COVID-19 DETECTION TESTS Clarifications about the different tests that exist

Recently, new rapid antigen detection tests have appeared on the market, which raises the question of which would be the most appropriate test in each case.

Among all current methods, nucleic acid-based kits (PCR) are considered the most reliable because of their excellent sensitivity and specificity.
Therefore, to date, it is the method of choice at the beginning of a viral outbreak.
This is not surprising since these methods target unique sequences in the viral genome for identification.

At this stage, it is critical to identify and isolate all potential patients before the virus enters a stage of exponential growth.
Therefore, it is important to reduce false-negative diagnostic results.
To achieve this, high sensitivity is necessary.

On the one hand, the PCR amplification used can detect reactions as low as 100 copies for a reaction volume of 100 μL.

On the other hand, it should be noted that high sensitivity is often accompanied by an increase in false-positive results.

When a PCR technique is performed for the detection of a virus, for example, Covid-19, it must be indicated how many amplification cycles have been performed.

What amplification does is that the genetic material is copied over and over again until it reaches detectable levels, revealing even very small amounts of the virus.

If by “doing better” more cycles are done, you can have positives that with fewer cycles are negative.
Doing many cycles could give false positives in patients who have already passed the infection, with positive IgG, and who could continue to test positive in the PCR test.

As an example, with 25 cycles we obtain about 67 million copies of the amplified fragment and with 30 cycles 2,147 million copies.

Cycle threshold readings are often inconsistent between different types of PCR-based tests, and even between laboratories that use the same test.

Both PCR and rapid antigen tests have another limitation: they look for pieces of the virus’s anatomy (remnants of the pathogen) and cannot determine whether the virus is still active.

Some people who have been infected by the coronavirus may register as PCR positive for weeks, even months, after they are no longer contagious or sick , Simply because the tests are detecting harmless memories of a past infection.
They are not real live virus measures.

But, let’s go back to reviewing what types of tests are available today:

RAPID TESTS: they do not require equipment, and the result is available in 15 minutes

1)Rapid Antigen TEST

The ANTIGEN rapid diagnostic test detects molecular fragments of proteins from the membrane of the SARS CoV-2 virus (Corona Virus 2019 (COVID-19)).

It allows you to know in 15 minutes after taking the test, if a person is a carrier of the virus and is, therefore, potentially contagious.

The SARSCoV AMP Rapid Antigen Test is a rapid immunochromatographic test for the qualitative detection of SARS-CoV-2 nucleocapsid protein antigen in human nasopharyngeal swab samples.

How is it done?

· Insert the swab into the patient’s nostril until it reaches the posterior nasopharyngeal surface (where most of the secretion is present).

· Rub by turning the swab several times.

· Place the swab in the tube along with 10 drops of buffer.

· Pour the final solution into the cassette window with the pipette.

· After 15 minutes, the results will appear

Advantages

1. Cheap, fast and easy to use: the result is obtained in just 15 minutes, making it a valuable tool for efficient detection of SARS-CoV-2 infection.
2. Speed is important in a situation of “panic”.
3. They can be used on a massive basis at the point of care (POCT).
4. Due to its sensitivity, it is recommended to use from the onset of symptoms up to 7 days.
   From this moment on, perhaps it would be more useful to do an antibody test where we would begin to see IgM appear.

Limitations

1. Not all tests have good sensitivity and specificity.
   For this reason, it is advisable to check the quality certificates before buying, see sensitivity and specificity section
2. Unlike PCR-based tests, antigen tests do not include a step in which they amplify their targets, which makes them faster and more convenient, but less reliable in finding the virus when there is little abundance, but in this case it could be an advantage by not detecting false positives, which is why the detection limit of the test or test is important.

Detection limits:

It is important when selecting the test/manufacturer as they can vary.
For example, if we compare:

Test Brand A: 1.15 x 102 TCID50/mL.

Test Brand B: 2.4 x 102 TCID50/mL.

Test Mark C: 2.5 x 102 TCID50/mL.

How do we understand this?

Well, the lower the value, the better the detection level.
This would answer some questions as to why some tests fail to detect some positives (false negatives)—therefore the lower the value.

We have not put the name of the brands but A, B, or C (they are the ones we have cross-checked)

What the buyer of these tests must do is precisely review the quality parameters of each brand, in terms of not only the level of detection but also what refers to sensitivity and specificity

For example, a test that has a sensitivity of 97% means that out of 100 positive patients it would detect 97 and 3 would be negative.

A specificity of 100% would mean that in all cases what it is detecting is the COVID-19 virus

2) Rapid Test for the Detection of IgG and IgM Antibodies.

The IgG/IgM antibody test is an excellent screening method for both patients who have passed the COVID-19 infection, either with a PCR+ test or under suspicion of having passed it, or without PCR performed.

The IgG/IgM detection test is useful to check if you are passing the infection (IgM positive) or if you have developed Immunity (IgG positive) for both carriers with symptoms or asymptomatic and thus prevent transmission.

