IFA is positive for syphilis. Enzyme immunoassay blood test with decoding of indicators. What does the analysis of ifa ag mean?

(ELISA) is a method of testing blood in the laboratory, based on the search for special cells - antibodies to various diseases. The method allows not only to identify the pathogen, but also to determine at what stage the pathological process is. The latter is very important for prognosis and further treatment patient.

Advantages and disadvantages of the method

Among all modern methods diagnostic ELISA is the most innovative and technically accurate. Its main advantages are:

1. The ability to search for all existing antibodies to infectious diseases in the patient's blood.
2. High accessibility of the research method. Today, ELISA tests can be performed by any medium-sized laboratory.
3. Almost 100% specificity and sensitivity of the method.
4. The ability to search for antibodies and antigens, as well as establish the stage of the pathological process and track its dynamics, thanks to comparison of quantities.

This number of advantages over other tests completely overshadows the one and only drawback of the test: it is able to detect antibodies, but not the pathogen itself.

Key terms for assessing analysis

In order to understand what ELISA analysis is, what it is and how it is performed, you need to get acquainted with the basic terms used by specialists.

1. Antibody- a protein that is produced by cells of the human immune system (type B lymphocytes). They respond with a specific reaction to the entry of a foreign agent or substance into the body. Another name for antibodies is immunoglobulins; they belong to different classes: A, E, M, G. They differ from each other in mass, reaction speed, half-life and a number of other characteristics. Normally, human blood contains mainly class G immunoglobulins. If any infection occurs, the amount of immunoglobulins A and M increases sharply. Immunoglobulins E take part in allergic reactions.
2. Antigen- a foreign agent of organic origin and high molecular weight. Most often it represents pathogens or their biologically active substances.
3. The antigen-antibody complex, or immune complex, is the direct combination of a foreign substance and immunoglobulin, which gives rise to the development of an immune reaction.

The essence and scope of the method

Patients often have a question: ELISA analysis, what is it, how is it performed and what is it for? You can talk about the method in an accessible way by briefly describing its stages.

Preparatory stage. The laboratory doctor uses a special plate with 96 wells. The antigen of a specific pathogen is applied to the surface of each well.

Stage 1. Blood is drawn, which is then applied drop by drop to the well. The well initiates a reaction between the antigen and the antibody in the blood.

Stage 2. The reaction with the formation of immune complexes is in full swing in the hole. As a result, a substance of a certain color is formed. The intensity of the color depends on the amount of antibodies in the patient’s blood to each specific pathogen.

Stage 3. Evaluation of the result by photometry. For this purpose, a special device called a spectrophotometer is used. It is used to compare the density of the material in the hole and the control sample. Next, the device generates a result through mathematical analysis.

Evaluation of results and purpose of ELISA

The interpretation of the result depends on several important nuances:

1. Optical density of the well.
2. Manufacturer of the plate with wells (test systems).
3. Laboratory in which the study was performed.

Given these nuances, you should never compare two results from different test systems or from different laboratories.

Another important point affecting the ELISA analysis is the so-called antibody avidity. This parameter characterizes the amount of antigen and the strength of the bond in the antigen-antibody complex. Its definition is based on the processing of the immune complex with urea in order to resolve protein structures. This allows you to destroy weak bonds between antigen and antibody and leave only strong ones. The significance of the avidity study is that it can be used to determine the duration of infection. This information is extremely important for making a diagnosis in pregnant women.

A blood test using the ELISA method serves:

1. To search for various pathogen antigens.
2. To study hormonal levels.
3. For examination for the presence of autoimmune pathology.
4. To detect markers oncological diseases.

Varieties of ELISA

ELISA analysis has the following types:

1. Indirect.
2. Direct.
3. Competitive.
4. Blocking method.

But in fact, today only a method called ELISA (enzyme linked immunosorbent assay) is used. It is based on the above-described reaction of the formation of an antigen-antibody complex with a change in color on the surface of the well.

