Covid 19 screening test: what can they say …

As the covid 19 epidemic continues, continues to threaten our health and weigh heavily on our business, screening for infection is highlighted as one of the measures that can curb the spread of the virus and us. approaching a way out of the crisis.

Diagnosis and screening are two aspects of the same process. Where the diagnosis seeks the explanation of a declared disease, screening seeks to detect the possibility of the onset of the disease and its severe forms, with an individual prevention or public health objective and, in the case of a communicable disease, to detect the presence of the infectious agent to avoid transmission. The means used to achieve either goal are often of the same, if not identical, nature. The requirements on the characteristics of the tests (sensitivity, specificity, acceptability, accessibility, speed of execution and results, cost) may however differ depending on the use. In the case of covid 19, these tests are designed to highlight the same SARS-CoV-2 or “trace” it leaves in infected people, usually antibodies. There are other means to reach the diagnosis, highlighting signs, clinical or radiological, or specific biological disorders of the disease, but it is reasonably believed that only the presence of the virus in the respiratory tract should be detected. taken into account to take isolation or protection measures to prevent transmission.

We wanted to extend screening, make it widely accessible, combining it with other measures, with the aim of controlling the evolution of the epidemic and achieving its extinction. This needs to be well understood, well done, and lead to proper behavior.

What do we expect from screening?

It affects people who are in good health (asymptomatic) or who show signs of what could be covid 19 (symptomatic). On an individual level, screening can be reassuring when out of circumstances conducive to contamination. If positive, it will allow referral to adequate treatment, depending on the presence or absence of symptoms, associated risk factors (old age, pre-existing diseases). Depending on the type of first-line test performed, a more efficient confirmation test may be needed.

The test is also of community interest, as it is intended to be used to stop the transmission of SARS-CoV-2. It therefore aims to detect all virus carriers, symptomatic not yet treated and above all asymptomatic, likely to contaminate the people with whom they come into contact. These are obviously health professionals, people who live or work in contact with fragile people, candidates for travel, people unable to comply with physical protection procedures (mask, distance) such as athletes. Screening in the general population, offered in France on a voluntary basis, increases the scope of the measure. It is also necessary to know how to use the results and then apply the appropriate measures.

What can the tests indicate?

Serological tests, conventionally performed on blood samples (a drop taken from the fingertip may be sufficient; some antibodies – IgA – could be found in saliva), show the antibodies produced against the virus in infected people, which ‘may or may not they may show symptoms. The first antibodies do not appear until 3-7 days after the onset of the infection, their level increases only slowly, reaching a level that is all the more high as the infection produces severe disease. This “kinetics” means that they can remain undetectable, at least for relatively insensitive tests such as so-called “rapid” tests, for 2 to 3 weeks after the onset of infection, during which virus transmission occurs. possible. It is therefore understandable that serological tests are of little interest when it comes to preventing a virus carrier from passing it on to his contacts.

Only tests that directly detect the presence of viruses can be used for this purpose. These are the tests that detect the viral genome (RNA), thanks to a reaction called RT-PCR (reverse transcription followed by amplification), or those that detect the antigens of the virus (its proteins; we speak of antigen because they are proven from antibodies used as reagents). Both types of tests are performed on a sample taken where the virus is likely to be present in large quantities and where its presence indicates a risk of transmission: in the case of SARS-CoV-2, it is the mucosa located in the back of the nasal cavity. There is often confusion in the media between “test” and “sample” for screening, but we could perfectly do both types of tests on other samples, such as blood or saliva. However, SARS-CoV-2 is not abundant in the blood, and a blood test is more complicated to organize and perhaps less well accepted than a nasopharyngeal swab. As for the saliva, extracting it would be less painful than the swab, but so far it has not achieved sufficient sensitivity.

Sensitivity (ability of the test to detect small quantities of viruses) and specificity (its ability to indicate only the presence of SARS-CoV-2) are the fundamental qualities of tests, they must be known for a good interpretation of the results. Well developed, RT-PCR is an extremely sensitive and specific method: the RT-PCR test carried out on a nasopharyngeal sample (swab) is therefore today the most reliable method, the one that will be used if necessary when awaiting confirmation of the result of a other test. However, in some cases it may remain negative despite the infection: the sample may have been taken when there was not enough virus in the nose, it may have been performed incorrectly (swab not reaching the posterior wall of the nasopharynx), poorly stored or for too long, products may have been entrained that prevent the good performance of the RT-PCR (in principle they are detected during the reaction). Conversely, RT-PCR can give a “false positive” result, very often in the case of contamination of the sample with viral RNA from another reaction (again, very strict procedures limit the risk). Its sensitivity is such that it can detect traces of viral RNA when no infectious virus remains in the body.

RT-PCR tests require sophisticated equipment and in principle a good deal of experience, although they are always easier to perform. Their cost remains relatively high, but above all, being carried out in the laboratory, they often impose significant delays, of the order of 24 or 48 hours.

So-called “antigenic” tests (for antigen detection) are simpler to perform. After the nasopharyngeal swab (there is no escape!), The collected material is placed on a strip and the antibodies indicate the presence of viral proteins in about twenty minutes. These tests are accessible to everyone, even without a doctor’s prescription, they can be performed by pharmacists, nurses, doctors. They have good specificity, but their sensitivity is of the order of 70-80%, much lower than that of RT-PCR. The possibility of false negatives is therefore high, which has several consequences:

• If the test is positive, it indicates the presence of viruses in large quantities and imposes isolation and protection measures for at least 7 days.
• If the test is negative in a symptomatic person, or over the age of 65, or weakened by another medical condition, an RT-PCR test should be done.
• A negative test does not rule out the possibility of infection and therefore a risk of virus transmission and possibly future disease.

What can extended screening do?

Well done and followed, the detection of virus carriers certainly offers the possibility of interrupting transmission, waiting for vaccines that will protect us from infections or at least from disease. However, the execution of the available tests requires compliance with some rules:

• People deemed positive who are not hospitalized must take the necessary measures to avoid transmission of the virus. The duration of isolation is 7 days from the date of the positive test, extended by 7 days from the date of onset of symptoms if necessary (in this case, treatment may be considered). In case of fever at 7^th day, isolation should be maintained for up to 48 hours after the fever subsides. The search for contact persons must be carried out. The news reports the control measures envisaged by the authorities.
• In case of a negative RT-PCR test in a person at risk 7 days after the last contact with a positive case, the duration of the quarantine is also 7 days.
• Regardless of the test carried out, a negative result does not exempt you from protective measures, especially towards frail people (safety distance and mask). In fact, the negativity is valid only for the moment in which the sample was taken; it could be a question of false negativity (antigen tests), or contamination and multiplication of the virus could have occurred after sampling.

Screening tests cannot in any way replace barrier gestures and will not allow you to refrain from them during the end of year celebrations.

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