Advantages

1. It is a test that has a relatively low cost
2. Easy to perform, no appliances required
3. Fast in time, result in 15 min
4. It is performed with a capillary blood sample.
5. As it is less expensive, it allows it to be repeated several times, either as a control of whether the infection has started or if antibodies are being created while being asymptomatic.

Limitations

1. It is a qualitative method, the result would come out positive or negative
2. They have somewhat less sensitivity than the laboratory test.

WHEN TO TEST AND TO WHOM?

Initially it makes sense to do them to those with symptoms.

The most common symptoms are: fever, dry cough, tiredness

Other less common symptoms: sore throat and soreness, diarrhea, conjunctivitis, headache, loss of sense of smell or taste, rashes, or loss of color in the fingers or toes.

Serious symptoms include shortness of breath or shortness of breath, chest pain or pressure, cardiovascular problems, inability to speak or move.

The Antigen test must be done if we have come into contact with someone positive or have symptoms

The Antibody test must be done if we have been in contact with someone positive for at least 6-7 days or if we believe that we had symptoms more than a week or time ago and we want to find out if we are passing the infection asymptomatically or we want to see if we have developed immunity

Also for those people who, having been infected, want to find out if they have developed immunity.

ASYMPTOMATIC PHASE

The problem arises in the asymptomatic phase where the person may have the presence of the virus and be contagious but not have symptoms

This asymptomatic phase can last 14 days and that is why the initial confinement arose, where we all stayed at home supposedly assuming that we could be infectious, to prevent the possible spread of the virus.

Fever and symptoms may start later.

It was assumed that if we all stayed at home, it would be known at the end of these 14 days who had symptoms and therefore when they were identified they would be treated in health centers, clinics and hospitals.
The problem is that many of the people who were active out of necessity (essential workers) were able to continue transmitting the disease and that is why the confinement was extended.

Currently, and after the summer where everyone has socialized with larger groups, the virus has spread again.

In our opinion, rapid tests are undoubtedly the easiest tool due to their speed and price to be able to detect most of the population early, either in the workplace, or before celebrating a wedding or event, to travel, and before entering a panic situation, to resolve the doubt that people are positive.

When do positive results come out?

The virus/antigen appears positive from the time we have the infection and disappears when we are cured and do not have the ability to infect others.
But as we have said before, since existing virus detection tests detect traces of dead virus, the results could still come back positive once we have been cured.

How do we know then that we have been cured?

It would be by adding two factors:

1. The disappearance of symptoms after at least 10 days of isolation.
2. The appearance of positive IgG (antibodies).

Antibodies can begin to be produced from the 6th day of the onset of symptoms.
First the IgM appears, which would mean that the infection is active, and then the IgG would appear, which is when the body begins to develop immunity antibodies.

After 7 days, almost half of the cases have antibodies, especially at the beginning of the IgM and after 15 days almost 100%, both mild and severe, are positive, being at this time when IgG antibodies begin to appear, which are the ones that will last the longest conferring immunity.

Thus, antibody tests seek to find patients who have either been suffering from an infection for days (IgM) or those who have already developed immunity (IgG).

It is also possible to find patients who have passed the infection in a more or less mild or asymptomatic way and who do not develop enough IgG antibodies.

Tests carried out in an Analysis Laboratory (require equipment), it takes several hours/days to issue the result.

1) PCR: detects the presence of the RNA of the virus, i.e. the Antigen

In addition to what was mentioned in the introduction of this article, we highlight:

Advantages

1. Capable of detecting fragments of the virus from the first days of infection.

Limitations

1. The result is in 1-2 days
2. A sample is taken from those collected from the nasal cavity or pharynx with swabs.
   The sample must be sent to a laboratory
3. High cost: 120-150€ although it has recently dropped in price and can be obtained cheaper for large groups (around 90€ – 100€).
4. If it is negative, the user will not repeat the test as screening due to the cost.
   The main problem is that taking the test today and being negative does not guarantee that tomorrow we will be in contact with someone and we can be contaminated… and we cannot be taking the test every day!
5. Because the PCR test is a reverse transcription polymerase chain reaction (RT-PCR), it is a nucleic acid amplification assay that has been routinely used for a long time for the detection of RNA viruses in clinical settings. The detection limit can be as low as 4 to 10 copies

2)IgG and IgM antibody TESTS performed in a laboratory

As with rapid tests, the IgG/IgM antibody test is an excellent detection method for both patients who have passed the COVID-19 infection, either with a PCR+ test or under suspicion of having passed it, or without PCR performed.

The IgG/IgM detection test is useful to check if you are passing the infection (IgM positive) or if you have developed Immunity (IgG positive) for both carriers with symptoms or asymptomatic and thus prevent transmission.

Advantages

1. It is a quantitative test and therefore quantifies the antibody value produced.

Limitations

1. The result takes 1-2 days.
2. As it is performed by a laboratory, it will normally be done by drawing blood into a vein.
3. If you would like to know more about rapid Antigen or IgG or IgM Antibody detection tests, please do not hesitate to contact Salengei

salengei@salengei.com

info@salengei.com