The quantitative ELISA blood test itself deserves special attention. This is not a type of analysis, but a way of evaluating results. Thanks to it, the number of antibodies is calculated and their classes are determined. The result depends on the optical density of the sample, the test system on which the ELISA analysis was performed, as well as on the laboratory.

Diseases detected by ELISA

ELISA is a blood test that can detect a huge number of different infectious diseases. Moreover, both viral and bacterial diseases are detected with equal accuracy. For example, using the formation of immune complexes, one can prove the presence of antigens of pathogens of the following diseases:

In addition, ELISA allows you to detect:

1. Cancer markers - TNF (tumor necrosis factor), PSA (prostate-specific antigen), CEA (carcinoembryonic antigen), CA-125 (ovarian tumor marker)
2. The pregnancy hormone is hCG (human chorionic gonadotropin).
3. Violations reproductive system: hormones of the female and male reproductive systems.
4. Pathology of the thyroid gland.

It is important to mention that the ELISA test for HIV today is the main way to diagnose this dangerous disease.

Material for ELISA and sampling technique

To perform ELISA, blood is drawn from the patient on an empty stomach. Next, serum is obtained from the blood, which is directly used for analysis. In addition, ELISA can be performed on cerebrospinal fluid (CSF), cervical mucus (cervix), amniotic fluid, and even vitreous fluid (eyeball).

Before donating blood, the patient is warned that he should not take any medicines, and treatment with antibiotics and antiviral drugs It is recommended to finish at least two weeks before blood collection.

Deadlines for receiving and decoding results

The time it takes to receive an answer from the laboratory does not depend on the speed of its work, but on what stage the disease is at and what antibodies have already appeared in the blood. So, for example: immunoglobulins M appear approximately 2 weeks after taking blood for analysis and mean that the process is at the stage of primary infection or an exacerbation of chronic infection has occurred. At the same time, antibodies of classes M and G appear during primary infection. Moreover, the latter can be detected after 4 weeks.

IgA appears after 2-3 weeks either alone or together with M, meaning acute infection, or together with G, indicating a chronic process.

Such different terms The appearance of antibodies in the blood will force the patient to wait a long time for the result. It is acceptable to wait more than a month after the ELISA test is performed. Deciphering and interpretation by a doctor also takes a certain period of time.

Content

To assess the body's ability to resist infectious diseases or a blood test is used to determine the phase of the pathology. The ELISA method occupies an important place among laboratory tests; it helps to comprehensively study the activity of the protective function of the blood, determine immunodeficiency in infectious diseases, blood ailments, hormonal, autoimmune processes.

What is an enzyme immunoassay blood test?

This method refers to laboratory research that determines the presence of protective blood factors of a protein nature (antibodies) to certain pathogenic agents (antigens). An enzyme-linked immunosorbent test detects immunoglobulins, which can be found in the form of immunocomplexes. They appear when complex neurohumoral reactions of the human immune defense occur, which become a response to the introduction of foreign antigens.

The body produces specific antibodies against each type of pathogen. Next, the pathological microorganism or antigen binds and a complex “antigen-antibody” compound is formed. Then it is neutralized, enzymatic lysis occurs, a phagocytosis reaction occurs, and the process ends with removal from the body. The presence of specific complexes, determined by ELISA, indicates the type of pathogen harmful substance at the patient.

Immunoglobulin classes

Scientists have discovered and studied 5 types of immunoglobulins: IgE, IgD, IgG, IgM, IgA. There are other classes, but they are still at the research stage, and their role is not fully understood. In practical medicine, A, M, G are important. The information content and accuracy of determination is based on the time intervals during which they appear, reach a maximum and disappear.

Indications for blood testing using ELISA

Using this analysis, you can evaluate the effectiveness of treatment, carry out comprehensive study before transplant operations, determine the state of immunodeficiency and antibodies to more than 600 types of allergens. Blood testing using the ELISA method is used as an additional method for detecting cancer cells. An analysis is prescribed if it is necessary to detect antibodies to microbes that provoke sexually transmitted pathologies:

• trichomoniasis;
• syphilis;
• toxoplasmosis;
• mycoplasmosis;
• ureaplasmosis.

