Types of tests

There are two main sorts of Covid tests available: PCR tests and rapid antigen tests.¹

PCR tests have essentially no false positives and ok-ish false negative rates.  They’re processed in lab; overnight if you’re lucky and the lab isn’t overworked. Rapid antigen tests are less accurate: they have rare but non-negligible false positives and they turn positive later in the infection process than PCR tests.  As it says on the tin, they’re rapid: you don’t need a lab and can get results in minutes. The two test types are useful for different purposes.

In New Zealand until recently, Covid was rare and occurred in limited clusters. In this setting, when the public health system is doing zero-tolerance control, there’s not much use for rapid antigen tests².  If you are symptomatic or a contact you should isolate and get PCR tests; if you are asymptomatic and not a contact then you’re very unlikely to have Covid and any positive test is likely to be a false positive.  Anonymous at-home testing allows someone who has symptoms or is a contact to test positive and keep it quiet — maybe stay away from Nana’s 90th birthday party or drinks at the pub, but still go to work and go to the supermarket. The public health system would like this not to happen.

When Covid is everywhere, as in the US and the UK, quite a lot of people will be exposed and not know about it, so asymptomatic community testing is no longer useless. The problem with PCR is that it’s slow, especially when the labs get overworked: you won’t get the result until at least tomorrow and perhaps a few days later.  Rapid antigen tests are very valuable because they are rapid — they tell you, with imperfect but useful accuracy, whether you are infectious right now.  If you’re planning an in-person party or meeting or movie or flight or date, that’s what you want to know.  In the UK and Europe, the ‘lateral flow test’ type of rapid antigen test have been very useful; they miss some infections, but they catch quite a lot.  In the US they would have been useful, but they’re expensive and availability has been limited. The US government has just moved on making the tests a little more available — they’ve bought half a billion tests, nearly two for each US adult.

In New Zealand at the moment the situation is more complicated.  We’re on the boundary between zero Covid and low-level suppression.  The government is still trying to keep control of test results, which has clear benefits in contact tracing and elimination, but removes the ability for everyone to use rapid tests to reduce their individual risk of spreading Covid.  Whether you think the government is making the right decision here depends a lot on how much you trust the public health system, and on how much you trust other people.

¹ yes, and a whole bunch of other minor options
² except perhaps for border workers or customer service people during an outbreak