Screening test for HIV. Methods for diagnosing HIV infection
A common diagnostic method today is screening. HIV can be easily detected using this method even in the early stages. With the help of such a study, it is possible to determine the presence of antibodies to this dangerous disease in the patient’s blood. What HIV screening diagnostics are used in modern medicine and what do you need to know about it?
Screening method for diagnosing HIV infection: description
Today, a range of different reagents are used for screening studies, with the help of which it is possible to diagnose the immunodeficiency virus. A few years ago, third generation enzymes were used for this. Their effectiveness and sensitivity were not high enough. The fourth generation reagents currently used show better results. With their help, it is possible to detect antibodies and antibodies to HIV 1, 2 (screening is carried out in laboratory conditions). The main advantage of this type of study is the ability to detect not only antibodies, but also antigens. This allows medical professionals to determine what type of immunodeficiency virus a person is infected with. With the help of such screening for HIV infection, it is also possible to identify carriers of the virus in whom it does not manifest itself, but can be transmitted to other people.
The most common test used to identify this disease in public and private institutions is the ELISA test. It is recommended to carry it out three to four weeks after infection. We are talking about unprotected contacts with a new partner, blood transfusions, and so on.
What else should you know about HIV screening tests?
It is important to know that such a study is performed by adding the patient’s blood to an indicator with special reagents. Blood sampling is carried out on an empty stomach. At least eight hours must pass from the last meal to testing.
A screening test plays an important role in diagnosing HIV infection. It can be used to determine the presence or absence of the immunodeficiency virus in the early stages.
The results of such a study can be positive, negative or questionable. The latter are extremely rare. A questionable result indicates that the procedure was carried out incorrectly or the reagents were of poor quality. If the result is positive, additional research is carried out. We are talking about immunoblotting. A negative result does not require confirmation.
Testing or screening for HIV antibodies has two broad but very clearly defined goals—case detection and surveillance. When cases are identified, the first step is to clarify the HIV infection status of each individual in order to prescribe appropriate treatment or follow-up with appropriate measures.
The purpose of epidemiological surveillance is to estimate the prevalence of HIV, the distribution of cases of infection and its trends in a group or population.
The sensitivity of an HIV antibody test is a measure of its ability to accurately detect these antibodies in a sample, and the specificity of a test is a measure of its ability to accurately confirm the absence of antibodies when they are not present in the sample. Ideally, the sensitivity and specificity of the test should reach 100%. In practice, no biological test satisfies this requirement, and yet the tests used for HIV antibodies are among the most sensitive and specific tests currently available
Laboratory diagnosis of AIDS consists of conducting virological, serological, and immunological studies of material from sick individuals suspected of having this disease.
In virological studies, primary cultures of mononuclear blood cells can be used to isolate the virus. Isolation and identification of the virus are methodologically complex and can be performed in specialized laboratories. The most effective diagnostic method currently used for routine mass examinations is the detection of antibodies to the human immunodeficiency virus. Antibodies to HIV may appear by the end of the first month of infection. According to a number of authors, the development of seroconversion requires from 4-7 weeks to 6 months or more. The presence of antibodies has diagnostic value for AIDS or indicates the risk of its development. Antibodies are not only a serological marker of AIDS. Detected in the preclinical phase of the disease, they allow for its early diagnosis. Their presence is of particular importance for the detection of carriers. Antibodies are detected over many years, almost throughout life. Researchers have established parallelism in identifying the virus and antibodies to it, i.e. the presence of antibodies to the immunodeficiency virus indicates a high probability that a person is a virus carrier.
Antibodies to the HIV antigen, having appeared during the incubation period, continue to be intensively produced with the development of the disease, since antigenic irritation is stimulated by virions released from infected lymphocytes, and subvirion components that enter the bloodstream during the disintegration of infected cells, and infected lymphocytes. At the same time, the provirus integrated into the genome of infected cells remains inaccessible to specific antibodies. This explains the seemingly paradoxical fact: the more antibodies to the human immunodeficiency virus in the blood serum, the easier it is to isolate the virus itself from the patient. This happens because the antibodies produced in response to a virus infection are not neutralizing and, as a result, do not have a noticeable effect on the virus, but are simply present in the body along with it. To detect antibodies (AT) to the AIDS virus, a number of tests have been developed that allow research to be carried out at a sufficiently high level of specificity and sensitivity. These are methods of solid-phase radioimmunoassay, radioimmunoprecipitation, immunofluorescence, enzyme-linked immunosorbent assay and immunoblotting. The most widely used methods in practice are enzyme-linked immunosorbent assay (ELISA), which is characterized by high sensitivity, the ability to quantitatively and visually record the results of the reaction, which makes the method accessible to laboratories of any level. ELISA uses foreign and domestic test systems.