At helminthic infestations in the ELISA analysis an increase in the amount of immunoglobulins will be noted. Studies are carried out to confirm whether the patient has:

• Epstein-Barr virus;
• herpetic infections;
• cytomegalovirus;
• group of viral hepatitis.

Blood test using ELISA method

Enzyme immunoassay blood testing is not the only option for determining immunoglobulins. Sometimes for this study, cerebrospinal fluid, vitreous tissue, and amniotic fluid are collected. When using blood, it is collected from the antecubital vein using an injection needle. The test must be taken on an empty stomach; before ELISA, it is not recommended to take medications that could affect the result. You should give up alcohol, smoking, and drug use before donating biomaterial. Test result options:

1. If the immunoglobulins IgG, IgM, IgA are negative, doctors say there is no pathology or initial stage. The same result (negative) will be after complete recovery after a long period.
2. If IgG is positive, but IgM and IgA are not detected, this indicates the formation of immunity after vaccination or an infectious disease.
3. With high titers of IgM and negative IgA, IgG, a diagnosis of acute infectious disease.
4. If IgG, IgM, IgA are positive, doctors talk about the acute phase of a relapse of an existing chronic disease.
5. For a chronic infection that is at the stage of abating (remission), the ELISA test shows negative IgM titers, while IgA and IgG will be positive.

Advantages and disadvantages of ELISA analysis

The main negative aspect of this study is the possibility of obtaining false positive or false negative results. The reason for unreliability is the use of medications and technical defects in the laboratory. The process of metabolic disorders in the body can falsify the analysis. The main advantages of the ELISA analysis are:

• accuracy, diagnostic specificity;
• low cost of analysis;
• speed of obtaining results;
• the possibility of dynamic monitoring of the stage of pathology and the effectiveness of treatment;
• ease of research;
• the ability to perform mass examinations of foci of infection;
• painlessness, safety for the patient;
• application in information technology processing.

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ELISA for syphilis is one of the most effective methods, used to diagnose venereal disease caused by Treponema pallidum. The analysis allows you to determine the relationship of an antigen with an antibody and is carried out using enzymes. This highly accurate method detects infection in 90% of cases. The test system used in enzyme immunoassay diagnostics is designed to detect both syphilis and other diseases caused by pathogenic microorganisms.

When to take it

An ELISA test for syphilis is prescribed 3-4 weeks after the suspected infection.

It is often used to confirm the diagnosis after obtaining a positive result in a manner based on the Wasserman reaction. Enzyme immunoassay is carried out if:

• The patient has a rash in the genital area;
• there were cases of unprotected sexual contacts;
• the woman is pregnant;
• a man wishes to become a semen donor;
• the child was born to a mother suffering from syphilis;
• the patient has an elevated temperature for an unknown reason;
• hospitalization or surgery is planned;
• A medical examination is carried out for employees of child care institutions and catering.

How to prepare for the ELISA test

In order for the reliability of the enzyme immunoassay to be maximized, patients must be aware of the preparation rules, which, if not followed, may result in a false positive result. In order to minimize the error, you need to:

1. Donate blood on an empty stomach.
2. For two to three days, refrain from drinking alcoholic beverages, eating with increased content fat
3. Limit physical activity a few days before blood sampling.
4. Three hours before donating the biomaterial, stop smoking.
5. Tell your doctor about medicines currently used, as well as about existing diseases and recent vaccinations.

It is better to ask the laboratory workers about how long the analysis can take. The timing of the event is related not only to its workload, but also to the availability of special equipment.

How is ELISA analysis performed?

In medical centers, the detection of spirochetes using the enzyme immunoassay reaction method is carried out indirectly and directly. The laboratory technician places serum isolated from the blood collected from the patient in the morning into a tablet with rounded indentations. After adding chromogenic dyes using tables with color schemes included in the ELISA kit, the test is deciphered. In this way, a laboratory worker determines the levels of immunoglobulins and their number. In order to evaluate the outcome of the reaction, a spectrophotometer is also used, which allows you to determine the density of the biomaterial and compare it with a control sample.

