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Gold nanoparticles, Covid and Rapid Tests

The science behind Prime Minister Boris Johnson’s “moonshot” project in dealing with Covid19 is actually doable. i.e. cheap disposable tests taking 20 minutes rather than expensive lab-based PCR tests that take several hours. The problem is sensitivity. Rapid antigen tests as they are called are already in use for diseases where sensitivity is not a problem. There are now commercial rapid antigen test kits for covid19 on the market already. The UK is currently trialling some of these tests in a mass testing exercise in Liverpool. They can detect around a minimum of about 105 virions/ml compared to 102 / ml for PCR. The range in patients can be from 102 to 1011 per ml depending on the period after infection (roughly day 3 to day 20). The key material in these rapid tests is antibody labelled gold nanoparticles that detect a viral protein and show up as a red line when the test is positive, just like pregnancy tests.
Public Health England stipulated that tests had to have 98% specificity and sensitivity to be useful. Many rapid tests are only 70-80% sensitive. However, it is known that as the viral load of patients’ increases over a period of days to its peak, it is detectable by rapid tests. This provides a window of opportunity for the tests to be useful, but testing every few days would be required to catch the peak in asymptomatic people. A recent publication modelling high-frequency use of cheap less sensitive tests states it is likely to be more beneficial than expensive slow PCR tests with all the bottlenecks we have observed.*
A delayed PCR test result is not much use in preventing spreading of the disease. The paper suggests rapid testing could be used as an initial mass screening approach reducing the demand for PCR testing in the first instance and thus allowing PCR confirmation of patients with ambiguous results that could be false positive or false negative depending on their symptoms status. Mass testing with rapid tests would also pick up a percentage of asymptomatic infectious people if they were in the peak phase of viral replication. Positive testing asymptomatic individuals could then have a confirmatory PCR test.
In summary, the rapid tests are very useful for detecting the virus at peak viral load when an individual is most infectious regardless of symptoms. It may lead to false negatives if tested too early in the cycle, or false positives which all diagnostic tests suffer from, but at very low levels. Meanwhile, the debate about which days you are actually infectious goes on. It is generally assumed to be days 3 to 10 post-infection. Regular testing every few days would help mitigate this problem.

* https://doi.org/10.1101/2020.09.01.20184713

Nano Biosols provides a service to prepare custom conjugated gold nanoparticles with the antibody of your choice for rapid test development or research purposes. We are happy to make small scale batches for pilot studies.

10/11/2020 - Posted by | Category 1

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