Swabs, serological tests, rapid tests: some clarity
Interview with Graziella Calugi, a specialist in molecular genetics, microbiology and virology and Scientific Head of the Lifebrain Group
Summer is over, the cold weather is upon us and the second wave of Covid-19 is lying in wait: a few days before the beginning of the school year, many are afraid of the “school effect” with a significant increase in infections. Although it is true that we have greatly improved the 3T strategy (test, trace and treat) after seven months of pandemic, with an average of over 100,000 swabs per day nationwide, the need for reliable, sensitive and rapid tests is urgent. In the beginning there was the swab test, then came serological tests, and more recently rapid molecular and antigenic tests were announced, which promise an almost instantaneous diagnosis. We’ll try to clarify the differences, and the advantages and disadvantages of the various types of tests.
Dr Calugi, in recent weeks we’ve often heard about rapid tests which provide a result in less than 30 minutes. Could you please explain what these are?
First of all, we mustn’t confuse rapid serological tests with the new rapid tests: the antigen test and the molecular saliva test.
Antigen tests are based on an immunochromatography and detect SARS-CoV-2 virus proteins from a nasopharyngeal sample (swab): they are proving to be very useful in mass screening (for example in airports) due to their low cost and speed of response: 15–30 minutes maximum.
There are great hopes for molecular salivary tests which, like the swab test, detect the virus RNA but with niftier technology, which reduces the time and complexity of the analysis giving results within an hour.
The real challenge for rapid, accurate mass diagnosis of SARS-CoV-2 will be a low-cost and sufficiently sensitive saliva test, repeatable periodically with minimal discomfort for the patient, which does not require specialised personnel for sampling.
Has anything changed in recent months in the diagnosis of Covid-19 infection?
The gold standard for the diagnosis of SARS-CoV-2 is still the molecular or swab test, which directly searches for the RNA of the virus using molecular biology techniques (Reverse Real-Time PCR) using a sample from the patient’s airways. The test identifies positive individuals as infectious, whether they are symptomatic or not.
Could you remind us who has to undergo the swab test?
Ideally, to contain the infection, as many patients as possible should carry out the molecular swab test. However, we are talking about a very complex test, which can only be carried out in public and private laboratories authorised by the Ministry which guarantee adequate technical skills and standards of biosecurity.
Thus, to date, the swab test is an absolute must for those who have contact with individuals who are positive for the virus; in addition, the molecular test is advisable for at-risk categories, such as the health professions, or those suffering from chronic diseases.
If the result is negative, can we be reassured?
The negative swab indicates the non-detectability of viral RNA in the patient, that is the presumed absence of contagion. However, this result must always be assessed in the light of the patient’s symptoms, to exclude possible false negatives related to the taking of the swab sample or to low amounts of virus.
And what are the serological tests for?
Serological tests are carried out with a simple blood collection and detect the individual’s antibodies in response to the SARS-CoV-2 virus: it is an indirect test which does not search for the virus but tells us if an individual has come into contact with it at some time in the past.
It is still a very useful tool for screening and for the epidemiological assessment of immunity to Covid-19 for certain population groups or professional categories. Examples include teachers, school staff or many companies which routinely use it for health surveillance.