Health

Why do I have Covid but the test is still negative?


Rapid tests can be falsely negative due to incorrect sampling, testing too early in the exposure phase, or poor Omicron sensitivity of the kit.

When she saw her son come into contact with someone infected with nCoV and had typical symptoms such as fever, cough, and headache, Ayesha Charagulla, who lives in San Jose, USA, let him have a quick test at home. The results were repeatedly negative.

But when she took her son to the PCR testing center, he was diagnosed with Covid-19.

“Fortunately, we put her in isolation from the very beginning. But how many students who are going to school with my child who tested positive for the virus knew they had the disease,” Charagulla said.

Charagulla and her son are not the only ones to have this experience. Brittany Prock, 22, of Campbell tested three times with three different results. The first time the test kit showed two faint lines, the second time the result was negative. But the final PCR test confirmed that she was positive for nCoV.

This is phenomena false negative. According to Dr. Gerald W. Fischer, the false negative rate depends on the type of test kit and the stage of nCoV infection. Normally, PCR test is more sensitive and diagnostic, but it is expensive and has a long time to wait for results.

“If the virus is in the early stages of replication (i.e. the person is testing too soon), the antigen kit can also give a false negative result,” he added.

He cited a study by the US Centers for Disease Control and Prevention (CDC) showing that the rate of false negative tests in symptomatic patients is 20%, in asymptomatic people is 59%. The PCR test has a lower false negative rate.

Earlier Johns Hopkins research also showed that taking samples too early in the early stages of the disease could lead to false results. Some patients’ improper handling also causes false negatives.

Jaquelin Dudley, associate director of the LaMontagne Center for Infectious Diseases at the University of Texas, stresses the importance of carefully reading and following the manufacturer’s instructions. She advised people to wait a few days after being exposed to F0 before testing, helping to reduce the risk of false negatives.

“The most important thing is to test for the virus when it is most prolific, just not when the immune system has brought it under control,” she said.

“Once an oral anti-nCoV drug is approved, limiting the number of false-negative tests is very important for patients with and without symptoms. The pill molnupiravir prevents the virus from replicating,” said Dr. Fissher. The earlier an individual is detected, the more effective the drug is in reducing both symptoms and the potential for transmission.”

Covid-19 rapid test kit in the US. Photo: NY Times

Most antigen tests can detect mutated infections. But there are studies that show that some kits are less sensitive to Omicron, especially in the early days of Covid-19.

According to preliminary data from a study published on medRxiv with 700 participants, a rapid test common in the US found that more than 95% of people with Omicron had a high viral load. But when viral loads are low, it misses about 35% of infections.

Data released at the end of December 2021 by the US Food and Drug Administration (FDA) confirm that the antigen test is still effective for Omicron infections, but the sensitivity has decreased.

This announcement is based on data from Emory University, part of the National Institutes of Health. Rapid test performance comparison study based on Omicron and Delta samples. Bruce Tromberg, director of the NIH’s National Institute of Biomedical Imaging and Bioengineering, said the rapid test kit detects the Omicron variant worse than Delta.

According to another test posted on medRxiv, in people with symptoms and diagnosed with nCoV infection by PCR, the rapid test kit takes three days to give a positive result. But this is just a small project, not yet approved.

There are people mistakenly thought he had Covid-19 because its symptoms were similar to common illnesses.

After two years of living in a rabies epidemic, the community is more fearful and more alert to symptoms of fever, cough or headache. Experts say these could be symptoms of seasonal flu, viral fever or illnesses other than Covid-19.

“When schools and companies reopen, the number of people with Covid-19-like symptoms will certainly increase. My niece recently showed symptoms of nCoV infection such as sore throat, stuffy nose and fever. But they have negative results on both antigen and PCR tests,” Dr. Fissher said.

Janice Johnston has received many similar cases. “Common illnesses like flu have symptoms that coincide with Covid-19 are body aches, fever, chills, cough, headache and stuffy nose,” she said. She encouraged people to get more tested for influenza and viral fever if symptoms persist but are negative for nCoV.

Experts say that symptomatic F1s should not be subjective even though they have tested negative for nCoV. “If you have any unusual symptoms, you should stay at home until the fever is gone and you feel better. You can go to the doctor for testing, treatment and appropriate prescription,” Ms. Johnston said.

Thuc Linh (Follow Forbes, Mercury News, WSJ)

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