Types of diagnostic tests for HIV infection
Types of diagnostic tests for HIV infection
Acquired Immune Deficiency Virus (HIV) belongs to the lentis family of viruses and there are two types of viruses called type I and type II.
The pathogenesis of the virus progresses in three main stages:
- Acute
- chronic
Acquired Immune Deficiency Syndrome (HIV) or Acquired Immune Deficiency Syndrome (HIV)
The HIV virus first targets and multiplies monocytes / macrophages, then multiplies in CD4 + lymphocytes.
An HIV-infected person may appear healthy and may have no symptoms or infection for a long time.
For this reason, it can be said that the only way to diagnose HIV infection is to have a blood test.
The time it takes for a virus to enter a person's body until a bloody test can be detected is called the latent period or window period.
Because the disease is usually diagnosed by detecting antibodies against the virus, the results of an HIV test can be negative during this period. However, despite the negative nature of the test, it is possible to transmit the virus.
In this section, we take a brief look at some of the common tests used to diagnose HIV.
The time interval between the entry of HIV into the body and the positiveness of a laboratory test is called the window period. This time varies depending on the nature of the tests.
The first and most common way to diagnose an infection was to detect antibodies that developed over time.
Thus, the first generation of EIA (Enzyme Immune Assay) was first developed, which only 8 to 10 weeks after infection could detect IgG class 1 antibody against HIV, although there were many false positives and negatives in the results of this test. .
The second generation of EIA tests was developed, which could detect approximately 4 to 6 weeks after IgG antibody infection for both HIV-1.2 types.
The emergence of the third generation of EIA tests stronger than the previous two generations, in addition to IgG antibodies, could also detect IgM approximately 3 weeks after infection.
Finally, the fourth generation of EIA, which in addition to antibodies could also detect the P24 antigen, was used, which has been confirmed by the FDA, and can diagnose the disease 16 to 20 days after infection.
Early detection of acute infection is critical to initiating early drug therapy as well as controlling and preventing the spread of infection.
A) Antibody testing tests:
- Elisa test
- Western blot test
tip :
Antibody tests detect antibodies against the HIV virus and are unable to detect the virus directly.
When the virus enters the body, the immune system responds by producing antibodies to HIV infection. These antibodies are used to detect HIV infection.
Given that it takes some time for such tests to detect the presence of antibodies in the blood, a positive result indicates that the person has been exposed to the virus in the past.
Therefore, antibody tests are limited to detect cases of recent exposure.
Because the antibody enters the fetus through the infected mother and remains in the body for up to 18 months, these tests are not suitable for diagnosing infection in the baby.
These tests for screening:
* Women at risk for HIV who have not previously been tested for HIV should be screened with a rapid test during or immediately after delivery to prevent, if necessary, antiretroviral drugs for infants or to give birth to a pregnant woman. Be prescribed.
When the mother's HIV status is unclear after delivery, and especially the mother or father has high-risk behaviors, rapid testing of the baby is recommended at the earliest opportunity after birth to prevent antiretroviral drugs for the affected baby.
* Quick HIV testing is recommended for orphaned infants as well as orphaned infants whose HIV status is unknown.
In cases of HIV infection, the source can be tested quickly to determine the need for medication.
The positive result of any rapid test should be determined by another confirmatory method before the final diagnosis.
According to national guidelines, if the ELISA test is positive, the test (preferably with another laboratory kit) is repeated, and if it is still positive, the Western blot test is performed. A positive test result is a sign of infection.
Special cases:
* If the ELISA result is positive for the first time and negative for the second time, the ELISA test will be performed again three months later, and if the negative result is repeated, the HIV person will be considered negative.
* If both ELISA tests are negative, the person is considered HIV-negative unless the person has been at risk for HIV transmission in the last six months. If the patient has a recent exposure (high-risk behavior over the past six months) and the results of the initial ELISA tests are negative, the test should be repeated three months later. If all of these tests are negative, the patient can be considered HIV-negative.
In a person who is not at risk for HIV but has positive test results and a negative Western blot, the test result is likely to be false positive. If the person is at risk for HIV, and his or her ELISA is positive and his or her Western blot is reported to be negative, it is recommended that he or she be re-tested for Western blotting one month later. If the result of Western blotting is still negative in this repeat test, the person can be considered HIV negative.
