In order to have a global view of each case and to be able to apply a diagnostic algorithm, it is necessary to use all three tests together: PCR, IgM and IgG.
It is very important to do all the tests to know 100% of the status of the infection.

A common mistake patients make is to ask for “just the antigen test” (rapid test) to find out if they are infected or not (it is the cheapest test). However, they are only useful to know if you is or has been in contact with the virus. If antibody tests are positive, we have no way of knowing the overall extent of infection without PCR testing, and cannot know whether or not it may be contagious.

Another common mistake is to ask for “just the antigen test” (rapid test) to find out if you are infected or not (it is the cheapest test). However, this test provides only preliminary results, and a negative result does not exclude infection by the virus so it cannot be assured whether it can infect or not.

PCR

Coronavirus 2019-nCoV/SARS-CoV-2 RNA is studied in different types of samples obtained, both respiratory and serum. Samples are subjected to viral RNA isolation where they are subsequently studied. two sequences of the virus Nucleocapsid gene (N1 and N2)The results were obtained by retrotranscription and real-time PCR (rRT-PCR, Real-Time (r) Reverse Transcription (RT) PCR assay) following the recommendations of the US CDC (Centers for Disease Control and Prevention).

Antigen

The rapid test for SARS-CoV-2 antigen determination is an immunoassay that provides preliminary results. A negative result does not exclude SARS-CoV-2 infection and should be confirmed by PCR.

Immunoassays are used to detect antibodies against the protein antigens of the virus in serum/plasma samples. The types of antibodies tested are IgM (immunoglobulin M) and IgG (immunoglobulin G). IgM is an early response to infection and tends to decrease as the disease progresses. IgG is of later onset and tends to increase as the disease progresses. It could generate future immunity.

For PCR and the rapid antigen test, a respiratory sample is taken through the nostrils and for antibody detection, a blood sample is taken. Fasting is not required.