Caution must be exercised in relation to children born from infected mothers. In the absence of a clinic, a child is considered infected if AT to HIV persists after a year. When receiving a positive result in ELISA, it is necessary to test sera that gave single positive results three times and confirm the positive result in an independent system - immunoblotting
Detection of AT in an ELISA reaction does not provide sufficient information, since it does not indicate the condition of the person being tested, but only indicates incubation, illness, or the presence of an asymptomatic infection. Immune blotting provides more information, since the presence of AT to many HIV antigens is characteristic of a severe disease, while a reaction with 1-2 antigens is more typical of a mild infectious process
It is informative to count the number of T (helpers) and the ratio of T4 to Te (suppressor) lymphocytes, determined using mono-body antibodies. An important criterion for the disease can be a sharp increase in the amount of immunoglobulins, especially A and V. In a general clinical blood test, the disease may be indicated by lymphopenia, leukopenia, erythropenia, thrombocytopenia, and eosinophilia.
HIV tests used for epidemiological surveillance do not need to be as accurate as those required for clinical purposes. However, if HIV prevalence in the population is very low, all positive samples must be re-examined in additional tests.
Blood collection for an HIV antibody test or for screening may be accompanied by registration of the names of the subjects (named collection), or may be carried out without registration of names or individual identification information (anonymous collection) (Table 2).
Anonymous, non-identifying screening has the following characteristics: it uses blood samples collected for other purposes; Anonymity is guaranteed due to the fact that identification data is not collected or taken into account; there is no requirement to obtain the consent of the subjects; no contact with counseling or social services required; Finally, and most importantly, errors in statistical estimates depending on the level of population participation are minimized.
Although anonymous HIV testing can provide more accurate data, this method has the following disadvantages: it cannot eliminate potential selection bias; data on high-risk behavior and other important variables are not available and cannot be collected retrospectively; it is impossible to contact people affected by HIV to inform them about their condition; The examination can only be carried out in groups of people from whom blood is taken for other purposes.
In areas where the prevalence of HIV infections is considered very low, public health surveillance should focus primarily on individuals or populations with the highest risk of infection behavior. sexual partners
Blood for HIV testing in this risk group is most easily collected at centers specializing in the treatment of sexually transmitted diseases or similar institutions. If intravenous drug use is also common, blood samples should be taken from drug users in special institutions. Blood collection once every 3 or 6 months from the highest risk groups in geographic areas where such groups are most abundant will usually be sufficient. An exception may be for risk groups such as drug addicts who practice intravenous drug administration, for whom more private examinations may be required.
WHO is currently developing a disease classification (staging) system for clinical trials that can also be used in treatment trials, which may also have prognostic value. However, such a system is not intended to replace existing AIDS definitions used in health care surveillance.
Currently, planned (routine) HIV surveillance systems are being developed everywhere. These systems need to be adapted to the current epidemiological situation; Thus, sampling methods in populations with very low viral prevalence must necessarily differ from those used in populations where prevalence is moderate or high.
Such surveillance involves routine examinations of well-defined and accessible population groups. It should primarily include those groups that are at greatest risk of infection, and from each of these groups a constant, predetermined number of individuals should be selected for examination.
In recent years, anonymous, identity-blind screening has become increasingly common as an accurate and cost-effective method for HIV surveillance in health care settings.
Timely diagnosis of HIV infection becomes an extremely important measure, since early initiation of treatment can largely determine the further development of the disease and prolong the patient’s life. In recent years, there has been significant progress in identifying this terrible disease: older test systems are being replaced by more advanced ones, examination methods are becoming more accessible, and their accuracy is significantly increasing.
In this article we will talk about modern methods for diagnosing HIV infection, knowledge of which is useful for timely treatment of this problem and maintaining a normal quality of life for the patient.
HIV diagnostic methods
In Russia, a standard procedure is carried out to diagnose HIV infection, which includes two levels:
- ELISA test system (screening analysis);
- immunoblotting (IB).
Other methods can also be used for diagnosis:
- rapid tests.
ELISA test systems
At the first stage of diagnosis, a screening test (ELISA) is used to detect HIV infection, which is based on HIV proteins created in laboratories that capture specific antibodies produced in the body in response to infection. After their interaction with the reagents (enzymes) of the test system, the color of the indicator changes. Next, these color changes are processed using special equipment, which determines the result of the analysis performed.
Such ELISA tests can show results within a few weeks after the introduction of HIV infection. This test does not determine the presence of the virus, but detects the production of antibodies to it. Sometimes, in the human body, the production of antibodies to HIV begins after 2 weeks after infection, but in most people they are produced at a later date, after 3-6 weeks.