Using ELISA it is possible to detect antibodies belonging to different classes (A, G, M). When the level of M-immunoglobulins increases, doctors determine the time when infection with pale spirochetes occurred. Typically this period does not exceed 90 days. If a patient has an increased titer of class A, this means that the infection occurred within the last 30 days. G-immunoglobulins make it possible to detect acute process, treated disease.

A summary test, designated in medical centers as “syphilis igg ELISA,” is prescribed to people who have not previously been examined for this venereal disease. The method allows you to detect it when the first symptoms appear and in the initial stage of infection.

Decoding the results - norms and deviations

Having received the transcript in hand, the patient cannot always correctly interpret its meaning. Only a doctor is able to determine the results and make an accurate diagnosis. The laboratory form may contain the following information:

• IgA+ IgM- IgG- – infection occurred recently;
• IgA+ IgM+ IgG- – the pathogen entered the body less than 30 days ago;
• IgA+ IgM+ IgG+ – infection lasted more than a month;
• IgA- IgM- IgG+ – venereal disease has been treated.

In some cases, if the test result is positive, the doctor may order additional tests using other methods.

ELISA accuracy

The method for detecting an infectious disease caused by Treponema pallidum is considered highly sensitive. Manufacturers of ELISA tests provide a guarantee of quality and confirmation of it through experiments in which the results of other specific diagnostics were compared. The accuracy is 98%.

A false positive result is possible in the following cases:

1. Pregnancy. During this period, there is a possibility of obtaining distorted results, since the genetic materials of the father and the fetus itself may be regarded as a foreign agent.
2. Chronic disease. A questionable result is possible if the patient has launched form tuberculosis, autoimmune processes, liver pathologies.
3. Recent vaccinations. The vaccine affects the synthesis of immunoglobulins.
4. Injury. A patient with recent injuries develops inflammatory processes that can cause a false-positive test.
5. Old age. Such people often have several chronic pathologies which have a strong effect on the immune system.
6. Newborns. In young children up to one and a half years old, serodiagnosis using the ELISA method can show positive results if women were infected with treponemes during pregnancy. The mother's immunoglobulins circulate in the baby's blood for a year or more.

Price issue

Depending on the laboratory, prices for specific ELISA tests vary from $6.1 to $11.4. Some diagnostic centers charge $2.50 for taking blood for analysis.

In many public medical institutions, ELISA can be taken free of charge. If laboratories are very busy and there is a need to get a quick result, it is better to take a syphilis test in private laboratories.

Where to get tested

An ELISA test to detect syphilis can be taken at Invitro. The cost of services is $14, including blood sampling in the treatment room. Completion time - 7 days.

The CMD also provides sexually transmitted disease testing services. The cost of the analysis is $9.2. Completion time: up to 3 days. If necessary, the institution's laboratory technicians can take blood at home.

You can get tested for syphilis in the Gemotest medical laboratory. The price of the RPG (passive hemagglutination reaction) test for syphilis is $10.3. Completion time - 1 day.

All diagnostic centers guarantee complete anonymity.

A syphilitic test based on an enzyme-linked immunosorbent reaction allows one to accurately detect a sexually transmitted disease caused by treponemes. The test is prescribed if a non-specific method for detecting infection shows a negative result when the patient has symptoms or has had contact with infected persons.

Treatment is carried out only if there are positive results obtained by several methods.

Enzyme immunoassay blood test is a common method for identifying various pathologies. The result of a blood test, the ELISA interpretation of which was interpreted by a doctor laboratory diagnostics, is reliable in most cases.

What is ELISA

ELISA is a modern diagnostic method that is used to diagnose infections, hormonal and immune disorders, and cancer in laboratory conditions. The method allows you to detect antibodies to infection already at early stages diseases. This method belongs to indirect diagnostic methods, as it reveals the body's immune response. Among the advantages of ELISA in comparison with other diagnostic methods is a high level of manufacturability, which reduces the likelihood of errors. The method is highly sensitive and is used to diagnose diseases in both children and adults. There are a wide variety of different modifications of ELISA.

