Rapid Antigen Testing

About

The rapid antigen test (RAT) is a testing instrument much like what we currently use for pregnancy and flu diagnosis. The RAT is able to diagnose COVID 19 in a patient who is asymptomatic or has symptoms of the disease. If the person tests positive, they can be advised on isolation protocols, follow-up medical review, further testing also the notification of public health officials so that contact tracing can begin immediately.

The test is accurate, can be used onsite , is cost effective compared to the nasal swab PCR test and allows early detection of the virus. It is fast (takes 15 minutes for the results rather than many hours to days with the nasal PCR swab) and is a powerful tool in detecting COVID-19 .

PCR has a relatively high false positive rate due to the detection of fragment viral RNA not associated with active or transmissible infection. By comparison rapid antigen tests have an extremely low false positive rate – in the order of 0.3% or less.

When it comes to the false negative results, both PCR and rapid antigen tests are not perfect; in some studies, as high as 40% false negatives. The false negative results usually related to variations in sample collection, sample timing (in relation to the infection cycle) or lab error. For well collected samples in the first 7 to 10 days post infection, rapid antigen tests are now showing false negative rates very similar to that obtained by PCR. Indeed, once the rapid antigen saliva tests are approved for use in Australia, the sample collection errors will reduce substantially, and rapid antigen tests will have false negative performance extremely close to nasopharyngeal PCR tests.

Some manufacturers of rapid antigen tests have optimised the positive cut-off values (the limit at which the test positive line is visible on the test cassette) to correlate with the PCR cycle count (Ct score) of 30. There is mounting evidence to suggest that at a Ct score of 30 or above, the viral load is below that likely to be infectious. The point being that just because you can detect viral RNA, it does not necessarily follow that an individual is infectious. This is especially so from about day 12 post infection; where a PCR test will generally show a weak positive (Ct score 30 or more) but the rapid antigen test will be negative.

Onsite Testing

Hemisphere Management Group can provide your production with an onsite team for Rapid Antigen Testing to register, test and process results.

Why and when?

Rapid antigen tests are not designed to take the place of PCR. They are designed as a screening test, best used in quite specific ways.

Rapid antigen tests are best used where you need to take rapid action and they are best used when you can repeatedly test the same cohort of people. The false negative rate reduces dramatically when repeat testing is performed.

The rapid antigen tests are best used in high-risk environments, such as hotel quarantine, where the prevalence is much higher than in the Australian community. The healthcare environments treating COVID patients. Or in situations such as events and tv productions where people need to work close together.

In these environments rapid antigen tests should be used to screen people every 3 – 4 days. The test result should guide an immediate action. In the case of the quarantine worker, repeated negative tests (every 3 or 4 days) would allow the worker to return home to their family, with minimal risk of propagating a community infection. Lab based PCR, cannot perform this role as it takes too long to get the results back. Currently the lab-based PCR tests are the mainstay for state health departments and will remain so for the moment. At the very best, a lab result takes 4 to 6 hours to return a result but normally 24 – 48 hours.

This is not practical in the repeat testing environments when a rapid decision needs to be made.

To summarise

  • Rapid antigen test performance is at least as good as PCR for specificity (false positive rate); antigen tests have an extremely low false positive rate.
  • Rapid antigen test performance is approaching that of PCR for sensitivity (false negative rate) and the new saliva tests will bring this even closer. Especially in the first 7 to 10 days post infection where they have a total accuracy of between 97 and 98%.
  • Rapid antigen testing should be used in situations where you need to work closely together and is best used when you can repeatedly test the same cohort of people.
  • Rapid antigen tests are best used as a screening test where repeat testing and rapid action are required. Especially in asymptomatic and pre-symptomatic individuals because it can be done at point of care and can give a rapid result. People who have symptoms should be referred to PCR testing. RAT can still be carried out on symptomatic people, but in most screening settings, we want to keep symptomatic people away and refer them to government PCR testing facility.