Western blotting is a more accurate test, but due to its higher cost, it is used as a confirmatory test in cases where the results of the ELISA test have been positive twice.
Ripa is another confirmation test. This test is similar to Western blotting and is used in cases where the antibody level is very low or when the results of Western blotting are unclear.
Like Western Blot, Ripa is a costly test that requires time and expertise.
HIV-ELISA test
The standard HIV test is an "ELISA test." This test is a global standard for use in hospitals, blood banks or blood transfusion organizations.
This test does not directly search for the virus itself. In this method, the amount of antibodies that the infected person's body has produced against HIV is measured.
The ELISA test is relatively inexpensive and very accurate (although not 100%).
When HIV enters the body, a special chemical is produced by the immune system called an antibody. Antibodies are the body's response to infection. So if a person's body produces antibodies against HIV, it means that the person is infected with HIV.
The production of antibodies in the body takes between a few days and three weeks.
A test that can be trusted to determine the outcome of a person's health is recommended in some sources between 6 weeks and 6 months, and in others between 8 weeks and 6 months after a dangerous relationship.
After 22 days of HIV infection, the ELISA test is positive in 50% of cases, positive in 95% of cases after 6 weeks, and positive in 99% of cases after 3 months.
Eliza's sensitivity is more than 99.9%.
To prevent false positive results, repeated results must be confirmed by westernblot testing.
Reasons for the false positive result of the ELISA test:
Influenza vaccine injection
Diseases such as connective tissue disease
Normal biologic variants
The main purpose of affirmative tests is to ensure that test-positive tests, which are used by the neuroscience department, are positive. The most common test used for this purpose is the Western Ballet test.
Western blot test
Used as a confirmatory test. Which examines the presence of IgG against several types of viral proteins. This test is more specific than ELISA and is less sensitive.
In other words, in this method, HIV type I proteins are separated by electrophoresis gel and then react with the patient's serum. .
The test is positive 35 to 40 days after the body comes in contact with the HIV virus.
Because the Western blot test is relatively expensive and difficult to perform. It is rarely performed as the first test and is more commonly used to confirm positive or similar ELISA tests.
The specificity of the Western Bluetooth test when combined with the ELISA is more than 99.9%.
The screening test used for HIV is ELISA.
In Iran for HIV screening:
- The first test 8 weeks after the dangerous behavior
- Repeat the test 6 months after the dangerous behavior
It is recommended for the first time 8 weeks after the dangerous behavior.
But because the speed and amount of blood antibody production in a person's body varies, and to perform a precise test that can be deduced and given a definitive answer, the test must be repeated 6 months after the dangerous behavior.
The indeterminate test results in the Western blot test are as follows:
- Early stages of HIV infection
- HIV-2 infection
- Autoimmune disease
- Pregnancy
- Recent administration of tetanus toxoid
All HIV-positive tests should be checked by additional tests, usually a Western blot test, which will take several days to several weeks to complete.
You do not need to be fasting to get tested for HIV.
- RT test - PCR
- P24 antigen test
- DNA-PCR & RNA-PCR test
Note: The preferred test for the diagnosis of neonatal infection is the RNA-PCR test.
Most types of HIV tests use a person's blood sample to check for HIV, but there are also tests that use saliva or urine for a sample.
HIV-PCR testing
Quite simply, this test directly detects the presence of HIV in the blood.
The word PCR is equivalent to Polymerase Chain Reaction
HIV-PCR testing detects virus (DNA) genes in the blood. If a person is infected, the DNA of the AIDS virus will be in their blood.
Because HIV is the first genetic marker to be detected in the bloodstream approximately 9 to 11 days after being infected with the virus, it can be used to diagnose the infection early.
This is positive at all stages of the infection, even during the window period and when the EIA is negative.
The two methods of HIV Nucleic Acid Test (NAT) are performed in both qualitative and quantitative ways.
This test is used when the EIA result is unknown or even when a person is suspected of having an acute HIV infection and the EIA result is negative or is used to confirm a chronic HIV infection.