The ELISA method is based on the specificity of immunochemical reactions, as well as the physicochemical patterns of reactions of antigen-antibody complexes. Reactions are carried out with the participation of specific enzymes, which are markers for the detection of antibodies. As a result of an immunochemical reaction, strictly defined antibodies bind to the corresponding antigens. Blood testing using enzyme immunoassay almost eliminates the possibility of obtaining a false positive result. Laboratory diagnostic specialists judge a positive or negative result based on whether the solution is colored during the enzymatic indication of the antigen-antibody complex. If the solution turns coloured, then the antigen interacts with the antibody, the ELISA result is positive.

What diseases can be detected by ELISA?

Research using enzyme immunoassay allows:

• identify a number of infectious diseases;
• diagnose autoimmune diseases;
• detect the presence of oncology;
• identify hormonal disorders;
• carry out other research.

A blood test using the ELISA method allows you to determine the presence of the following infections:

The method is used to identify antigens of pathogens of a number of infections, as well as to identify antibodies of various classes. The ELISA method has become very popular for detecting syphilis, HIV, and viral hepatitis. Determining the presence and level of antibodies in blood serum for the purpose of primary diagnosis of STIs is not recommended. In this case, the presence of antibodies in the blood can only be a sign that the patient’s body has been in contact with the infectious agent in the past.

Diagnosis of various autoimmune diseases using ELISA is carried out by examining:

• antinuclear bodies;
• antibodies to double-stranded DNA;
• antibodies to soluble nuclear antigens (ENA screen);
• anticardiolipin antibodies;
• IgG to citrullinated peptide;
• rheumatoid factor;
• C-reactive protein;
• autoantibodies to neutrophil cytoplasmic antigens (ANCA screen).

Specific immune complexes are characteristic of certain autoimmune diseases. For example, double-stranded DNA antibodies are characteristic of a disease such as systemic lupus erythematosus.

Cancer is determined by enzyme immunoassay of blood serum for specific tumor markers, such as PSA, CA-125.

PSA indicates the presence of prostate adenoma and prostate cancer. CA-125 is a tumor marker malignant tumor ovary. The value also increases with cancerous tumors of the uterus, mammary glands, and endometrium.

Preparing for analysis

To ensure maximum accuracy of the results, you need to follow certain rules for preparing for the study. An analysis for laboratory diagnostics using the ELISA method is usually taken in the morning from the antecubital vein. It is necessary to donate blood strictly on an empty stomach. In addition to this simple instruction, the following preparation recommendations should be followed:

• 24 hours before the test it is necessary to avoid drinking alcohol and smoking;
• avoid heavy physical activity;
• remain in a calm state;
• avoid nervous tension;
• donate blood for ELISA no earlier than 10 days after stopping the medication;
• notify your doctor about taking any necessary medications.

In addition, a few days before the test, it is recommended to adhere to a diet. At the same time, exclude from the diet fatty foods, fried food. Before analysis viral hepatitis exclude from the diet not only fatty and fried foods, but also citrus fruits, as well as orange vegetables.

It should be noted that the results of a certain range of hormonal studies are influenced by such factors as the phase menstrual cycle. The need to take a test at one or another phase of the menstrual cycle should be discussed in advance with your doctor. Otherwise, you may get unexpected results. For example, the normal level of luteinizing sex hormone in women varies depending on the day of the cycle:

• 1-12 days - 2-14 honey/l;
• 12-14 days - 24-150 mU/l;
• from the 15th day before the start of a new cycle - 2-17 mU/l.

Decoding ELISA results

The analysis allows us to determine the presence of antibodies of various classes in the body. There are 3 classes of antibodies:

The production of these antibodies occurs at various stages of the disease. IgM antibodies are the first to be produced in the body after infection. Their presence is an indicator of the disease in any case. This class of antibodies is absent in the body of a healthy person.

