A recent article published in the British Medical Journal dated 7 October 2020 looked at this issue.
Rapid diagnostic tests are integral to the government’s Moonshot plan to carry out up to 10 million COVID-19 tests a day by early 2021.
Jacqui Wise published her findings of the options being developed and trialled.
It is known that the United Kingdom’s testing system is currently under an immense strain, partly owing to a lack of laboratory capacity for the gold standard PCR test.
The government has wanted mass testing of the population and alongside increased PCR capacity, they are also looking to validate, buy, trial and scale new technologies.
This is basically a laboratory in a box that uses PCR technology. A nose swab is inserted into a handheld reader, and the result is provided in less than 90 minutes.
The kits have been trialled in eight London hospitals and have been useful in specific care pathways where a quick answer is required.
However, the specificity is 100% but the sensitivity is only 94%. This means that 6/100 samples tested will be falsely negative and will in fact be positive.
The LamPORE test takes about six and a half hours from sample to result and can come in a laboratory version or a portable version to process swab or saliva samples. The specificity was found to be 99.6% and the sensitivity was found to be 99.1%. This is clearly very encouraging.
Lateral Flow Antigen Tests
In September the WHO approved two lateral flow antigen tests, which display a result like a pregnancy test. They are small and portable and deliver a result in just 15 to 30 minutes.
Made by the South Korean company SD Biosensor and the US company Abbott, they are the first rapid tests to meet WHO specifications.
An independent evaluation of the SD Biosensor test by the Foundation for Innovative New Diagnostics found that it had a clinical specificity of 99.3% and a clinical sensitivity of 76.6%. This clearly has a large degree of false negatives and is not indicative of the specificity and sensitivity data provided by the company.
They are clearly helpful in evaluating positivity as if a patient is found to be positive then clearly they are accurate.
However, if a patient is found to be negative, this does not preclude a positive result and following this PCR should be performed.
The study felt that the Abbott and SD Biosensor tests were about 20% less sensitive than PCR. They have suggested that they miss cases with lower viral loads as they do not have the amplification stage like a PCR test. However, in the presence of symptoms, it is helpful to have this as a useful tool within the practice to identify whether patients may or may not be positive.
Saliva versus swab samples:
It is essentially faster to use saliva samples than nasal swabs and does not require a skilled operator. It is also likely to be more acceptable if repeated testing is needed.
Halo, a UK biotechnology company, has developed a saliva based quantitative PCR test potentially combining the benefits of PCR testing with easier use and a faster result. The test is 100% specific and results can be sent directly to a phone app in as little as seven hours.
The Halo test works similar to a PCR test used globally for testing people for the Coronavirus using a swab.
It purifies the genetic material from saliva and uses PCR reaction to detect the virus in the lab. Internal tests correctly identified the virus in 100% of tested samples when the viral load was 1000 times lower than amounts typically found in a patient’s saliva. The data from the company suggests that the sensitivity is of the order of 10,000 copies per ml of saliva with sensitivity detecting COVID-19 in patients with viral loads as low as of 1000 copies per ml with a CT greater than 98%.
The test is specific for the off lab gene which all known SARS-CoV-2 strains contain. There was no cross reactivity with other known viruses. From their own data, they were all an internal blind trial in which 38/96 saliva samples were spiked with a known amount of retrovirus, 38 spiked samples came out positive and no false negatives were identified.
This is a company which has a turnaround time of 20 minutes for nasal throat swabs or saliva. The machine looks for tiny traces of the virus in their DNA and the machine amplifies DNA billions of times chemically so that it can detect the virus with extreme sensitivity. This can also be used with saliva samples. The device has been shown to be as accurate as PCR swabs in clinical trials by Public Health England. The machine amplifies the DNA billions of times chemically so that it can detect the virus with extreme sensitivity.
Dr Paul Ettlinger
The London General Practice