There are four generations of ELISA tests with varying sensitivities. In recent years, third and fourth generation test systems have been increasingly used, which are based on synthetic peptides or recombinant proteins and have greater specificity and accuracy. They can be used to diagnose HIV infection, monitor HIV prevalence, and ensure safety when testing donated blood. The accuracy of generation III and IV ELISA test systems is 93-99% (tests produced in Western Europe are more sensitive - 99%).
To perform an ELISA test, 5 ml of blood is taken from the patient’s vein. At least 8 hours must pass between the last meal and the analysis (usually it is performed in the morning on an empty stomach). It is recommended to take such a test no earlier than 3 weeks after the suspected infection (for example, after unprotected sexual intercourse with a new sexual partner).
ELISA test results are obtained in 2-10 days:
- negative result: indicates absence of HIV infection and does not require contacting a specialist;
- false negative result: can be observed in the early stages of infection (up to 3 weeks), in the later stages of AIDS with severe suppression of the immune system and with improper blood preparation;
- false positive result: can be observed in some diseases and in case of improper blood preparation;
- positive result: indicates HIV infection, requires carrying out an IB and the patient contacting a specialist at the AIDS center.
Why can an ELISA test give false positive results?
False-positive HIV ELISA test results can occur due to improper blood processing or in patients with the following conditions and diseases:
- multiple myeloma;
- infectious diseases caused by the Epstein-Barr virus;
- state after ;
- autoimmune diseases;
- against the background of pregnancy;
- condition after vaccination.
For the reasons described above, nonspecific cross-reacting antibodies may be present in the blood, the production of which was not provoked by HIV infection.
In recent years, the frequency of false-positive results has decreased significantly due to the use of generation III and IV test systems, which contain more sensitive peptide and recombinant proteins (they are synthesized using genetic engineering in vitro). After the introduction of such ELISA tests, the frequency of false positive results decreased significantly and is about 0.02-0.5%.
A false positive result does not mean that the person is infected with HIV. In such cases, WHO recommends conducting another ELISA test (necessarily IV generation).
The patient’s blood is sent to a reference or arbitration laboratory with the mark “repeat” and tested using a IV generation ELISA test system. If the result of the new analysis is negative, then the first result is considered erroneous (false positive) and IS is not carried out. If the result is positive or questionable during the second test, the patient must undergo IB after 4-6 weeks to confirm or refute HIV infection.
Immune blotting
A definitive diagnosis of HIV infection can only be made after obtaining a positive immunoblotting (IB) result. To carry it out, a nitrocellulose strip is used, on which viral proteins are applied.
Blood sampling for IB is performed from a vein. Next, it undergoes special treatment and the proteins contained in its serum are separated in a special gel according to their charge and molecular weight (manipulation is carried out using special equipment under the influence of an electric field). A nitrocellulose strip is applied to the blood serum gel and blotting (“blotting”) is carried out in a special chamber. The strip is processed and if the materials used contain antibodies to HIV, they bind to the antigenic bands on the IB and appear as lines.
IB is considered positive if:
- according to American CDC criteria - there are two or three lines gp41, p24, gp120/gp160 on the strip;
- according to the American FDA criteria, the strip has two lines p24, p31 and a line gp41 or gp120/gp160.
In 99.9% of cases, a positive IB result indicates HIV infection.
If there are no lines, the IB is negative.
When identifying lines with gr160, gr120 and gr41, IB is doubtful. This result may occur when:
- oncological diseases;
- pregnancy;
- frequent blood transfusions.
In such cases, it is recommended to repeat the study using a kit from another company. If after additional IB the result remains doubtful, then observation is necessary for six months (IB is carried out every 3 months).
Polymerase chain reaction
A PCR test can detect the RNA of the virus. Its sensitivity is quite high and it allows detecting HIV infection within 10 days after infection. In some cases, PCR may give false-positive results, since its high sensitivity may also respond to antibodies to other infections.
This diagnostic technique is expensive and requires special equipment and highly qualified specialists. These reasons do not make it possible to carry out mass testing of the population.
PCR is used in the following cases:
- to detect HIV in newborns born from HIV-infected mothers;
- to detect HIV in the “window period” or in case of doubtful IB;
- to control the concentration of HIV in the blood;
- for the study of donor blood.
The PCR test alone does not make a diagnosis of HIV, but is carried out as an additional diagnostic method to resolve controversial situations.
Express methods
One of the innovations in HIV diagnostics is rapid tests, the results of which can be assessed within 10-15 minutes. The most effective and accurate results are obtained using immunochromatographic tests based on the principle of capillary flow. They are special strips on which blood or other test fluids (saliva, urine) are applied. If antibodies to HIV are present, after 10-15 minutes a colored and control strip appears on the test - a positive result. If the result is negative, only the control strip appears.
