Providing help for people with HIV and AIDS

Laboratory Diagnosis of HIV

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Diagnosis is defined as the demonstration of HIV 1 and HIV 2 antibodies or antigens in blood or body fluids. Antibody detection tests are the most commonly used tests. Test to detect the virus are available but have largely remained research tools and confined to monitoring progress of infection of response to therapy rather than for diagnosis. However these test are ideal for the diagnosis of HIV in infants and during the window period.

For the diagnosis of HIV infection, at least two test systems of different antigen preparation or test principles must be used for each sample. Tests with a high specificity must be used to minimize the rate of false positive results. If the two tests are concordant, the result is reported, if discordant, the tests are repeated. If they continue to be discordant, a supplemental test or a tiebreaker is used to determine HIV infection status.

a) What is the Window period?
The window period represents the period of time between initial infection with HIV and when the body builds a measurable antibody response. During this period, HIV replicates in the blood and lymph nodes, but the infected person has no measurable antibody response. The virus can be detected during this phase by laboratory tests used to identify the virus itself. The window period can last from 2 to 12 weeks.

b) Types of HIV Tests: There are two main types of tests:

  1. The first is the antibody test, which is often just referred to as the HIV test. This shows whether a person is infected with HIV, the virus that causes AIDS.
  2. The second is the viral test. This test shows the level of virus in the blood. It is usually used to monitor the health of someone who already knows they are infected.

The sensitivity of test means the percent of those identified by the test as having the infection actually have it. The specificity of a test means the percent of those identified by the test as not having the infection that are actually infection free. With never methods of test and reagent development, the currently available assays have excellent sensitivity and specificity.

HIV tests are grouped into three categories:

  1. Screening tests
  2. Confirmatory or supplemental tests
  3. Antigen screening tests

Most commonly used screening tests are Enzyme linked immunoassays (ELISA) and simple/rapid tests.

ELISA is most appropriate for large laboratories where large numbers of samples are processed per day. ELISA’s however suffer from the disadvantage that they require skilled scientific/technical skills and specialized equipment, which can be expensive and usually are not readily available at smaller health institutions.

There are four types of rapid or simple tests:

  • Agglutination assays
  • Comb/dipstick assays
  • Lateral flow membrane assays
  • Flow through membrane assays

Rapid tests are most appropriate for the smaller health institutions where only a few samples are processed each day. Rapid tests are quicker and do not require specialized equipment. Rapid tests, by definition, take up to 10 minutes. Most are dot/blot immunoassays or agglutination assays requiring no instrumentation or specialized training and are fast to perform. Most rapid tests have to be given on the same day as testing thus reducing the number of visits made by the clients. There is also an increased likelihood of clients receiving test results as opposed to the numbers who may not return when same day testing regimes are not used.