Screening Tests to Diagnose HIV/AIDS: Types and Their Accuracy

Laboratory tests to diagnose HIV/AIDS infection are done to rule out the presence of the viral infection after an exposure to any of the HIV risk factors.

A positive result of the test indicates that you are infected by the human immunodeficiency virus (HIV). You are then labeled as HIV positive or as immunocompromised.

Why is HIV difficult to diagnose from symptoms?

It is difficult to diagnose HIV from its symptoms because they resemble those of cold and flu and the doctor and the patient are more likely to label the ailment as one of the two.

People who are unaware that they are HIV positive pass through the progressive stages of this infection and land up in the last stage, which is the stage of AIDS, in a few years.

It may be too late by then for any treatment to be effective. Your immune system is damaged beyond repair and you succumb to this disease. Late diagnosis of HIV therefore, may not be of much help.

Unknowingly, you also infect others, which endangers the lives of your sexual partners and your spouse. Getting diagnosed earlier will help get treatment promptly and give you a full life.

Therefore, the HIV diagnosis guidelines laid down by the Centers for Disease Control and Prevention (CDC) recommend that everyone between the ages of 13 and 64 be tested for HIV at least once as part of routine health care. A general rule for those with risk factors is to get tested every year.

An early diagnosis and early treatment make the difference between a normal lifespan and a premature death.

Testing methods

There are various testing methods done on the patient’s blood, oral fluid (not saliva) taken with a swab, and urine.

All these tests work on one principle: They detect the antibodies to the virus, or the antigen itself, or the RNA. Urine testing, however, is rarely done due to inaccuracy.

HIV (Human Immunodeficiency Virus) screening tests should form a routine part of all testing programs. Even a flu-like picture should warrant HIV testing in the lab. (Read HIV/AIDS symptoms to understand why)

Though this may sound simple and unconvincing, this adherence may well save a lot of lives.

When testing for HIV/AIDs becomes most necessary?

If you are living a lifestyle, which is described here, you need to test yourself for HIV/AIDS not just once but periodically. These risk factors are listed below:

  • If you have visited prostitutes even once
  • Even if you have used a condom, you must test yourself. Use of a condom reduces the risk but does not eliminate it 100%.
  • If you have multiple sex partners
  • If you drink alcohol or take drugs, it can make you reckless and take on any partner without knowing his or her sexual history
  • If you take intravenous drugs and use shared needles. Shared needles are one way by which HIV is transmitted
  • Pregnancy

Why are HIV screening tests done?

  • To screen donated blood before transfusion
  • To keep the prevalence figures up to date
  • To diagnose the infection in individuals, especially the high-risk group
  • In pregnant women, to prevent the passing of the virus to the baby.

Principles for testing a person for HIV

The ethical principles as advocated by UNAIDS/WHO have to be followed when testing for HIV:

  • Confidentiality has to observed
  • Counseling for those who test positive
  • Informed consent of the person should be taken

HIV and AIDS Laboratory Tests

HIV tests are carried out on the blood serum, oral fluid or urine of the person to be tested. All these tests are designed to detect the HIV-1 virus, which is the most common HIV.

Most test kits also detect the HIV-2, which once was found mainly in West Africa but an increasing number have been detected in Europe, India and the United States.

Blood tests to diagnose HIV/AIDS are of three types:

(A)   Antibody tests, which give results by detecting the antibodies (proteins produced by the body to fight against the  HIV  virus) produced by the body to the virus.

(B)   Antigen tests, which detect the genetic material (DNA/RNA) of the HIV in the infected blood.

(C)   A combination test, which detects both the antibodies and the viral protein called p24.antigen.

(A) Antibody tests for HIV

These tests are more commonly used and they detect the antibodies to HIV in blood rather than the HIV itself.

When one is infected with HIV, the immune system of the body produces antibodies, which are proteins to fight the virus.

Though the weakened antibodies cannot neutralize the virus, they serve as markers, which reflect in the HIV antibody tests.

Third generation HIV tests (ELISA antibody)

Enzyme-linked immunosorbent assay (ELISA) or enzyme immunoassay (EIA) test is performed usually using the blood serum of the person, but can also be done on the oral fluid or the urine sample.

Testing results with urine are not as accurate as results obtained with blood or oral fluid.

This test detects the antibodies to the HIV and accordingly, the result is obtained usually in two weeks.

If the test result is negative, it is repeated again to confirm after 6 weeks, 3 months and 6 months to be absolutely certain that HIV is not present in the individual. That is its window periods.

The ELISA test is highly sensitive and relatively accurate. A negative result does not need confirmation with other tests.

Chances of false results are extremely low especially after the window period for the ELISA test of four to six weeks has elapsed.

However, if the test after four weeks is negative, a repeat test should be advised after 3 months especially in persons living a high-risk lifestyle.

A positive result is confirmed with a repeat test. If the second test too, is positive another test called the Western Blot test is performed.

This confirmation is necessary in view of the fact that the HIV positive patient has to be put on antiretroviral therapy all his entire life – what with its cost and side effects!

Secondly, a positive result on ELISA does not necessarily mean that the person has HIV. A false positive result for HIV can also occur in certain conditions such as lupus, Lyme disease, and syphilis.

Reports of the Elisa test take two weeks in coming.

