In recent weeks, there are more and more cases of patients with Covid symptoms but who test negative for the swab. The phenomenon, which emerged with the new wave of infections led by the Omicron 5 variant, caught the attention of the Federation of the Medical Order which formulated three hypotheses to explain it. According to the first, the symptoms could precede the positive test result since today, after two years of pandemic and acquired immunity, the immune system would be more reactive, activating faster against the virus. According to the second, the daughter of some studies, the new sub-variants, Omicron 4 and 5, would lead to a lower accumulation of viral particles in the cells of the nose, making false negatives more likely. Finally, according to the third hypothesis, the greater use of do-it-yourself swabs would give a higher percentage of false negatives as with the self-test it is easier to collect the biological material incorrectly.
But these remain only hypotheses, also because to date there is no official estimate of cases showing symptoms similar to Covid but which obtain a negative result from the swab. NapoliToday explored the topic with Alessandro Perrella, UOC Director of Emerging Infectious and Highly Contagious Diseases of the Cotugno Hospital in Naples.
Dr. Perrella, there are cases of patients who have symptoms compatible with that of COVID-19 but the swab is negative, why?
“In reality there are several infectious diseases that can give clinical manifestations similar to those of COVID-19, in particular when it comes to a flu like syndrome (flu-like syndrome). Not for all these syndromes specific laboratory tests exist, nor is it generally used to resort to such tests when a symptomatology of this type is present. Let’s not forget that there are also other Coronaviruses that can mimic symptoms of COVID-19, such as Parainfluenza 4, which are not detectable by the antigen or molecular test for SARS-COV-2 “.
What has changed in the immune system response since the start of the pandemic?
“In vaccinated subjects a better performance in the ability to respond to the virus. It is as if the body were training in anticipation of a sports competition ”.
Is the molecular swab still the most reliable one compared to do-it-yourself?
“There still remains the most accurate test in discerning the type of SARS-CoV-2, variants included. Buffers are reliable especially the molecular ones; the antigenic ones are good and, in principle, in their capacity of positive predictive value they are affected by the epidemic trend “.
Yet there are cases in which the do-it-yourself swab is positive, while the molecular is negative. How can this happen?
“This can happen and depends on various factors in the diagnostic process, for example: different collection of biological material in terms of quantity, operator experience, etc.”.
Given the current situation, so a negative swab, in the presence of symptoms, shouldn’t it be a pass to get out?
“If there are no elements suggestive of positivity to a diagnostic test, it is an infection like any other, however clinically relevant, and does not necessarily require isolation”.
Could the result of the serological test (which reveals the presence of the two types of IgG and IgM antibodies) resolve the uncertainty of the swabs and understand if you have contracted Covid or not?
“If the subject is vaccinated it makes no sense, but even in the presence of unvaccinated subjects the test may not help.”