PCR tests work by detecting specific genetic material within the virus.
If you wake up one morning with a fever, shortness of breath and a cough — in other words, symptoms of the new coronavirus — you’ll likely wonder how you can get tested for it and what that entails.
If you’re in the United States, chances are a health care worker will use a long Q-tip to swab the back of your throat and then send that sample off for testing. If you’re in a country that has developed an antibody test, such as China, you may get blood drawn.
What happens next to these samples is very different. The throat swab is well suited for polymerase chain reaction testing, also known as PCR, while the blood sample will be mined for antibodies specific to the new disease, known as COVID-19.
Both tests take just hours to run, meaning results could be ready within a day, said Dr. Amesh Adalja, an infectious-diseases specialist and a senior scholar at the Johns Hopkins Center for Health Security in Baltimore. However, once the rapid form of these PCR and antibody tests are created, results could be ready in under an hour, he said.
“There are rapid PCR tests, but they’re not quite yet available” for the new coronavirus in the United States, Adalja told Live Science.
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The COVID-19 tests developed by the Centers for Protection and Disease Control (CDC), as well as those developed in Washington and New York, are PCR tests.
PCR tests work by detecting specific genetic material within the virus. Depending on the type of PCR on hand, health care workers might swab the back of the throat; take a saliva sample; collect a liquid sample from the lower respiratory tract; or secure a stool sample.
Once a sample arrives at the lab, researchers extract its nucleic acid, which holds the virus’s genome. Then, researchers can amplify certain regions of the genome by using a technique known as reverse transcription polymerase chain reaction. This, in effect, gives researchers a large sample that they can then compare to the new coronavirus, known as SARS-CoV-2.
SARS-CoV-2 has nearly 30,000 nucleotides, the building blocks that make up DNA and RNA. The PCR test developed by the University of Washington School of Medicine (UW Medicine) targets just 100 nucleotides that are specific to SARS-CoV-2, Dr. Alex Greninger, an assistant professor in the Department of Laboratory Medicine and an assistant director of the Clinical Virology Laboratory at UW Medicine, told The Seattle Times.
These 100 nucleotides include two genes in the SARS-CoV-2 genome. A sample is considered positive if the test finds both genes, inconclusive if just one gene is found, and negative if neither gene is detected.
Tests from UW Medicine that are either inconclusive or positive are sent to Washington’s Public Health Laboratories and the CDC for further testing, The Seattle Times reported.
In contrast, serological tests look for specific antibodies the body has produced to fight the virus. “If they detect those antibodies, [the test] gives a positive result,” Adalja said. Singapore has developed an experimental antibody test for COVID-19, Science magazine reported, and China has licensed several, as well, according to STAT News.
A serological test is advantageous because it can detect antibodies even if a patient has recovered, whereas a PCR test can detect the virus only if the person is currently sick. However, both tests might miss cases if samples are taken too early, when the viral load is too low or if the person’s body hasn’t produced antibodies against the virus yet, Adalja said.
For the serologic test, it can take about a week before the body produces ample antibodies, although “as we get more serologic tests, there will be guidance on what timeframes are needed to do a serologic test on [COVID-19],” Adalja said.
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