Testing for Covid19
The Difference between tests for Covid19/Coronavirus.
These are usually what is referred to as PCR tests, though this is the technique and nor Covid19 specific.
Antigens come from the virus itself. In the presence of symptoms, proof that a subject is carrying antigens is taken to mean active infection. As antigens are parts of the virus persistent carriage of antigens may mean that a subject is potentially infective to others, though this is uncertain as incomplete virus parts may not be as infectious.
It is a test to see if virus particle is therefore present. This test uses a long swab to collect material from the back of the nose where it meets the throat. A swab is therefore passed up the nose and a second though the mouth. It can be uncomfortable and provoke some gagging but failure to reach far enough back can yield inconclusive or false negative results.
A positive result indicates that viral genetic material is present. It is a specific test for Covid19 and not a test to identify other causes of respiratory illness. A negative result indicates that the SARS-CoV2 virus that causes the Covid19 disease was not found. It is possible to have a very low level of the virus in the body with a negative test result (false negative).
In conclusion, this test is needed to identify the presence of the SARS-CoV-2 virus that causes the Covid19 disease.
Tests in general
For a test to be useful it must be accurate, available, easy to perform and acceptable to the patient and ideally cost effective. The larger the sample drawn the less likely an error in the test either giving a positive result when it shouldn’t (false positive); or a negative result when infection is present (false negative).
Covid19 (Coronavirus) Molecular (Swab) Test
Covid19 (Coronavirus) Antibody (Serology) Test
This is a blood test. It can be performed on a formal blood draw sample or by a finger prick or using some systems. It is designed to detect antibodies (immunoglobulins, IgG and IgM) against the coronavirus that causes Covid19.
Antibodies are proteins produced by a subject. It is the response of the immune system to an infection and are specific to that particular infection. They are found in the liquid part of blood specimens, which is called serum or plasma, depending on the presence of clotting factors. The use of antibody tests is the basis of the talked about concept of an ‘immunity passport’. While the data is unclear a positive antibody test may mean a resistance to further infection and may indicate a lower risk of passing it on to others. While not yet universally applied it may reduce the need for wide social distancing or form a basis for decision making about quarantining. This may also once/if a vaccine is found be used to confirm successful vaccination.
When to have an antibody test – if:
you may have been exposed to the coronavirus which causes Covid19 based on your current or previous signs and symptoms (e.g. fever, cough, difficulty breathing);
you live in or have recently travelled to a place where transmission of Covid19 is known to occur;
it can be useful if you have been in close contact with an individual suspected of or confirmed to have Covid19; or
you have recovered from Covid19.
Antibody Test for IgG
This test detects IgG antibodies that develop in most patients within seven to 10 days after symptoms of Covid19 begin.
IgG antibodies remain in the blood after an infection has passed. They are the memory against further infection with the same pathogen, but do not guarantee future immunity. These antibodies indicate that you may have had Covid19 in the recent past and have developed antibodies that may protect you from future infection.
It is uncertain at this time how much protection antibodies might provide against reinfection.
Antibody Test for IgM
This test detects IgM antibodies. IgM is usually the first antibody produced by the immune system when a virus attacks.
A positive IgM test indicates that you may have been infected and that your immune system has started responding to the virus.
When IgM is detected you may still be infected, or you may have recently recovered from a Covid19 infection.