The expression “false negative” for the result of a Covid-19 test is widely used. However, the term does not exactly mean that there is a mistake in the test result. Infectologist Evaldo Stanislau explains what happens and how to proceed after a test is negative for Covid-19, although the patient still has persistent symptoms.
Stanislau, who works at the Hospital das Clínicas at USP and is director of the Paulista Society of Infectious Diseases (SPI), says that the “false negative” is related to the time the test was performed on the patient.
“What we find with some frequency is the test – which they call a false negative – because it was collected too early or too late. The antigen test has an ideal detection time, which is in the first week of symptoms”, says the infectologist.
He adds that when a person starts to show symptoms, in one or two days, in general, they already have a positive antigen test.
“What we have observed is that, with Ômicron, this period of antigen expression has been perhaps a little shorter. So you have a lot of viruses, but detection is more elusive than usual,” he says. That is, the detection period is shorter.
“Sometimes, even within the first seven days of symptoms, we find patients who are negative in the antigen test. In this situation, the recommendation is to make another technique [como o RT-PCR]”, adds Stanislau.
how to proceed
The principle of the test is to detect the Covid-19 virus. This virus can be detected by the antigen screening technique or by a molecular technique.
“If we don’t find it, it means that the virus does not exist or that there was a problem at the time of collection or during the test”, says the infectologist.
Therefore, in case of clinical suspicion and presentation of symptoms, even with a negative antigen test, the recommendation is to repeat the procedure within 48 hours.
He says that in the case of someone with symptoms, “you can’t, just with an antigen test. [negativo], at the current moment, we say that she does not have Covid. You need to repeat this test in 48 hours, because you may have collected the test too early or too late”, warns the doctor.
Two tests were negative, now what?
With two negative tests, what we can say is that at this moment the person is not transmitting. This is because, in principle, when the antigen test is positive, it means that there is a sufficient amount of virus to be transmitted, as explained by Stanislau.
“But to know whether or not a person had Covid, then I use a PCR test, with two negative antigens, which is the molecular biology technique”.
“The PCR test can be positive for up to 17 days and there are patients who reach 83 positive days”, he adds.
What is the most accurate test for detecting Covid-19?
In the expert’s assessment, the most accurate test is the PCR test. However, it has the aforementioned characteristic of being able to maintain the positive result for a long time and, often, outside the contagion period.
“The ideal is to always do the antigen research, to see if the patient is transmitting and is positive, and leave the PCR to cover those situations in which the antigen did not answer our question”, suggests the doctor.
Do I need to retake the antigen test to come out of isolation?
Stanislau explains that if the patient wants to shorten the isolation between 5 and 7 days, the recommendation is to only do this if a new test is performed and the result is negative.
“If the person does 10 days of isolation, on the 11th day he can leave without the need for another test”, says the specialist.
The doctor’s recommendation is for the patient with symptoms to wear a mask, avoid contact with other people, seek medical advice and do the scheme of two sequential antigen tests plus the PCR complement.
“These actions are so that we can safely exclude that she has not had Covid or that she is not, at the moment, transmitting the virus. When in doubt, wear a mask and maintain isolation until further notice”, says Stanislau.
Source: CNN Brasil
