Human Immunodeficiency Virus (HIV) is a virus that attacks the CD4 cells (immunity cells). This attack may, over time, result in a full-blown Acquired Immunodeficiency Syndrome (AIDS). Although it infects the entire body tissues, it is only transmitted through body fluids such as breast milk, vaginal fluid, saliva, blood, and semen.
Diagnostic tests are carried out for several reasons such as public health surveillance, the safety of tissue products and blood protection, as well as individual diagnosis. There are different testing methodologies as there are also reasons for testing, meaning that the nature of the test adopted is heavily dependent on several conditions such as convenience, the group to which test subjects belong, and the characteristics of the test.
These tests are standardized procedures put in place to determine the presence of HIV in the saliva, blood serum, or urine. The detection of HIV-I and several other shades of the virus in the RNA target sequence, p24 antigens, or HIV antibodies are widely accepted as an indication of HIV infection except for special cases where subjects may have been recipients of experimental vaccines, and likely presence of other infectious agents with antibodies similar to those of HIV leading to false positive results (i.e. when test results indicates the presence of HIV in a non-infected subject).
Types of Testing
Generally, several types of tests are adopted to screen blood or other body fluids to detect the virus. However, these established diagnostic tests have their respective limitations since early infections are hardly detectable. The window period (i.e. the time it takes the virus to reproduce to levels where they can be detected by a test) varies across the various types of tests.
The average window periods for the tests are:
- 12 days for Nucleic Acid Testing (NAT)
- 16 days for antigen testing
- 25 days for antibody testing
The implication of the above is that cases of a false negative (i.e. when test results indicate that an infected subject is free of the virus) may arise from tests carried out on subjects earlier than the indicated window periods.
Besides window periods, another parameter that has helped refine standard test algorithms over the years is the “efficiency of test methodology”. Their efficiency is generally described in terms of specificity (i.e. the percentage of negative results when HIV is absent in the subject) and sensitivity (i.e. the percentage of positive results when HIV exists). Therefore, sensitivity tests are applied for screening while specificity test is a follow-up test for confirmation of results.
This type of inexpensive and highly accurate diagnostic test is specifically designed for adults. The test screens for antibodies (a certain kind of protein produced by the body in response to HIV infection). These antibodies in most people take about 30 days after infection to develop. This may lead to false negative results if tests are carried out during the window period.
This high sensitivity test is also called enzyme-linked immunosorbent assay (ELISA) or immunoassay. With this method, blood sample, and in some cases urine or other mouth fluids are screened. The rapid version of these tests available for use in ‘point-of-care’ can generate results in less than 30 minutes.
The various types of antibody screening include:
- ELISA dongle
- Western blot
- Rapid test (commonly done by individuals at home)
This test is carried out to screen for the presence of p24 protein, a component of HIV particle – capsid. Because of its early development and production, the p24 antigen can be detected in subjects immediately after infection. Though this test is adopted for the early detection of HIV, the test is less reliable for the later stages of the infection and as such cannot subsist as a standalone test.
The sensitivity of this test diminishes because the rapid production of the p24 protein usually recorded at the early stages of infection is not sustained – it dwindles with time. This makes the detection of the p24 difficult in much later days after infection. This is why the test is usually performed alongside an antibody test for the purpose of precision.
Antibody/Antigen Combination Tests
This early detection test is a combination of the antigen and antibody tests and is capable of detecting HIV between 2 to 5 weeks after infection. It is a recommended test designed to screen for HIV antibodies as well as the p24 antigen. The rapid version of the test can give results in less than 20 minutes.
Nucleic Acid-based Tests (NAT)
Unlike the antibody, antigen, and the antibody/antigen combination tests, this test does not involve the screening for p24, or antibody, or both; it screens for the virus. It is an expensive test that can detect and quantify the precise amount of HIV in a subject 10 days after infection.
Indications for Testing
HIV test may be indicated for persons with the following:
- People with behavioral risk such as multiple sex partners, sex with an infected person, injection of drugs through syringe and needles used by other persons.
- Occupational risk arising from a workplace injury.
- STI infections, TB, as well as hepatitis and herpes.
Routine tests and general health screening are the recommended means to stay ahead of HIV. Get tested today!