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New Antigen and Antibody-Based Kits Bolster Coronavirus Fight
By Ishani Dasgupta, Ph.D.
The COVID-19 pandemic has brought about an unprecedented impact on global health and safety, with several countries reporting a steady increase in new cases. This further reinforces the importance of rapid and sensitive diagnostic tests. The scientific community has been working relentlessly towards developing different types of detection kits, ranging from the accurate PCR-based molecular tests to the rapid, large scale antibody-based tests. Now, a third kind has made its entry.
PCR-Based Molecular Detection Tests
The complete genome sequence of SARS-CoV-2 made available during the early onset of COVID-19, enabled scientists to develop the RT-PCR assay for COVID-19 detection which continues to be the standard, reliable diagnostic test. This method ensures the detection of viral RNA in the nasal, nasopharyngeal swabs, sputum, or lower respiratory tract aspirates collected from suspected COVID-19 patients.
First, the RNA isolated from patient samples is reverse transcribed to cDNA. If the sample contains viral DNA, the primers designed for the viral nucleocapsid gene regions will anneal to the target sequence and result in their amplification. A probe in the mixture would then bind to the amplified DNA and generates a quantifiable fluorescent signal in real-time. When the intensity reaches a predetermined threshold frequency, it is deemed as a positive test result. The number of amplification cycles needed to reach the threshold determines the viral load and severity of the infection.
Since RT-PCR assays are based on DNA amplification, it requires time and trained personnel to conduct them. Although newly developed genetic amplification systems like the ones by Abbott Laboratories’ generate rapid test results, it is largely prioritized for hospitalized patients. Therefore, the availability and accessibility of such tests is a major concern. Another limitation is that only one sample can be processed at a single time. Also, results are prone to false-positives owing to the presence of nucleic acids from dead virus particles and it is hard to predict when a patient first acquired the infection.
Serological (Antibody-Based) Immunodiagnostic Tests
To combat a pandemic of this proportion, large scale testing programs have to be in place. With over 68,000 deaths in the USA alone, the current scenario necessitates 20 million tests instead of the paltry 150,000 available daily. However, it is difficult to scale up molecular tests to such high numbers and miniaturize it to point of care test formats. This further emphasizes the need for additional tests beyond the conventional PCR. This is where immunoassays come into the picture.
Antibody-based tests use the patient’s blood to detect the presence of antibodies against SARS-CoV-2. These tests can detect whether a patient was infected previously but cannot confirm the current status. Although test results increase in accuracy only a week after infection (when enough antibodies are developed), the ease of use, quick result, and lower variation make serological tests a practical option.
In light of this emergency situation, several antibody tests are made available for use, even without an FDA clearance. However, the agency requires healthcare providers to be aware of limitations and not use it as the sole basis to diagnose COVID-19 infection. The tests that did bag an FDA authorization are listed on the Emergency Use Authorization (EUA) page.
On May 3rd, Roche announced that its new Elecsys® Anti-SARS-CoV-2 antibody test was issued EUA. Earlier, Severin Schwan, the CEO of Roche was critical of some of the low-quality antibody kits available in the market and called them “a disaster”. Roche’s antibody test reportedly has a specificity greater than 99.8% and 100% sensitivity (14 Days post-PCR confirmation).
“Thanks to the enormous efforts of our dedicated colleagues, we are now able to deliver a high-quality antibody test in high quantities, so we can support healthcare systems around the world with an important tool to better manage the COVID-19 health crisis,” said Schwan. ”I am in particular pleased about the high specificity and sensitivity of our test, which is crucial to support health care systems around the world with a reliable tool to better manage the COVID-19 health crisis.”
Immunoassay-Based Antigen Tests
The region of the pathogen that stimulates an immune response is called an antigen. An antigen-based kit is designed to detect the viral antigens from the nasal swabs of patients using a simple, rapid, inexpensive, and scalable lateral flow assay method. In the case of COVID-19, the target is the spike protein of the SARS-CoV-2.
E25Bio, a Cambridge, MA-based biotech startup has developed a rapid antigen test for COVID-19 led by MIT scientists, Lee Gehrke and Irene Bosch. Akin to an over-the-counter pregnancy strip test, this rapid diagnostic test kit can examine if the virus is present in the patient sample within 15 minutes without the need for any sophisticated instrument and trained personnel. Nasal swab sample solution from a patient is placed onto one end of paper strips that can adsorb and transport the analytes present in the sample.
The paper strip contains antibodies that recognize SARS-CoV-2 antigens. SARS-CoV-2 antigens migrate up the paper strip and conjugates to the test antibodies, generating a visual readout. This is integrated with a mobile app that organizes the test results by sex, age, location, and time which will provide valuable information about disease spread. Taken together, this metadata can prove resourceful to the public health bodies, enabling them to implement mitigation strategies against COVID-19.
E25Bio is presently working with a UK-based manufacturer, aiming to produce millions of tests daily. Once the test kit obtains clearance from the FDA, it will speed up the diagnosis and help track disease progression. Gehrke mentioned that the COVID-19 antigen test is based on a similar principle of their accurate tests established for Dengue and Zika. Also, the cost for each test would be as less as $10, which is much less compared to PCR based diagnostics. Although these tests have yielded high sensitivity in lab samples, their accuracy on patient samples remains to be validated.
These newly designed antigen test kits alongside the conventional PCR-based methods will surely assist in accelerating COVID-19 diagnostics. Antigen testing, if successful will provide a rapid, inexpensive, simple, and reliable testing platform which is the need of the hour.
Editor: Rajaneesh K. Gopinath, Ph.D.
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