These immunoglobulins are present in the blood serum for about 5-6 weeks.

Class G immunoglobulins present in the blood indicate that a person has either already suffered from the disease or is a carrier of the infection. These antibodies begin to be produced after class M antibodies, in most diseases 3-4 weeks after infection. Their presence in the body is possible for several years. And in some diseases (for example, syphilis), IgG is present in the blood for life.

If IgA is present in the body, it is necessary to fight the infection as intensively as possible. Antibodies of this class appear only in the case of a chronic infectious disease. The disappearance of IgA indicates the destruction of the infection.

If an ELISA test is performed in children under 1.5 years of age, the following feature must be taken into account: the child’s blood contains mother’s IgG for various infections. This does not mean that the child is sick. In this case, this is rather the norm. The presence of IgM is evidence intrauterine infection or an infection acquired after birth. Mother's IgM cannot enter the baby's body through the placenta.

The table shows possible combinations of the presence or absence of antibodies of 3 classes in the body and their interpretation.

When decoding tests (+) indicates a positive result, and (-) indicates the opposite, that is, negative. A result showing the presence or absence of a substance in the body is called qualitative. It can be supplemented quantitatively. The quantitative result reflects the quantitative content of various substances in the body.

It is worth noting that test systems have their own reference values, defined by the manufacturer, that characterize the indicators. Exceeding the reference values, as a rule, means the presence of certain pathologies in the body of the test subject.

After receiving the ELISA results, the values ​​obtained should be deciphered by the attending physician. Only he will be able to correctly assess the results obtained and determine the stage of the disease.

– modern laboratory test, during which a search is conducted for specific antibodies in the blood or antigens to specific diseases in order to identify not only the etiology, but also the stage of the disease. ELISA results can be given qualitatively and quantitatively.

Currently, ELISA is used in the following situations:

1) Search for specific antibodies to any infectious disease;
2) search for antigens of any diseases (infectious, venereological);
3) study of the patient’s hormonal status;
4) examination for tumor markers;
5) examination for the presence of autoimmune diseases.

Advantages of the ELISA method:

1) High specificity and sensitivity of the ELISA method (more than 90%).
2) The ability to determine the disease and track the dynamics of the process, that is, comparing the number of antibodies in different time periods.
3) Availability of ELISA diagnostics in any medical institution.

Relative disadvantage:

1) Detection of the immune response (antibodies), but not the pathogen itself.

Basic Concepts

Before we clarify the essence of the ELISA method, let us briefly understand some concepts.
Antibodies (or immunoglobulins - Ig) – specific proteins produced by B -
lymphocytes (immune cells) in response to any infectious pathogen (viruses, bacteria, fungi, etc.) entering the body. There are immunoglobulins A (IgA), immunoglobulins E (IgE), immunoglobulins M (IgM), immunoglobulins G (IgG), immunoglobulins D (IgD). They differ from each other in molecular shape and weight, half-life, participation/non-participation in infectious processes, and timing of detection from the moment of infection. If we consider the molecular weight, then IgM has the largest weight - it is a pentamer (950,000 daltons) in contrast to other Igs (from 150 to 200,000 Da), due to which IgM simply cannot pass through the placental barrier. Therefore, the detection of IgM in a child of 1 year of age is always a sign of the presence of infection in the fetus. In blood serum, the bulk of immunoglobulins is represented by IgG (75-85%), and the lowest is IgE (0.003%). Only IgA, M, G are directly involved in the infectious process. IgE is a sign of allergic reactions and diseases, and IgD can only be found in the tissue of the lymph nodes and tonsils and plays a role in the formation of local immunity.

Antigens – high-molecular substances of organic origin, in particular pathogens of infectious and other diseases, as well as substances of various altered cells formed during a particular disease (autoimmune diseases, oncology).

Immune complex – an antigen-antibody complex involved in the immune process.

What is the ELISA method based on?

There are several types of ELISA (direct, indirect, blocking method, competitive), but in practice the heterogeneous solid-phase immunoassay or ELISA (enzyme linked immunosorbent assay) is most often used.

