An international study has shown that the swab (RT-PCR) currently used for the Coronavirus test (SARS-CoV-2), although technically very reliable, is inefficient if the test is performed in the phase preceding the onset of symptoms. The spread of the virus begins slowly from the respiratory tract and it is also possible that at an early phase the infection will only persist in individual regions of the throat; this significantly reduces the chance of finding Covid-19 virus genes in a swab carried out too early and very often it leads to false negative results.
A research team at Johns Hopkins University in Baltimore (USA), led by Lauren Kucirka, has carefully reviewed 7 previous studies in which more than 1330 swabs (RT-PCR) taken from Covi-19 patients (at different stages of infection) were analyzed.
The study conducted by the US team shows that:
on the first day of the suspected infection, viruses were never detected in the swab, the false negative result rate was 100% (with a 95% confidence rate). On the fourth day of infection, the false negative test rate dropped to 67% and even on the day of the first symptoms the average false negative rate was still 38%. The optimal time to detect a Covid-19 infection is day 8 (which is usually day 3 after the onset of symptoms), but even then, the false negative rate is still 20%. Already on day 9 there was an increase in the false negative rate to 21% and on day 21 the rate rose again to 66%. Theoretically once the patient has overcome the infection, his body no longer produces viruses and he should no longer be contagious. However, this only applies to the part of the body in which the swab was made. It cannot be excluded that the patient may transmit viruses in other ways (e.g. through excretion or sexual intercourse).
Therefore, in essence, the American research team believes that it is not advisable to rely solely on the results of the swab to make a diagnosis.