Health

Does Omicron ‘evade’ the quick test? – VnExpress


Experts say that the question of whether Omicron “escapes” the test quickly is still controversial; In fact, many people have Covid symptoms but negative rapid test may be due to poor kit sensitivity, low viral load or wrong sampling operation…

Doctor Nguyen Hong Ha, Vice Chairman of the Vietnam Infectious Diseases Association, noted that many people now have obvious Covid-19 symptoms such as cough, sore throat, cold sores, chills, but many rapid tests are negative. while the PCR test is positive. The doctor said: “The Omicron strain is gradually dominating the community, but the false-negative test results are not caused by the virus ‘evading’ the rapid test, but by many other causes.”

The first, according to Mr. Ha, is the sensitivity of the test method. The rapid test kit is less sensitive than the PCR test, so the virus concentration in the body must be high for the rapid test to be positive. Meanwhile, PCR testing technique detects viral gene (RNA) fragments in patient samples from patients or suspected infected people, then replicates and multiplies many times to look for viruses. Therefore, the PCR test is more sensitive than the rapid test.

Monday is test kit quality. Currently, on the market, there are many types of rapid tests from many manufacturers in many countries, the sensitivity and specificity of each type is different. Users must have fake goods, fake goods, poor quality goods… then the test will give false results.

Tuesday, Quick test results depend on when and how samples are taken. For example, during the incubation period or when you have just been exposed to an infectious source, you may be infected but have a low viral load. At this time, when taking a test sample, the possibility of a negative result is high because the virus has not multiplied enough for the test kit to detect. In addition, when testing yourself, you may perform the wrong operation, sampling technique, and test procedure. In the family, the elderly and young children cannot take samples by themselves, other people can help.

Doctor Nguyen Huy Hoang, Vietnam – Russia Hyperbaric Oxygen Center, Ministry of Defense, also shared that the question “whether the quick test cannot detect Omicron” is still a controversial issue. He also agrees with Dr. Ha’s analysis, the fact that many people perform the wrong self-sampling operation, take the wrong sample at the wrong time, as well as the poor sensitivity of the quick test… which makes the results misleading. “The viral load in the body must be equivalent to a CT index of less than 25, if the PCR test is to make the quick test results clear; if the CT is at 25-30 or above 30, which is low, the rapid test is almost difficult. detect the virus,” explained Dr. Hoang.

Another hypothesis put forward by experts is that actually the person tested did not have Covid-19. Symptoms such as sore throat, cough, chills can be caused by weather causing other diseases. If in doubt, these cases need PCR testing to check for accuracy.

Medical staff in Ho Chi Minh City test for nCoV with a test kit. Photo: Quynh Tran

Around the world, data at the end of December 2021 of the US Food and Drug Administration (FDA) recognized Antigen testing is still effective for Omicron infections, but sensitivity is reduced. Bruce Tromberg, director of the National Institute of Bioengineering and Biomedical Imaging of NIH, USA, also said that the rapid test kit for detecting the Omicron variant was inferior to Delta.

Research by the Johns Hopkins Institute indicates that sampling too early in the early stages of the disease can lead to false results. In addition, some kits are less sensitive to the Omicron strain, especially in the early days of Covid-19. According to preliminary data from a study published on medRxiv, which surveyed more than 700 people, a common test in the US found that more than 95% of people infected with Omicron had a high viral load. However, when the viral load is low, the test kit misses about 35% of infections.

Dr. Gerald W. Fischer cited a study by the US Centers for Disease Control and Prevention (CDC), showing that the rate of false negative tests in people with Covid has symptoms is 20%, people without symptoms is 59%. .

For quick testing with accurate results, bDoctor Khong Minh Tuan, deputy director of the Center for Disease Control (CDC) in Hanoi, advises that the first thing is to buy an antigen test kit licensed by the Ministry of Health. This list has been publicly posted on the website of the Hanoi Department of Health. Then perform the following steps:

Step 1: Read the manufacturer’s quick test instructions carefully. The person taking the sample must clean and disinfect their hands.

Step 2: Remove the test tray from the pouch, place it on a clean horizontal surface and use it within one hour. Take the nasopharynx stick out of the pocket.

Step 3: Collect samples properly to avoid inaccurate results.

If using the oropharyngeal test kit, the person being sampled should sit still, blow his nose gently into a tissue, and then tilt his head back at an angle of 70 degrees. Gently insert the probe into the nose, pushing and rotating to help the sampler go deep into the probe easily. The depth is about half the length from the tip of the nose to the earlobe on the same side. Hold the probe in place for 5 seconds to ensure maximum infiltration, then slowly rotate and withdraw the probe and insert it into the tube already containing the extraction buffer.

If using a nasal swab test kit, you should sit in the same position when collecting nasopharyngeal fluid. Hold the sampler probe gently into the nose about 2 cm deep, rotate the sample probe into the nasal wall for about 3 seconds, then pull it out, and put it into the tube containing the sample extraction buffer. Do the same with the other side of the nose, using the same sampler.

Step 4: Sample extraction.

Dip the probe tip into the extraction tube, rotate the tip of the probe against the wall and bottom of the tube 10 times, and immerse the tip in the solution for one minute. Then, squeeze the two walls of the tube to squeeze the tip of the rod and then slowly rotate, withdraw, and squeeze the tip of the rod to collect as much fluid as possible. Next, tightly cover the tube with the dropper cap and shake vigorously horizontally 10 times to mix the sample well, avoiding the solution from reaching the filter tip in the dropper cap. When finished, place 3 drops of sample extracted from the tube into the sample receiving cell (S) of the test tray and start counting the time.

Step 5: Read the results.

The time to read the results depends on the type of rapid test. Normally, rapid test takes about 15-30 minutes to give results. If negative, the test tray will only appear a red line in the letter C. If positive, the test tray will appear two lines in the letters C and T. In the case of the test tray, there will be no lines in the letters C and T or only the line appears in the letter T, the result is not valid.

In the context that Omicron is gradually dominating in Vietnam, doctors recommend that people with typical Covid symptoms but a negative rapid test should not be subjective. These people need to be monitored for 5-7 days, limiting contact with people around.

6 steps to quickly self-test Covid-19 at home

Instructions for self-collecting samples for home testing by CDC Hanoi.

Thuy Quynh