The pandemic of a koronavirusny infection of 2019 (COVID-19) continues to have an impact on the most part of the planet. Knowledge of diagnostic tests on SARS-CoV-2 in the course of an operating time, and a clear understanding of the mechanism of these tests and also the correct interpretation of their results is extremely important. This article explains how to interpret two types of the diagnostic tests which are usually used for detection of the infection caused by SARS-CoV-2: PTsR with reverse transcription (OT-PTSR) and IFA for definition of IgG and IgM and also how their results can change eventually.
Today the most often used and reliable test for diagnostics of COVID-19 is PTsR with reverse transcription. As material for a research most often use a smear from a nasopharynx or other biosamples from the upper airways, for example, a smear from a stomatopharynx or saliva.
With symptoms of a koronavirusny infection virus RNA in a smear from a nasopharynx is found in most of people from the first day of emergence of symptoms and becomes maximum within the first week. Level of positivity begins to decrease by third week of a disease and gradually becomes not defined. In hard cases positive PTsR can remain within three weeks from the moment of the beginning of a disease whereas in the majority of mild cases shows a negative take. However, positive PTsR reflects only presence of virus RNA and optional indicates existence of a viable virus. In certain cases virus RNA is defined by the PTsR method even in six weeks after the first positive test. Also several cases with positive takes which were carried out after two PTsR consecutive negative tests which are carried out with a difference at 24 o'clock were recorded. Not clearly, it was a mistake in a research, repeated infection or reactivation.
The timeline of positivity of PTsR changes if as samples use not a smear from a nasopharynx, and other samples of biomaterials. In particular, in a phlegm the positivity of PTsR decreases more slowly and can be revealed even after smears from a nasopharynx show a negative take.
Infection with a coronavirus can be also defined indirectly — by a research of an immune response of the patient. The serological diagnosis is especially important for patients with easy or medium-weight forms which late ask for the help, two weeks later from the beginning of a disease. Serologic assay also becomes the important tool for understanding of scales of distribution of COVID-19 among the population and identification of persons, immunized and "protected" from infection.
The most sensitive and earliest serological marker is the general level of antibodies. The IgM and IgG levels begin to increase with the second week from the moment of emergence of the first symptoms; IFA for their definition usually becomes positive even earlier, for the fourth day after emergence of symptoms, reaching the maximum level in 2–3 weeks of a disease.
Specificity of IFA for definition of IgM and IgG at diagnostics of COVID-19 more than 95%. Initial PTsR, a research of pair serums and the second research in two weeks can increase diagnostics accuracy. As a rule, the majority of antibodies are produced against the most intensively produced protein of a virus - a nucleocapsid. Therefore tests which reveal antibodies to a nucleocapsid have to be the most sensitive. On the other hand, protein of RBD-S is a protein of attachment from the owner's organism, and the research of antibodies to it has to be more specific. Therefore, the research of one or both antigens for identification of IgG and IgM will lead to increase in the general sensitivity. However, antibodies can show cross-responsiveness concerning SARS-CoV-2 and other coronaviruses.
Rapid tests are eurysynusic and available to identification of antibodies, having at the same time different level of quality. Many producers do not open the nature of the used antigens. These tests seldom show result in a qualitative sense and, as a rule, they can only indicate existence or lack of antibodies to SARS-CoV-2.
Thus, there is a certain timeline of diagnostic markers for identification of COVID-19. The majority of the listed data are relevant for adult not immunokomprometirovanny persons. Time of detection of positive PTsR and seroconversion can change at children and some groups of adults, including a large number of asymptomatic patients, diagnostics of a koronavirusny infection from whom demands active intervention. There are many questions, for example, as the immunity at the transferred COVID-19 of persons with symptoms and without those and search of answers to the matters — a problem of the subsequent researches in this direction long remains.