The basis of enzyme immunoassay is the immune reaction of antigen and antibody with the formation of an immune complex: antigen-antibody, which results in a change in the enzymatic activity of specific marks on the surface of the antibodies.

In simple terms, this process can be divided into several stages:

1) On the surface of the wells of the tablet of the doctor conducting the examination, there is a purified antigen of a certain pathogen. When biological material (blood serum) from the patient is added, a specific reaction occurs between this antigen and the desired antibody (immunoglobulin). This compound will act as a “special antigen” in the next stage.

2) At this stage, the formation of IC (immune complexes) occurs - a reaction between a “special antigen” and a conjugate (this is an immunoglobulin labeled with the enzyme peroxidase). A special chromogen is added. The result of this enzymatic reaction is the formation of a colored substance in the well of the tablet, the color intensity of which depends on the amount of immunoglobulins (antibodies) contained in the patient's material.

3) Next, the result is evaluated: photometry using a multichannel spectrophotometer, comparison of the optical density of the material under study with the optical density of control samples, mathematical processing of the results. The amount of antibodies in a patient directly depends on the height of the optical density of a given well.

Typically, 96-well plates are used in practice.

When measuring the optical density (OD) of the test liquid, the amount (or concentration) of antibodies in a certain unit of volume is calculated. The result is then compared with a control sample.

Need to remember:For each test system, individual indicators are developed to record the results, indicators of normality and pathology (that is, “reference values”). This must be taken into account when assessing the results of each specific study. It is incorrect to interpret the results of one laboratory based on the “reference values” of another laboratory. It is also incorrect to compare the results of different laboratories with each other.

When performing ELISA reactions, the concept of antibody avidity is also important.
Antibody avidity - this is the strength of the connection between the antibody and the antigen and the amount of antigen that is in relationship with immunoglobulins (antibodies). Avidity has great value when assessing the expected duration of infection, which is extremely important when diagnosing primary infection in pregnant women.

The basis of the antibody avidity test consists of treating the immune complex (antigen-antibody) with a urea solution to destroy the protein. High-avidity bonds remain intact, while low-avidity bonds are destroyed. The result is given as an avidity index expressed as a percentage (%).

What diseases are detected using ELISA diagnostics?

2. Markers of autoimmune diseases and indicators of human immunity(total IgE, total IgG, total IgA, total IgM, total IgD, secretory IgA, IgG 2, IgG4, CEC-circulating immune complexes, IgA and IgG to gliadin and others)

3. Oncological markers(TNF - tumor necrosis factor, CEA - carcinoembryonic antigen, PSA - prostate-specific antigen, hCG - human chorionic gonadotropin, CA 125, alveomucin and many others)

4. Reproductive disorders I (estradiol, progesterone, prolactin, testosterone, AFP-alphafetoprotein, FSH - follicle-stimulating hormone and others)

5. Thyroid diseases(free and bound T3, T4, thyroglobulin, thyroid peroxidase - TPO, thyroid-stimulating hormone - TSH).

This list does not represent all diseases that are diagnosed using enzyme immunoassay.

Material for ELISA analysis and rules for its collection

The most common material for the ELISA reaction is the patient's blood serum taken on an empty stomach. The material can also be cerebrospinal fluid, amniotic fluid, vitreous contents, mucus cervical canal and urethra, smears.

Preparing patients to submit material for ELISA

Production time for ELISA

Enzyme immunoassay of the material is carried out quickly, within 24 hours. Delays may occur in different laboratories due to the accumulation of a certain amount of serum.

Possible results of ELISA diagnostics

When assessing the results for specific infections, the class of antibodies detected and their quantity are important. Not only the question of the etiology of the infection (whether it exists or not), but also the expected stage of the disease (acute, chronic), as well as the presence of an active infection (acute or exacerbation of chronic) at the time of examination depends on this.

What is the approximate time frame for the appearance of antibodies (immunoglobulins - Ig)?

