WHICH TEST SHOULD I GET?

Here is a general overview table comparing PCR tests with Rapid Antigen tests such as Binaxnow.  

 A longer discussion about which test does what

There is a lot of noise in the discussion about testing. It is complicated, and different circumstances affect which type of test is most appropriate. 

They are all good, they are all better than guessing, none are perfect.

1 – Sample quality matters

All tests depend on a decent sample. Usually, if someone is infected with the virus and is showing symptoms, the swab should be able to pick up virus particles from the back of the nose.

However, sometimes the virus might be multiplying in places away from where the swab is, or there is not yet enough virus around for the swab to pick up.

So a negative test might mean that you don’t have coronavirus, or that you have coronavirus and it just isn’t detectable yet, or the wrong place was swabbed. At the beginning and end of infection the amount of virus varies (it is “sputtering”) and you should not swab too often. 

2 – Why lateral flow tests are more likely to pick up current infection

The antigen test and the PCR test are for different things. The PCR tests for viral RNA while the antigen test (aka “Lateral flow tests – LFT) checks for part of the capsule of the virus itself.

In the graphic above the amount of virus is shown over time. It starts on left side with exposure, through incubation period, into infection and recovery.

The actual number of days that this happens varies a lot with individual factors such as immunity, dose of exposure, variant of the virus just to name a few.

The PCR test will detect the COVID virus earlier than the antigen test, but because the results take longer to get back (1-4 days) it may be meaningless to prevent transmission.

The other problem with PCR tests arises at the far right side of the curve where you have low viral load, and the PCR test is still positive. This phase can last for a while. The body is breaking up virus RNA and it is around but may not be transmissible at all. 

This is why once you have a positive PCR test it is recommended you not retest with a PCR for 90 days.

3 – So what tests should I take, and when?

At present, we are recommending people use home tests to check their current level of contagiousness

• For school

• For work

• Before travel

• Before mixing with others

• If they have symptoms

• Or even if they are asymptomatic but are about to meet up with someone vulnerable.

• If you have had Covid, and want to find out if you are still contagious. Although the CDC says that asymptomatic people can come out of isolation on day 5, evidence shows that a fairly large proportion of these people are still contagious. It’s worth doing an antigen test on day 5 to check.

The antigen tests works best for this and is nearly 98% sensitive for COVID-19 if done as two tests — aka serial testing approximately 1-2 days apart. With two tests you know where you are — staying negative or now positive. That is why the kits come with two tests.

A test that misses the virus early will catch it on the second test especially if virus levels are rising.

So, a person that is symptomatic = usually a higher viral load = more transmissible = antigen test works very well.

When might you need a PCR test? 

If you have been exposed to Covid, and are able to get an appointment for a rapid PCR test that gives results within 24 hours, then it will catch infection earlier than an antigen test

If you have Covid symptoms but are getting repeated negative antigen tests it may be worth getting a PCR test if you can find one.

For travel – some countries are requiring a PCR test. REMEMBER: if you have had Covid within the last 90 days, a PCR test may still show as positive. For travel you may be required instead to show evidence of a recent positive Covid test instead. 

When it is required eg before going into hospital.