As with ELISA tests, the results of rapid tests must be confirmed by IB analysis. Only after this can a diagnosis of HIV infection be made.
There are rapid home testing kits available. The OraSure Technologies1 test (USA) is FDA approved, available over the counter and can be used to detect HIV. After the test, if the result is positive, the patient is recommended to undergo examination at a specialized center to confirm the diagnosis.
Other tests for home use have not yet been approved by the FDA and their results may be very questionable.
Despite the fact that rapid tests are inferior in accuracy to IV generation ELISA tests, they are widely used for additional testing of the population.
Tests to detect HIV infection can be taken at any clinic, central district hospital or specialized AIDS centers. On the territory of Russia they are carried out absolutely confidentially, or anonymously. Each patient can expect to receive medical or psychological consultation before or after the test. You will only have to pay for HIV tests in commercial medical institutions, while in public clinics and hospitals they are performed free of charge.
Read about the ways in which you can become infected with HIV and what myths exist about the possibilities of becoming infected.
The primary stage (screening) of HIV testing.
Attention! When passing the AT and AG test for HIV 1/2 for the primary screening study in accordance with the legislation of the Russian Federation It is mandatory to provide the following data and documents:
1) For residents of Moscow and the Moscow region
- Last name, first name, patronymic
- Day, month and year of birth
- Registration Information
- Passport
- Insurance policy (series and number of the insurance policy, name of the insurance company).
- Last name, first name, patronymic
- Day, month and year of birth
- Registration Information
- Insurance policy (series and number of the insurance policy, name of the insurance company)
- Passport
You can take the test anonymously: in this case, the patient is registered as anonymous with an individual order number (clause 2, article 8 of the Federal Law of the Russian Federation No. 38-FZ), with the obligatory indication of the year of birth and place of residence (subject of the Russian Federation).
We draw your attention!
That the results of the primary (screening) study are only the results of a laboratory test and are not the result of an examination or a conclusion about the presence or absence of HIV infection. Based on the results of the screening study, you should contact the municipal AIDS center to conduct a voluntary examination for the presence of HIV.
The results of an HIV test, regardless of their outcome, are issued only if the patient personally contacts the laboratory department. When examining minors (under 14 years old) - the legal representative specified in the order.
Results are issued upon presentation of a contract, estimate and identification document of the patient himself or the patient’s representative specified in the order.
Research results are not communicated by phone or email.
With this test, test serum/plasma samples can be simultaneously tested for the presence of HIV type 1 and 2 antigen and HIV type 1 and 2 antibodies.
The human immunodeficiency virus belongs to the family of RNA-containing retroviruses and the subfamily of lentiviruses, i.e. viruses of slow infections. HIV is genetically and antigenically heterogeneous; according to its structure, it is divided into types 1 and 2. When introduced into human cells, the virus forms a DNA section in their genome, which subsequently forms new HIV in unlimited quantities. Viral antigens appear on the surface of damaged cells. The response to their appearance on the part of the immune system is the formation of specific antibodies to the virus of the first or second type. Over the past three years, the number of HIV-infected persons officially registered in Russia has been growing exponentially. The rapid spread of HIV infection in the country may become uncontrollable. Therefore, one of the main tasks at present is the use of more advanced diagnostic methods to ensure the identification of infected individuals at the earliest stages of infection. These methods include laboratory serological tests that determine both the protein structures of the virus and specific antibodies to the virus.
Indications for the purpose of the study
1. Enlarged lymph nodes in more than two areas;2. Leukopenia with lymphopenia;
3. Night sweats;
4. Sudden weight loss of unknown cause;
5. Diarrhea for more than three weeks of unknown cause;
6. Fever of unknown cause;
7. Planning pregnancy;
8. Preoperative preparation, hospitalization;
9. Identification of the following infections or combinations thereof: tuberculosis, manifest toxoplasmosis, often recurrent herpesvirus infection, candidiasis of internal organs, repeated herpes zoster neuralgia caused by mycoplasmas, pneumocystis or legionella pneumonia;
10. Kaposi's sarcoma at a young age;
11. Casual sex.
Preparing for the study
It is recommended to donate blood in the morning between 8 and 11 o'clock, on an empty stomach (at least 8 hours must pass between the last meal and blood drawing, you can drink water as usual), on the eve of the study - a light dinner with limited intake of fatty foods.1-2 hours before donating blood, refrain from smoking, do not drink juice, tea, coffee, you can drink still water. Avoid physical stress (running, quickly climbing stairs), emotional excitement. It is recommended to rest and calm down 15 minutes before donating blood.