The earliest antibodies are IgM. They can be detected 1-3 weeks after possible infection, which characterizes acute phase infectious process. The second situation for the appearance of IgM antibodies is the activation (or exacerbation) of a chronic process. IgM antibodies circulate on average for about 3 months, then their number gradually disappears. However, in some patients, trace amounts of IgM can be detected within 1-2 years after infection.

Modern test systems are highly sensitive, resulting in nonspecific false positives(often in pregnant women). Therefore, in this group of patients, positive IgM must be rechecked!

IgA antibodies appear 2-4 weeks after infection, but in quantities sufficient for detection within a month. Serum IgA is synthesized by plasma cells of the spleen, lymph nodes and mucous membranes. Secretory IgA is concentrated on the mucous membranes to perform its protective function - they participate in local immunity.

From the 4th week after infection, IgG antibodies begin to appear. With most infections, their titer gradually increases with a maximum at different times (on average after 1.5-2 months), then the titer remains at a low level and indicates immunity. In some diseases (mycoplasmosis, chlamydia, trichomoniasis), the IgG level is not high and decreases significantly due to the lack of immunity in these infections.

Options for detecting antibodies of different classes:

Isolated detection of IgM antibodies suggests the presence of a primary
infection.
- Simultaneous detection of IgM and IgG in the blood is characteristic of primary infection
in the previous 2-3 months, as well as during exacerbation of a chronic disease. Therefore, during pregnancy, the presence of IgM is not always a sign of primary infection.
- Detection of isolated IgG may indicate immunity to this disease,
as well as for chronic infection. In the second situation, both the amount of antibodies (titer) and the change in this titer over time are important. Typically, studies are carried out at intervals of 2-4-6 weeks.
- Detection of IgA isolated or with IgM indicates a primary infection. At
The appearance of IgA along with IgG suggests activation of a chronic infection (on average 2 weeks from the moment of exacerbation).

Definition IgG antibody avidity is an excellent complementary stage in the diagnosis of primary infection from long-standing infection, which has its clinical significance, first of all, when assessing the risk of intrauterine infection of the fetus. The detection of low-avidity IgG indicates a primary infection and is detected on average 4-6 months after infection, rarely longer. Low-avidity IgG requires other laboratory confirmation of primary infection (IgM). High-avidity antibodies are either a sign of a chronic disease and its exacerbation, or of formed immunity.

Features in infants:in children up to one year old, and sometimes even 1.5 years old, maternal IgG to various infections circulates in the blood (that is, they penetrated through the placenta from mother to fetus during intrauterine development). They are not in themselves a sign of the presence of infection in the present. If IgM is detected at this age (remember that maternal IgM cannot penetrate the placenta), then this is a sign of intrauterine infection or an infection acquired after birth.

Quantitative ELISA method

The result of ELISA diagnostics (using an enzyme immunoassay analyzer) is given in certain units of measurement:
- Optical density (OD) of a sample – the concentration of specific antibodies per unit volume. The higher the OD of the sample, the higher the concentration of antibodies. Some results refer to the positivity coefficient (CP), which is also the optical density of the sample.
- Units of antibody concentration (nanogram/milliliter or ng/ml).
- In the form of serum titers: 1:20, 1:40, 1:100, 1:200, 1:400, 1:800, 1:1200 and so on. Diagnostic titers (at which the diagnosis of the disease is made, and not the fact of infection) are different for different diseases.
- In the form of symbols – “+”, “-”, “?” (+, ++, +++, ++++).
- In the form of a qualitative assessment according to a given criterion (positive or negative).

Only a doctor can correctly assess the number of antibodies, the option of class detection of immunoglobulins, and, therefore, set the stage of the disease and the need for treatment.

We must not forget that for any test system its own “reference values” (variants of the norm) are developed, when exceeded, a particular disease is diagnosed (variants of pathology). For different test systems, the “reference values” are different.

Correct comparison of ELISA results taken over time is only possible if they were produced in the same laboratory.

Infectious disease doctor N.I. Bykova