Western Blot test

This blood test is more complex but is necessarily done to confirm the positive result of two ELISA tests. A positive result on Western Blot after the two positive reports on ELISA almost certainly confirms that the person is HIV positive.

Western Blot will give a negative report if the ELISA test is false positive in conditions mentioned above such as syphilis, Lyme disease, and lupus.

This test can differentiate between the antibodies of HIV and these other conditions. The results of the western blot test take time and will be available after two weeks.

The Western Blot is used as a confirmatory test and not done routinely because it is difficult to perform and requires highly skilled personnel.

Rapid HIV antibody tests

Rapid HIV antibody tests are performed on blood serum, urine or oral fluid to detect antibodies to the HIV.

However, testing with urine may not give accurate results. This test is as accurate as the ELISA test and has an added advantage that the result can be obtained in 20 minutes as against two weeks for the ELISA test.

Although these tests are highly specific, false positive results are seen with these tests. A positive test report on the rapid test has to be followed with the Western Blot test to confirm that the person is HIV positive.

(B) Antigen Tests for HIV

Antigen tests are performed on the blood sample only. These tests are not as commonly done as the antibody tests because highly skilled personnel and expensive equipment are required. Results of these tests are available within one to three weeks.

Polymerase chain reaction (PCR)

This test is also referred to as the viral load test or HIV NAAT (nucleic acid amplification testing) or the HIV RNA test.

Unlike the antibody tests, this test detects the genetic material – the HIV RNA and HIV DNA.

It can detect acute and recent HIV infection even in the absence of antibodies in the blood, making the HIV RNA test the most sensitive and accurate early detection test available on the market today.

It can be performed as early as 9 to 11 days after the infection has been introduced in the body. In developed countries, PCR is done to screen blood donation samples for HIV.

PCR also helps to determine the viral load. Viral load is the amount of the virus in the blood.

The more the viral load the faster your CD4 cells get destroyed and your immunity falls. This increases your risk of developing complications of AIDS early.

Special RNA-PCR testing is also done in babies born to HIV positive mothers. Such babies carry the HIV antibodies from the mother’s blood for few months and other standard antibody tests would, therefore, show positive results.

But, the special RNA-PCR test will identify whether the babies are HIV positive themselves as early as in six weeks after birth.

If negative, the test is repeated after six weeks and again after 12 weeks. A negative result after 12 weeks can be a sure sign that the baby does not have HIV.

At home HIV tests

At home HIV testing kits are available over the counter or online for persons above 17 years of age. They are of two types:

1.     OraQuick In-Home HIV Test

2.   Home Access HIV-1 Test System

Oraquick In-Home HIV test on oral fluid

Oraquick In-Home HIV test is performed using oral fluid taken from the mouth with a swab. It is not used with blood or urine. It can detect antibodies to HIV three months after the person has been infected.

A special swab is wiped over the upper and lower gums to collect the oral fluid. The swab with the oral fluid is put in the test tube, which comes with the OraQuick test kit.

Results show up in 20 to 40 minutes. However, the U.S. FDA says that a positive result with this test must be confirmed with follow up testing in the laboratory.

False negative results can be obtained due to the window period delay of antibodies showing up in the oral fluid. High-risk candidates should be periodically tested.

Home Access HIV-1 Test System

This is not a true testing kit but a sample blood collection kit and offers the much-needed privacy one looks for in testing for HIV.

This testing kit is approved by the U.S. FDA. The procedure is short and simple.

You prick your finger using a sterile lancet, squeeze your finger and collect a drop of blood on a special strip provided by the kit. This strip is then sent to the lab for analysis.

Results are available in about a week. The test is fairly accurate but has a longer window period.

This test offers anonymity as you are identified only by the code number that comes with the kit.

(C) Fourth generation HIV tests (ELISA combined antigen/antibody)

The Fourth generation tests look for HIV antibodies and also for the p24 antigens.

What is p24 antigen? The p24 antigen is a protein that makes up most of the core of the virus (HIV). The serum concentration of p24 antigen is high during the initial few weeks after the person is infected.

This is what makes the HIV infected person most infectious during this initial period of infection.

Fourth generation tests can detect HIV from 11 days to 1 month after you have been infected. That is its window period.

According to the CDC, it can take 2 to 6 weeks for the infected person’s body to make antigens and antibodies that are enough for a combination test to detect HIV.

Testing for CD4+ Count

The CD4 count blood test is done in patients who are already identified as HIV positive. CD4 T cells are white blood cells and play an important part in the function of the immune system to protect the body against infection. The normal CD4 count is 500 to 1200 per cubic milliliter.

The Human Immunodeficiency Virus destroys the CD4 T cells in the blood and lowers the immunity of the body.

This makes one vulnerable to opportunistic infections which occur during the last stage of AIDS. This happens when the CD4 cell count falls below 200/mm3.

Knowing the CD4 cell count in a patient with HIV tells you how much damage the HIV has caused to the immune system or how well the ART medicines are working against the virus.

Testing for complications of HIV/AIDS

When your CD4 T cell count falls below 200, your damaged immunity makes you vulnerable to several opportunistic diseases, which otherwise your body would have fought off.

These are the complications that set in during the last stage of HIV infection – the stage of AIDS. They have to be diagnosed and respective tests are to be performed to confirm their diagnosis.

All this is described in opportunistic infections, the dangers of HIV/AIDS.

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