosorbent Assays)

ELISA is the most commonly used test to screen for HIV infection. It detects antibodies to HIV. ELISA may not be sensitive during the initial 3-4 wks of infection because the HIV specific antibodies become positive about 22 to 27 days after acute infection. 

False positive tests may occur in multiparous women, recent recipients of Influenza or Hepatitis B vaccines or multiple transfusions, those with hematological malignancies, multiple myeloma, primary biliary cirrhrosis or alcoholic hepatitis.

False negative ELISA occurs in very early or late in the course of HIV disease when antibody production is low. 

Laboratory Tests for HIV Infection

Manifestations of HIV

Management of HIV infection