Currently the test for the diagnosis of Covid-19 has rested on analysing a ‘scrape’ collecting cells from the mouth and nose for the presence of genetic traces of the virus. Reports yesterday suggest that taking a sample of saliva might be a better approach and certainly more acceptable. I discuss the science behind this story and the issues involved.
Current standard approach to diagnosis
- Current approach to diagnosis is based on using a swab to collect a sample of cells from the back of the throat (‘nasopharynx’) and the lining of the nose
- The cells at these sites from infected people should contain the virus
- Although the virus can be grown (cultured) from these cells, it is a much easier job to look for the presence of genetic material from the virus using what is call RTPCR technology
- The nature of the RTPCR test is that it requires specialist equipment and chemicals and can only be done in specially set up laboratories
- In theory the laboratory analysis can be completed within an hour; though given the pressure of numbers, the results take much longer to come through
Sample collection using swabs
- It is unpleasant to provide a sample: it is easy to ‘gag’ on the swab
- It is not so easy to do oneself and the scrape might not ‘hit the right target’
- If health care professional staff are used they need to be trained
- Such staff taking the sample are putting themselves at risk and facilities for collecting samples need to have stringent infection control
Have tests on saliva been considered in the past?
- There have been a very large number of studies showing that saliva can be used instead of a swab to diagnose other viruses and, more recently, Covid-19
- By the beginning of August over 26 research studies had compared saliva testing to the swab test
- Overall those studies showed that 91% of people who were positive on the swab test and had symptoms of Covid-19 had a positive saliva test
- A research paper published last week found higher amounts of virus genetic material in saliva samples than in the swab test.
- In that study, compared to the swab test, the saliva samples were positive for more days after diagnosis
- Given these results and the easier collection, health authorities in several countries are now suggesting saliva as a replacement for the swab test
Why aren’t saliva tests used up to now?
- There needed to be a standard way of the patient giving the sample in order to compare the results with the scrape test.
- It is not that the test can say ‘just spit into this pot’!
- For example one cannot eat or drink for 30 minutes prior to the sample
- There also needed to be a standard way for collecting the sample, and storing it so it can be posted or transported
- The RTPCR test that is used for the swab samples also needed to be adjusted for testing saliva
- All these issues are now sorted
But the way that saliva can be tested is also changing!
- This week the results of a tie up were announced between a UK biotechnology company MicroGEM and the University of Southampton in the UK
- MicroGEM produced a test, that does not need a laboratory to do the analysis and can provide the user with the result in 15 minutes
- My guess is that the kit might look like a pregnancy test and could be just as easy to use: anyone should be able to test themselves and get an answer
- Obviously, it needs to be robustly evaluated but early results look promising
- The US government have given MicroGEM $41m to expedite development and the means of widespread production of the test
- The test will also identify those with influenza (important when people have typical symptoms of a viral infection)
Point of care (POC) testing
- POC testing has been the goal since the start of the pandemic
- It means that patients, health care professional staff and groups such as school children, employees etc can self-test regularly and those infected can be identified very quickly
- I had previously suggested in my post (10 August) about returning to school that a test such as this could be the strategy needed to reassure parents, students and teachers
- A POC test aiming to identify proteins produced by the virus had been proposed earlier this year but there were too many false negative results and this approach had not been developed further
- This could be a ‘game changer’
- It is not a substitute for social distancing and other preventive measures but it will make it much easier to track the disease and control local outbreaks
- This technology could be used for diagnosing flu and other common virus outbreaks in the future