What you should know about Covid-19 testing
As Covid-19 spreads throughout the world, the need for more extensive laboratory testing increases. Testing and diagnosing people who have the disease is critical to tracking cases and managing the pandemic. But countries around the world have been unable to meet the demand to expand testing. Glenda Mary Davison answers key questions.
The molecular based tests used to detect SARS-CoV-2, the virus that causes covid-19, are mostly developed in university research laboratories and manufactured by a number of commercial companies. These manufacturers are urgently ramping up production to meet the worldwide demand.
After the first case of Covid-19 was reported in December 2019, scientists were quickly able to decipher the genetic sequence of SARS-CoV-2 and share it with the rest of the world. This led to the development of molecular based tests which are very specific but can take two to three days to process and produce a result.
At the moment, in most parts of the world, this type of test is still the only one available.
How does this test work?
Molecular testing is based on the reverse transcription polymerase chain reaction (RT-PCR). SARS-CoV-2 contains a single strand of RNA which holds the genetic code for the proteins the virus needs to replicate.
Before the test can be performed, viral genetic material must be extracted from the throat or via a nasal swab. The polymerase chain reaction, which is used to expand this genetic material, requires DNA, and so the viral RNA must first be converted to DNA using an enzyme called reverse-transcriptase. Only after this is done can the viral genome be copied millions of times until there is enough to be analysed and detected using a fluorescent dye.
The tests are mostly done in specialised laboratories. They require skilled experienced laboratory professionals and expensive equipment. In South Africa for example, the tests are carried out by the National Institute of Communicable Diseases, tertiary academic laboratories at the National Health Laboratory Service and some private laboratories.
What are the biggest challenges?
Not all laboratories are able to offer these tests. Another challenge is that the tests require skilled professionals and equipment. And there are limited test kits.
In the US the limited availability of tests means that many people who want to get tested can’t. In South Africa both public and private laboratories are struggling to keep up, and test kits are rapidly being depleted.
Because of the shortage of testing kits, the World Health Organisation has advised that tests should only be done when people show symptoms, are at high risk or are suspected of having been infected.
Another challenge is that the tests can take three to four days to process. This is a long time given the highly infectious nature of the virus.
What’s needed? What’s next?
There’s an urgent need for more rapid and widespread testing. In countries such as South Korea, where extensive testing was available, the government was more effective in controlling the spread and infection rate. Widespread testing of individuals – not only those showing symptoms – could reveal the true size of the pandemic and allow epidemiologists to predict and control the spread.
Another major breakthrough would be a test that could be performed at the point where the patient presents, and which can provide results within 15 to 20 minutes.
Many scientists are racing to develop antibody based tests which will produce faster results and are more cost effective. Antibody tests are cheaper and quicker to perform and may provide useful information on how widespread the infection really is.
One possibility is the serology tests which is used to detect other viruses such as HIV and Hepatitis.
Serology tests are positive in those who still have active virus as well as in patients who have already recovered but still have antibodies in their blood. These individuals may not be infectious as the virus has been eradicated. The testing of recovered individuals could provide important information on how the immune system responds and maintains immunity.
This type of testing could also be used as a screening test in the early stages of infection. This may be important as some individuals with few or no symptoms are still contagious.
How are the new tests being developed?
Researchers have begun by isolating and modifying sections of the spike proteins which are part of the outer surface of the virus. These proteins can be multiplied by using cells which are cultured in a laboratory. After this, the proteins are isolated and then fixed onto a solid surface.
The next step would be to add serum, which is separated from a blood sample. If antibodies to SARS-CoV-2 are present, they will bind to the viral proteins on the solid surface and can be detected using a chemical that will change colour. This test is called an Enzyme linked immunosorbant assay (ELISA) and has already been approved for use in some countries. But proper validation is still required. As soon as this is achieved, they will be distributed to other regions of the world including South Africa.
In China, after the massive spread of the infection, a point of care test was developed. It was called the covid-19 IgG/IgM Rapid Test Kit. This test didn’t have to be performed in a laboratory and could deliver results within 15 minutes. Although it’s available, its results aren’t considered as reliable as the molecular tests. Many countries are therefore wary of introducing it until it has been fully validated.
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