Coronavirus tests – the basics

Understanding what the different tests mean and how they can be used most appropriately underpins the test and trace process.

International Medical Information (IMI) interviewed two pathology experts – Mr Michael Bass, (PathDirect Laboratory Director) and Dr John Bolodeoku (chemical pathologist and PathDirect Clinical Director) – to get their views on the available tests for the coronavirus.

Both the molecular test and the protein test are to detect the presence of virus in the nasopharyngeal cavities so they require swabs. The molecular test uses a molecular (gene) probe and the antigen is detected using a protein probe, explains Dr Bolodeoku.

The molecular test relies on a PCR (polymerase chain reaction) method.  This is a technique that is usually carried out in molecular biology laboratories using specialised equipment with a high level of biological safety measures in place.  At present, PCR testing for covid-19 cannot be performed in chemical pathology laboratories that are set up for routine biochemical and haematological measurements. Research is under way to develop methods for PCR testing with smaller pieces of equipment but this is not yet available.

Antigen testing can be undertaken in routine laboratories and can be completed in as little as 12 minutes.

The PCR test starts with a tiny piece of molecular material from the virus and this requires multiple ‘cycles’ to amplify the material to produce a large enough quantity to be detectable; this process can take four-six hours.

An additional complication is that swabs have to be transported to specialised laboratories before the testing can take place and so this also contributes to the turn-round time for PCR tests. Essentially, “PCR testing is not set up for large-scale testing”, explains Dr Bolodeoku.

Relative costs of tests

PCR testing costs between 10 and one hundred times more than antigen testing   because of the analytical equipment, reagents and facilities that are required.

Given these considerations, antigen testing appears to be more suitable for large scale use, such as in ‘test and trace’ schemes.  In fact, in Italy authorities are now asking for antigen tests instead of PCR tests

See linked here a (PDF) summary table that compares the three types of coronavirus test.