A Covid-19 blog post for the non-expert
We’ve been hearing how the UK government wants to return to normal life under its £100bn Operation Moonshot Covid-19 testing programme. Headlines such as those in the Daily Mail have proclaimed “Prospect’ of 10-minute ‘rapid turnaround’ Covid tests” but others including the BMJ are not so sure. I thought, therefore, the time was right to look at the emerging Covid-19 tests Operation Moonshot is pinning its hopes on, and see how they work.
In a previous blog post I looked at two Covid-19 test techniques, one which detected the presence of the virus itself and another which looked for the presence of an antibody to the virus in a blood sample. Scientists don’t hang around during a pandemic and two new tests are coming into play, known as the Lateral Flow Test (LFT) and Loop-Mediated Isothermal Amplification, or a LAMP. In this post, I will look at what LFT and LAMP are, how they work, and what are their advantages and potential shortcomings.
Some media reports have given confusing accounts of these new tests, probably because they are based on existing technology and the technical terms describing them are very similar. In an attempt to avoid confusion here, I will first provide definitions of the four most important technical terms.
Antigen – this is what’s being tested. With Covid-19 the antigen is a protein on the SARS-CoV-2 virus.
Antibody – is a protein associated with the immune system which locks on to an antigen such one on the SARS-CoV-2 virus. Since the virus has many proteins, the body will raise a range of different antibodies, each sticking onto its own viral antigen.
Monoclonal antibody – this is an antibody made in the laboratory that targets one specific antigen. All the antibody molecules are clones of one another, with essentially the same molecular structure, which is where “monoclonal” gets its name.
RNA – Ribonucleic acid is the genetic material inside SARS-CoV-2. (Some viruses have DNA and others, such as SARS-CoV-2, have RNA). SARS-CoV-2 RNA comprises a chain of around 30,000 nucleic acids (known as bases) which codes for viral proteins.
The current gold standard test for Covid-19 is the Polymerase Chain Reaction, or PCR test (see previous blog post). In a nutshell, the method replicates SARS-CoV-2 RNA many times over, generating enough material for it to be reliably detected. PCR is used routinely in molecular biology and once we know the code-sequence of the genetic material, scientists can usually generate a PCR test within a few weeks. Running a PCR test however, is not straightforward and requires laboratory conditions and skilled analysts. In particular, PCR has multiple heating and cooling cycles to promote the required chemical reactions, and the reagents, being relatively complex, demand continuous preparation. Loop-Mediated Isothermal Amplification (LAMP) is still a PCR method, but operates at a single temperature (60-65 ºC – hence “isothermal”) and does not require the same fresh reagents. These, and other modifications, reduces the time to carry out a LAMP test to as little as 15-minutes, or an hour in the case of Covid-19. The test also has the potential to be run on a bench-top type device rather than requiring sophisticated laboratory conditions. While LAMP sounds like a great leap forward, it’s nevertheless a recent development in Covid-19 testing and so we have to remain cautious to its reliability. It no doubt has great potential but the traditional PCR will remain the gold standard, at least for the time being.
While LAMP detects viral RNA, LFT detects viral protein. It does this using monoclonal antibodies, which is probably the source of confusion between LFT and an “antibody test”. So to be clear, there is a test which looks for the presence of an antibody to SARS-CoV-2 in blood that shows if you’ve contracted the disease sometime in the recent past. LFT does not do this, it detects the presence of the virus itself. LFT technology has been around for some time and it’s used in home pregnancy testing kits. LFT is divided into two types known as sandwich and competitive. In the sandwich type, lines appear showing a positive test, in the competitive type the absence of lines indicate a positive test. The LFT for Covid-19 is the sandwich variety. The device comprises a flat plastic plate (see image), with a well at one end and a panel where lines appear if the test is positive. A sample from the nose or throat is placed into the well, where it migrates along the plate by capillary action. It reaches a line of monoclonal antibodies (made to recognise a specific SARS-CoV-2 antigen) which are attached onto a tiny particle, typically made of gold or carbon (the antibody-particle complex is known as a conjugate). We classify the particles as nanoparticles because they are about a billionth of a meter in diameter. The conjugate, along with the bound SARS-CoV-2 antigen, continues to migrate along the plate by capillary action until it reaches another line of monoclonal antibodies. These monoclonal antibodies don’t recognise the SARS-CoV-2 antigen however, but the monoclonal antibody conjugate. (You can see where things get confusing because the antibody conjugate is now essentially an antigen to the second monoclonal antibody). As the conjugate binds to the second line of monoclonal antibodies, they hold the particles upwards forming a visible coloured band – that is a positive test. Depending upon the test, there may be more than one line indicating a positive test and an indicator line showing the device is working correctly.
LFTs are tricky things to develop, which is why it’s taken until now for them to appear. Appropriate monoclonal antibody production is not straightforward and even variables such as capillary flow-rate within the device can affect reliability. The first LFTs have however, passed their assessment and should be available soon. They have the advantage that the test is complete in about 10-15 minutes and do not need a sophisticated laboratory to carry them out. Don’t get too excited however, because the announcement by the Prime Minister may have been a little over optimistic, in that it still requires some expertise and you are unlikely to be able to do them at home. LFTs have a higher rate of false positives than RNA-based methods and so a positive result may necessitate confirmation via PCR or LAMP. Nevertheless, with appropriate training LFTs should have a major impact on test and trace.
For both LAMP and LFT the one huge advantage is not so much the tests themselves, rather than the impact on logistics. The biggest problem we’ve had in the UK has been getting test results back within an appropriate time frame, which sparked arguments over government statistics on testing capacity versus actual tests returned. Both LAMP and LFT however, have the potential of testing while-U-wait. And of course, tests continue to evolve and perhaps the next big step might be a simple saliva test.
And I will leave this post with some optimism flavoured with realism. Despite politicians saying no one could have predicted Covid-19, in truth it was inevitable sooner or later. Equally so, is the occurrence of another pandemic sometime in the future. I hope that the words of Friedrich Hegal don’t come true, “we learn from history that we do not learn from history” and we retain Covid-19 testing capability ready to rapidly adapt to a new virus. Fingers crossed.
Graham Lappin 