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.

HIV/AIDS Disease

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AIDS stands for Acquired Immune Deficiency Syndrome. “Acquired” means not innate nor inherited but transmitted from person to person; “Immune” is the body’s system of defense; “Deficiency” means not working to the appropriate degree; and “Syndrome” means a group of sings and symptoms. AIDS is the advanced stage of HIV infection. It is a disabling and deadly disease caused by HIV. Because HIV progressively destroys the immune system, without any treatment most people, particularly in resource-constrained settings, will die within a few years of the first signs of AIDS.

After the HIV virus enters a person’s body, it infects and starts to replicate in the person’s cells (essentially CD4 T cells and macrophages). It is believed that in a small number of people, HIV may enter a latent phase with limited replication. In the majority of people, the replication is significant and infection with HIV induces the body’s immune system to produce antibodies specific to HIV. The period between the acquisition of the infection and the production of HIV detectable antibodies is called the “window period”. The window period can last for 2-12 weeks. During this period the person is highly infectious but may not test positive on common HIV antibody tests. Up to 30-50% of people have a recognizable acute illness at the time to infection characterized by fever, lymphadenopathy) enlargement of lymph nodes), night sweats, skin rash, headache, and cough.

HIV-infected people may remain symptomatic for periods to as long as 10 or more years. People in this phase potentially play an important role in the transmission of HIV as they remain infectious and can be identified only by screening their scrum for HIV antibodies. After a variable period of time that varies from one individual to another, viral replication resumes and is accompanied by a destruction of CD4 lymphocytes and other immune cells resulting in a progressive immunodeficiency syndrome. The progression depends on the type and factors as that may cause faster progression including age less that 5 years, or over 40 years, other infections and possible genetic (hereditary) factors. As HIV infection progresses and immunity declines, people become more susceptible to opportunistic infections (Ols). The three most commonly reported Ols in South East Asia are tuberculosis (TB), Pneumocystis carinii pneumonia and extra pulmonary cryptococcosis (usually meningitis).

WHO has proposed a clinical staging system for HIV infection and disease in adults and adolescents in 1989 in four clinical stages. In addition to the signs, symptoms and diseases, physical activity was added to the framework using the performance scale, a modification of the Eastern Co-operative Oncology Group score. Patients are classified according to the presence of the clinical condition, or performance score, belonging to the highest stage. The staging system is hierarchic: once a stage is reached, the patient cannot revert to a lower stage, he/she can only progress to a higher once. A laboratory axis measuring CD4 count was introduced in 1990.

Rates of progression to AIDS are influenced by plasma viral load and CD4 T cell count. The higher the viral load (the amount of virus in the body) the lower the CD4 count and the higher the chances of progressing to AIDS and death. Death may be due to HIV, Ols, or malignant diseases. The prevention and better management of opportunistic infections can have a beneficial impact on the progression of HIV infection.