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Diagnosing and testing for COVID-19: Update from ACB Microbiology Professional Group

04/04/2020 09:06:11

Rob Shorten, Consultant Clinical Scientist, Lancashire Teaching Hospitals NHS Foundation Trust
Dr Catherine Moore, Consultant Clinical Scientist, Wales Specialist Virology Centre
On behalf of the Microbiology Professional Group of the Association for Clinical Biochemistry and Laboratory Medicine

There have been numerous reports in the media in the last week regarding testing capacity for the SARS-CoV-2 (the novel coronavirus that causes COVID-19). This novel virus was first described in late December and the first UK case was seen a month later. Healthcare Scientists and other laboratory staff have been working hard for several weeks to procure, validate, and roll out these tests. This has involved rapid redeployment and training of staff to cover extended shifts.

It is worth covering the steps involved in diagnosing such an infection. There are three main methods: molecular detection of the virus’ genetic material, detection of proteins on the virus surface in patient samples, and detection of antibodies produced by the patient in response to infection. 

Molecular detection of viral genetic material (in this case RNA) is the gold standard for detecting active infection. These tests, known as nucleic acid amplification tests (NAAT, also called PCR; polymerase chain reaction) are sensitive and specific. These tests could not be developed, or scaled up until fundamental information about the virus was known, i.e. its genetic code. Reference laboratories around the world, including in the UK, were amongst the first to develop these tests. Commercial manufacturers also went about producing these. Of note, the SARS-CoV-2 virus has been designated as a Hazard Group 3 pathogen by the Advisory Committee on Dangerous Pathogens, which requires that laboratories have appropriate measures in place to ensure the safety of their staff. Prior to detecting the genetic material, the virus must be inactivated to make it safe to handle, followed by the extraction of the genetic material. This is a complex process that requires specialist equipment and reagents (or chemicals) that are in addition to the equipment and reagents required for the detection of the genetic material. Most manufacturers, including the largest producers, are based outside the UK. Additional consumables, such as tubes and pipettes also need to be sourced in large quantities from overseas. UK laboratories have been working hard to do this for several weeks, as have laboratories all over the world.

Detecting viral proteins (or antigens) from patient samples can be performed quickly, but these tests lack the sensitivity of molecular tests and are usually far inferior in diagnosing acute infections.

Detecting antibodies in samples from infected patients (sometimes called serology), is of limited use in diagnosing acute viral infections, especially those caused by respiratory viruses. The detection of antibodies shows that a patient has been infected at some time in the past. Some antibodies are produced early in the course of the infection (IgM) and whilst sensitive, are often not very specific and can cause false positive results. The antibody produced later in infection (IgG) is often more specific but can be less sensitive leading to false negative results. For both types of antibody test, false positive results can also occur to antibodies produced in response to infections caused by similar viruses, such as the seasonal coronaviruses that circulate annually causing cold symptoms. What is less clear for COVID-19 is when these antibodies appear and how long the protection from IgG is likely to last for.

These tests are only just coming to the market but require significant validation using blood from people known to have had confirmed COVID-19 to determine how best to use these tests. This work is vital to understand when antibodies are likely to be produced after the onset of infection for the test result to become reliable.

It is also important to test the blood from people who have never had COVID-19 to determine the specificity of these tests. Any test that produces a positive result in a patient who has never been infected could lead to a false sense of security, especially in a healthcare worker or other frontline worker who is required to work in a high risk environment.

Members of the ACB continue to work alongside the colleagues in sister organisations to source, validate and roll out molecular and serological tests at scale. Patient safety and the provision of tests that give consistent, high quality results are at the forethought of all our members.  

Agnes Becker

(This page is maintained by the ACB Publications & Communications Committee)

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