Health

Does the Covid-19 rapid test know the viral load?


The rapid test for Covid-19 antigens can still predict some of the viral load through the stage of the disease, according to experts.

Master Vo Tuan Linh, Deputy Department of Diagnostic Imaging, Functional Exploration, Center for Disease Control of Ho Chi Minh City (HCDC), said that viral load is the amount of virus present in the blood or secretions of a patient. High load means high viral load, multiplying, and high transmissibility. The high and low viral load in the Covid-19 test is usually known through the CT value of RT-PCR.

In which, the CT value is called the threshold period value in molecular biology tests, which is the number of cycles that the tester can detect the presence of the virus’ genetic material. The higher the CT value, the lower the amount of virus in the test sample, the lower the chance of transmission.

During the course of Covid-19 and home treatment for asymptomatic or mildly symptomatic F0, the viral load in the body increases and then decreases according to the natural course of the virus.

Stages of Covid-19 disease progression and viral load. Photo: HCDC

The stages of disease progression that reflect the viral load that cause Covid-19 are as follows:

Exposure stage: The patient has recently been exposed to the virus. At this stage the rapid antigen test will not be able to detect it. The duration of this stage can be 24-48 hours from the time of exposure to the pathogen.

Incubation period: The virus has begun to appear in nasopharyngeal secretions, which can be detected by a rapid antigen test. Viral load increases gradually from day 2 to day 4 of illness, the T line of the antigen rapid test will darken. At this stage, the infected person infects the people around them, starting the period of disease transmission.

Infectious stage: The ability of the virus to spread is highest at the 5th and 6th day, the T line is darker than the C line of the antigen rapid test. This phase lasts until the 10th day. The viral load gradually decreases from the 7th to the 10th day. Observe the rapid test results that the T line fades in color compared to the C line.

Post-infection and recovery period: After the 10th day, is the period when the detection threshold of the antigen rapid test does not detect the virus, which means that the infected person is no longer able to spread. The rapid antigen test is now negative.

The increasing viral load is shown by the T line of the antigen rapid test, where the T line is the test line representing the patient's line, the C (Control) line is the standard line of the test, N (Negative) is the result. negative, P (Positive) is positive.  Photo: HCDC

The increasing viral load is shown by the T line of the antigen rapid test, where the T line is the test line representing the patient’s line, the C (Control) line is the standard line of the test, N (Negative) is the result. negative, P (Positive) is positive. Photo: HCDC

According to Mr. Linh, those who have a negative rapid test, and a positive RT-PCR test with CT 31-36 should not be alarmed or worried. This result shows that the virus is positive and is not contagious. People with Covid-19 are healthy, but the virus remains in the body for more than 25 days or maybe longer.

At the end of January, the Ministry of Health instructed F0 to be treated at home, ending isolation when they have completed 7 days of treatment and have negative antigen rapid test results. If the rapid test results are still positive after 7 days, they will continue to isolate for full 10 days for those who have received the full dose of vaccine and 14 days for those who have not received the full dose of vaccine. It is not necessary to re-test.

Le Phuong

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