Flu tests are gaining in popularity. The influenza virus causes acute respiratory infections and kills several hundred thousand people worldwide every year. The elderly and immunocompromised people are particularly vulnerable to the complications of the disease. And it is in them that flu tests are of particular importance in identifying the virus. What are the types of flu tests? Which tests should be performed to obtain a reliable result?
Flu testsare the basis for the diagnosis of the disease. This is because, due to the similarity of the clinical picture and course of influenza (so-called flu-like symptoms) to other viral infections, it is impossible to diagnose the disease on the basis of symptoms alone. Influenza is a potentially fatal disease, especially in groups at high risk of complications, such as the elderly, young children and immunocompromised people. For this reason, it is important to quickly identify the virus and apply appropriate treatment, e.g. with neuraminidase inhibitors. However, flu testing is not usually done routinely, as most people are mildly ill and do not require pharmacotherapy.
Flu test - types of testing
Laboratory tests for influenza viruses include:
- molecular (genetic) tests aimed at confirming the presence of the genetic material of the virus in the collected sample
- serological (immunological) tests to confirm the presence of antibodies raised against the influenza virus in the sample taken
- quick diagnostic tests, the advantage of which is obtaining the test result within several or several dozen minutes
Flu testing - molecular testing
Molecular Testis the most sensitive and specific flu test. Testing is primarily based on the reverse transcription polymerase chain reaction (RT-PCR). The test is to detect the genetic material of the virus, i.e. RNA in the collected sample andis the so-called the gold standard in influenza diagnosis . The high specificity of molecular tests is guaranteed by the use of special fluorescent primers and probes that connect with sequences characteristic of the influenza virus.
The advantage of molecular testing is that it can be differentiatedinfections with particular types of influenza virus, and even specifying its subtype, e.g. A (H1N1) pdm09 or A (H3N2).
Properly collected material for the study is the key to obtaining a reliable result of the molecular test for influenza. The swab used for the swab cannot be made of cotton, cotton wool or wood. Most often, swabs made of Dacron (a type of synthetic fiber) are used for this purpose. In addition, the collection time, storage and transport conditions for the sample are important.
Abnormalities may result in false-negative results, i.e. the patient is infected with the influenza virus but tested negative. In this case, when the symptoms clearly indicate influenza, the test should be repeated from a newly taken sample.
Flu test - serology tests
In addition to molecular flu tests, serological tests can be performed to detect the presence of immune proteins (antibodies) in the patient's blood. However, their importance in clinical practice is marginal, as they do not confirm the active infection with the influenza virus. This is because the immune system needs several days to produce specific IgM and IgG antibodies against the virus.
For this reason, serological tests must be performed on two samples of the patient's blood taken at the beginning of the disease and during the convalescence period or several weeks after the infection. Testing for the flu once, or at the very beginning of an infection, may give you a false-negative result.
Serological tests are more important in assessing the effectiveness of vaccination against influenza viruses.
The most commonly used serological test for the diagnosis of influenza is thehemagglutination inhibition test . The method relies on the ability of antibodies raised against a protein on the surface of influenza viruses (haemagglutinin) to inhibit red blood cell agglutination (sticking together).
Another commonly used serology test ison the method of neuraminidase inhibition . This test is mainly used to assess the effectiveness of an influenza vaccine.
Flu Test - Rapid Diagnostic Tests
The Rapid Influenza Diagnostics Test (RIDT) relies on the direct detection of proteins (antigens) of influenza A and B viruses. Test result is available within 15-30 minutes, therefore the test can be performed by a doctor or nurse during the patient's visit to the office.
Rapid Influenza Diagnostic Tests are based on an enzyme immunoassay or immunochromatography method. They are currentlyone of the most widely used diagnostic tests for influenza viruses.
The disadvantage of rapid diagnostic tests is the high rate of false-negative results, which increases further during the flu season. On the other hand, during periods of low flu incidence (e.g. summer), the likelihood of false-positive results increases. Therefore, these tests should be treated as a preliminary test, and any questionable result should be confirmed with a laboratory test anyway.
The material for the test is a nasopharyngeal swab, a nasal aspirate or an upper respiratory wash, which should be collected in adults no later than 48 hours after the onset of the first symptoms of infection. In young children, the number of flu virus particles is usually higher and testing later (even after 5 days) is also possible.
Flu test - what does the collection of test material look like?
Correct sample collection is essential in getting a reliable flu test result.
In order to obtain a swab from the nasopharynx, the person taking the dacron swab introduces the patient's nostril to a depth of 3-5 centimeters. Then, after a few turns, the swab is taken out and placed in the transport tube. This method of extraction is usually used by adults.
In children, however, rinses or aspirates from the nose are more often taken. The aspirate is withdrawn by the person taking a small amount of sterile saline solution into the nose using a syringe. The saline is then aspirated back into the syringe. The aspirate obtained in this way is placed in a special virological medium.
The serological test is performed on the false blood taken from the elbow bend.
Flu test - when is the test performed?
- in hospitalized people suspected of influencing
- in people with flu-like symptoms at high risk of complications:
- elderly
- with chronic diseases such as cardiovascular disease, asthma, kidney disease and liver disease, diabetes
- with immunodeficiency e.g. HIV-positive, organ transplant
- in pregnant women
- about obese people
- in an epidemiological investigation that identifies the source of the infection and the routes of spreading the infection
- in the process of assessing the effectiveness of the flu vaccination
Flu test - how to interpret serological test results?
Increased concentration of IgM and IgG antibodies against virus proteinsinfluenza (e.g. haemagglutinin) indicates infection caused by flu. However, be aware that a single sample study may not be clinically informative. Hence, serological tests should be performed on two blood samples taken during the acute period of the disease (but not later than 7 days after their onset), and then during the convalescence period (2-4 weeks after the onset of the disease). This is to determine the changing dynamics of the antibodies.
Flu tests - how to interpret the results of molecular tests?
A positive result obtained by molecular methods means that the tested person has an active infection with the influenza virus. A negative result means no flu virus infection. However, in the event that the symptoms clearly indicate influenza, the test should be repeated from a newly taken sample.
Flu testing - where can it be done? Price
In Poland, molecular and serological tests for influenza are performed by several reference diagnostic laboratories, including National Institute of Hygiene in Warsaw. A flu test can also be done in commercial laboratories.
The price of a serological test(IgM and IgG antibodies) is about PLN 160
The cost of a molecular testwith a virus subtype is about PLN 200.
Flu test - and coronavirus
Experts call for an overlap between influenza and COVID-19 in the fall and winter. Both diseases are caused by viral infections and show a very similar clinical picture. Hence, it will be crucial to distinguish infection in people with flu-like symptoms. Several companies are currently developing molecular and serological tests to differentiate the two infections.
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