LOS TESTS PARA DETECCIÓN COVID-19
In the current context of the pandemic, there has been a lot of talk in the media about virus detection tests and immunity tests, but for most people it is not clear what uses and the information they provide.
Many doubts arise in interpretation in these tests as well as the belief and or hope of having passed the infection asymptomatically and being already immunized.
First I will define the type of tests that exist:
Tests carried out in an Analysis Laboratory (require equipment), it takes several hours to issue the result.
Normally in 24 hours you already have the result
1. PCR: detects the presence of the RNA of the virus, i.e. the Antigen:
Advantages:
- Capable of detecting the virus from the first days of infection.
Limitations:
- The result is in 1-2 days
A sample is taken from those collected from the nasal cavity or pharynx with swabs.
The sample must be sent to a laboratoryHigh cost: 120-150€
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!
2. IgG and IgM Antibody TEST performed in a laboratory
Advantages:
The IgG/IgM antibody test is for the detection of 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.
This IgG/IgM test therefore serves to identify antibodies that are formed at an early stage of infection or those that develop later that would provide immunity.
Limitations:
The result takes 1-2 days.
As it is performed by a laboratory, it will normally be done by drawing blood into a vein.
RAPID TESTS, do not require equipment, or go to a laboratory and the result is in 15 minutes
1. Detection of IgG and IgM antibodies
Advantages:
It is a test that has a relatively low cost.
Easy to perform, no appliances required.
Fast in time, result in 15 minutes.
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.
This IgG/IgM test therefore serves to identify antibodies that are formed at an early stage of infection or those that develop later that would provide immunity.It is performed with a capillary blood sample.
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:
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. Detection of the proteins of the virus, it would be a test equivalent to PCR but with less sensitivity
There are different test manufacturing laboratories that are developing this type of test to be able to offer an antigen test soon, designed for the rapid detection of the virus that causes COVID-19.
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
- Weariness
Other less common symptoms:
- Aches and pains
- Sore throat
- Diarrhoea
- Conjunctivitis
- Headache
- Loss of sense of smell or taste
- Rashes or loss of color on the fingers or toes
Severe symptoms include:
- Shortness of breath or feeling short of breath
- Chest pain or pressure
- Inability to speak or move
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 begin after these 14 days
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 or 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 has been extended.
Another way could have been, as they have done in some countries, which is by carrying out mass tests on the entire population to identify infectious patients and stop the pandemic more quickly and not paralyze the economy for so long.
When 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.
Antibodies can begin to be produced from the 6th day after the onset of symptoms and that is when the viral or antigen load begins to decrease, although this time may vary depending on the severity of the infection.
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).
The presence of antibodies does not exclude the possibility of further transmission of the virus.
To do this, it should be confirmed by PCR or rapid antigen test.
What is the COVID19 rapid test?
Why is early detection important?
The rapid IgG/IgM detection test is not only useful for checking the patient’s immunity but also for detecting both symptomatic and asymptomatic carrier patients in just 15 minutes and thus preventing transmission.
In asymptomatic people, the test may be negative, although in later days the result may be positive.
The antibodies that are synthesized as a defense are detected, not the COVID19 Antigen.
A positive IgM result can range from 5-7 days to 21-28 days after the infection and/or fever began.
Positive IgM may be an indicator of present infection.
IgG antibodies appear after 14 days and will remain for a long time.
Positive IgG can be an indicator of infection or a previous infection.
Kit Contents:
Test devices, lancets, micropipettes depending on the brand of the test.
Sometimes it will only be necessary to deposit the drop of blood, the well intended for the sample and reactive buffer.
Store at room temperature.
The box shows the expiry date.
This means that of 178 patients who were positive by the PCR test and confirmed to be COVID19 positive by CT, by the rapid test 176 gave a positive result for IgM and 2 who came out negative (false IgM negatives)
And of 82 patients who were negative by PCR, 74 were negative by the Rapid Test and 8 were positive (false IgM positives)
This means that of 91 patients who were positive by the PCR test and confirmed to be COVID19 positive by CT, by the rapid test 88 gave a positive result for IgG and 3 who came out negative (false negative IgG).
And of 89 patients who were negative by PCR, 81 were negative by the Rapid Test and 8 were positive (false positive IgG).
Conclusions:
Faced with so many possibilities, be it Antigen or Antibodies, slower or faster tests, what is clear is that we can carry out controls, without difficulty.
When there is doubt about possible contagion, the most appropriate one must be decided.
If we have been unwell for days or we think we have contracted the infection a few weeks ago, I would opt for the rapid test because it is cheaper.
If we were in contact with someone who later turned out to be positive 3 days or more ago, I would opt for the PCR to find out if they have infected me and I am still asymptomatic and could unknowingly be infecting other people.
Also, if I am recently feeling unwell, I would do a PCR, although if several days have passed, I could also take the rapid IgM test.
