Different types of tests are currently available for the detection of the SARS-CoV-2 infection, responsible for COVID-19. However, they do not have the same value and differ in terms of reliability and effectiveness. We talked about it with Dr. Maria Teresa Sandri, Head of Humanitas Clinical Analysis Laboratory.
The swab for the molecular test
The most reliable test for the diagnosis of SARS-CoV-2 infection is the molecular test that is performed on a sample collected by a swab from the respiratory tract. The nasal /oral-pharyngeal swab consists in collecting the mucus lining the surface cells of the mucous membrane of the nasopharynx (the upper part of the pharyngeal canal) and of the oropharynx (the part of the pharynx located behind the oral cavity). The sample collection is carried out with a cotton swab in a few seconds, it is minimally invasive and not painful, although the patient may experience some discomfort when the swab reaches to the mucous membrane of the nose and oral cavity. It is important to perform the swab correctly and, therefore, to rely on a trained professional who can ensure adequate sample collection.
The sample collected is then purified and examined in the laboratory, the specialists searching for the traces of viral RNA – the genome of the SARS-CoV-2 virus – through the molecular method of real-time Reverse Transcription- Polymerase Chain Reaction (RT-PCR). The laboratory analysis allows identifying the presence of the SARS-CoV-2 virus in the sample, thus confirming or excluding infection.
Rapid antigen test
Rapid antigenic tests look for the surface proteins of the virus (antigens) and not the viral genome, as in the case of the molecular test. The sample is still collected through a nasopharyngeal swab, but the result is available much faster, in about 15-30 minutes, while the molecular test requires about 24-48 hours to process. The sensitivity and specificity of the so-called rapid test seem lower than those of the molecular test: therefore those testing positive on the rapid test should then undergo the molecular test to verify the diagnosis and exclude the so-called false-positives.
In Italy, the rapid test was introduced by the Ministry of Health for the screening of passengers in airports and marine ports, and could be useful in other contexts, accelerating the diagnosis and, therefore, isolation and tracing of suspected cases of COVID- 19.
Serological tests, on the other hand, are performed on the patient’s blood sample. They can be rapid (a drop of blood is sufficient) or quantitative (where a full blood draw is necessary). Rapid qualitative antibody tests allow to see whether the individual has previously come into contact with the virus and his/her immune system has therefore produced the relative antibodies. Quantitative serological tests, on the other hand, can measure the specific quantity of the antibodies produced.
The antibodies in question are the IgM immunoglobulins (the first ones to be produced in case of infection) and IgG antobodies (which follow the IgM when the level of the latter drops). The detection of IgG antibodies in the blood sample means that the infection occurred in the past. It is still unclear, however, whether a person with IgG antibodies is immune.
The serological test, therefore, highlights the presence of antibodies against the virus and indicates any previous exposure to SARS-CoV-2. The positivity is evident not immediately following exposure – therefore, the antibody test is not suitable for detecting an infection in progress. It thus cannot replace the molecular test (the swab) to verify the positivity of an individual. However, this type of test can be useful in the epidemiological field to estimate the spread of the infection within a community.