Tasting can also be used to detect premature and acute infections in infants from a dried blood sample (DBS).
This test has the ability to detect infection, 6 days earlier than p24 antigen detection and 12 days earlier than antibody tests, and has considerable sensitivity and specificity compared to blot western.
The accuracy of this test is 95%. Some sources recommend this test two weeks after having a dangerous relationship (a relationship that leads to suspicious contact with blood) and others first four weeks later.
It usually takes between 5 days and a week to get the results of this test.
In terms of cost, this test is usually much more expensive than the HIV-ELISA test.
During this test, various viral components are placed on special strips on the test plate that are tested in the person's serum.
If the serum has an HIV antibody, it binds to viral bands, which change color. However, HIV testing is currently widely available. Extensive screening is not performed.
Due to the fact that in this test, the virus genome is studied, so it is useful in early diagnosis of the disease or the baby born to an infected mother.
Due to the high sensitivity, false positives of this test are high. Positive diagnosis with PCR, however, must be confirmed by serological testing.
Although this test has a very high sensitivity and reliability, its volume is not simple and the laboratory operation must be done accurately and use it carefully.
False-positive: ایشThe test of a small percentage of people, whether with ELISA or Western blot testing, can be wrong. Slight discoloration may be seen in some people who do not have HIV and may be mistaken for HIV.
If the virus concentration is less than 2,000 milliliters, there is a possibility of a false positive in HIV diagnosis.
False negatives: In some cases, the test may not be able to change color despite the antibodies, so someone who is infected with the virus may mistakenly be considered negative.
P24 antigen:
In this method, the antigen of the virus is examined. Therefore, it can help in early detection of infection. However, in the stages of asymptomatic infection or antiviral treatment, we may have a false negative due to the low number of viruses in the blood.
This test is initially used to screen for blood samples, but in some areas it is used as a diagnostic test for HIV.
The P24 antigen is a protein that is part of the HIV structure and is produced in large quantities in the early stages of infection and can be detected in the blood by diagnostic tests. The P24 test can detect HIV infection before HIV testing, so the P24 antigen test is used to diagnose HIV in the early stages.
RNA test:
The test looked at the genetic material of the HIV virus itself and used it to screen for blood sources as well as detect the virus in the early stages (9 to 11 days after exposure to HIV), as well as in very rare cases where antibody tests were able to detect They are not antibodies against HIV.
HIV viral load test:
The test shows the amount of HIV being divided, which is linked to disease progression and response to antiviral therapy.
This test is the best test available to diagnose active HIV infection. However, false positive results are common, especially when the viral load is low.
A series of tests has been proposed as the fourth generation of HIV testing, which is a combination of both antigen and antibody measurements, called 4th generation Ag / Ab combination assay, the most popular of which are:
HIV Ag / Ab combo assay test:
This is a high-sensitivity test for P24 antigen detection. It is also more specific or specific than other tests and is considered a test for diagnosing acute HIV infections.
The test specificity for P24 antigen and antibody against HIV type I and HIV type II is 99.8%.
Its sensitivity to:
- P24 antigen equivalent to 94.2%
- HIV type I antibody equivalent to 99.6%
- HIV type II antibody is equivalent to 98.2%.The test can detect HIV infection when the viral load in the blood is over 170,000 copies per milliliter of blood.
Complete blood test or CBC test:
In advanced stages of HIV, the following are common:
Anemia ٬ Some neutrophils (neutropenia) ٬ Some platelets (thrombocytopenia)
Number of CD4 lymphocytes:
It is most commonly used as a predictor of HIV progression.
The risk of progressing to opportunistic infections of AIDS or malignancies is high when CD4 levels are below 200 c C / .l.
Percentage of CD4 lymphocytes
The percentage of CD4s may be more reliable than their number.
The risk of progressing to opportunistic infections of AIDS or malignancies is high when the CD4 percentage is less than 20%.
The only safe way to diagnose HIV infection is to have an HIV antibody test, and HV infection cannot be determined by clinical signs.
Due to the availability of ELISA and Western blot in Iran, as well as high specificity (less false positives), the criterion for the definitive diagnosis of a person's infection is the positiveness of two ELISA tests and a Western blot test.
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