HIV tests are used to detect the presence of the human immunodeficiency virus in serum, saliva, or urine. Such tests may detect HIV antibodies, antigens, and nucleic acids. The Centers for Disease Control and Prevention (CDC) encourages voluntary HIV testing as a routine part of medical care for all adults age 13- 64 yrs.

 

Window period

The window period is the time from infection to antibodies to appear, it takes 12 weeks for HIV antibodies do develop.

An early infection is detected by HIV Antigen detection tests. These tests detect the presence of the virus in the blood.

Minute quantities of viruses can also be picked up by these tests.

• Polymerase chain reaction (PCR)
• Reverse polymerase chain reaction (RT-PCR)
• P24 antigen assay

HIV tests

Tests used for the diagnosis of HIV infection require a high degree sensitivity and specificity.

Antibodies detected by an initial test based on the ELISA method, and then a second test using the Western blot procedure determines the size of the antigens in the test kit binding to the antibodies. The combination of these two methods is highly accurate.

HIV antibody tests

ELISA

ELISA is the most widely used anti-HIV antibody test for screening HIV individuals.

The sensitivity of the ELISA is 100%, less specific compared to western blot test but false positive and false negative reactions occur.

Western blot

HIV Western blot is a confirmatory blood test used to diagnose chronic infection with human immunodeficiency virus (HIV).

Western blot testing is done in following conditions

• Screening people in high-risk groups (men who have sex with men, drug users and their sexual partners, and commercial sex workers)
• Screening people with certain conditions and infections (such as Kaposi's sarcoma, Pneumocystis pneumonia)
• Screening pregnant women to help prevent them from passing the virus to the baby

Importance of HIV testing

• A positive ELISA screening test does not necessarily mean that the person has HIV infection. There are certain conditions that may lead to a false positive result, such as Lyme disease, syphilis, and lupus.
• A positive ELISA test is always followed by a Western blot test.
• A positive Western blot confirms an HIV infection.
• A negative Western blot test means the ELISA test was a false positive test.
• The Western blot test can also be “indeterminate,” in which case additional testing is done to clarify the situation.

Spot tests or rapid tests (ora quick and ora sure)

• Several rapid tests can give highly accurate information within 20 minutes.
• These tests look for antibodies to the virus using a sample of blood or fluids collected on a treated pad that's rubbed on your upper and lower gums.
• The oral test is almost as sensitive as the blood test and eliminates the need for drawing blood.
• A positive reaction on a rapid test requires a confirming blood test.

HIV- home test

Home test approach ensures your privacy and anonymity.

The greatest disadvantage is that counseling is not offered that patients typically receive in a clinic or doctor's office.

HIV antigen tests

P24 antigen test

The p24 antigen test detects the presence of the p24 protein of HIV  the capsid protein of the virus.

Although HIV antibody tests are the most appropriate for identifying infection, alternate technologies can contribute to an accurate diagnosis, assist in monitoring the response to therapy, and can be used to effectively predict disease outcome.

Uses of the p24 Antigen Test

• Blood Screening
• Identifying Acute Infection
• Monitoring HIV Infection
• Detecting Infection in the Newborn

Major limitation of p24 antigen assay

• Low levels of antigen are difficult to detect
• Because antigenemia occurs only transiently during different stages of infection

Nucleic acid based tests

PCR test is a nucleic acid based test that detects the genetic material of HIV itself, and can identify HIV in the blood within two or three weeks of infection.

PCR tests come in two forms: DNA PCR and RNA PCR.

Babies born to HIV positive mothers are usually tested using a DNA PCR because the babies retain their mother's antibodies for several months, making an antibody test inaccurate.

Blood supplies are screened for HIV using an RNA PCR test, which can produce positive results several days before a DNA test.

When a person infected with HIV, may also have a viral load test to detect HIV genetic material and estimate the level of virus in the blood.

This can be performed using either an RNA or DNA PCR test.

PCR tests are not routinely used to test for HIV in adults, as they are very expensive and more complicated to administer than a standard antibody or P24 test.

RT-PCR: used to measure the viral load in HIV.

Other tests in HIV

CD4 T- cell count

• A CD4-Tcount does not check for the presence of HIV.
• It is used to monitor immune system function in HIV-positive people.
• Declining CD4 T-cell counts are considered to be a marker of progression of HIV infection.
• In HIV-positive people, AIDS is officially diagnosed when the count drops below 200 cells/μL or when certain opportunistic infections occur.