Covid-19 tests Q&A: No test can tell whether you are infectious or not

I see lots of private firms offering Covid-19 tests these days. Are these a good idea?

If you have symptoms, you need to contact your GP and arrange a test from the Health Service Executive. This is free and you should receive the results within a few days; you are required to self-isolate during this period.

In the well population there is a growing demand for Covid tests for reassurance. Some people might be visiting older relatives over Christmas, for instance, and would like to be sure they are not infectious. It seems an increasing number are prepared to pay for private testing, despite the cost – the standard test typically costs €129-€149.

Does this mean I can go visit the family over Christmas with peace of mind, once I have a negative test?

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No, things are more complicated than that. For a start, no test can tell whether you are infectious or not. People with Covid-19 seem to be most infectious just before they start showing symptoms.

The standard PCR (polymerase chain reaction) test can pick up genetic strands of Covid-19, indicating that you were or are infected, but the result is valid only at the point in time the test was taken. You might have a new infection that was not picked up in the test, or you could become infected some time after the test.

There seem to be lots of different types of tests out there. Which is the best?

The PCR test is regarded as the gold standard of Covid-19 testing, and is used by the HSE in hospitals and community testing. But, like any test, it is not perfect, and there can be false positives and false negatives; that’s why anyone with symptoms is told to stay at home even if they get a negative test result until the symptoms are gone for 48 hours, or why contacts have to restrict their movements for 14 days even if they get a negative result.

PCR is also the most expensive type of test.

Increasingly, various forms of rapid tests are being developed that give results in hours or minutes. Antibody tests will tell you if you had an infection in the past, but not whether you are currently infected, so they have no relevance in this context.

Antigen tests detect specific proteins from the virus, are cheap to run and give rapid results. They can be done at the point of care, whereas PCR tests have to be processed in the lab. They work best in people with high viral load, who are within days of showing symptoms. But for people with early infections, or where the sample was not taken properly, false negatives can result. Overall, their accuracy is lower than for PCR technology.

What if I pay privately for a test and it comes back positive?

Ouch! First, you have to self-isolate. Then you need to contact your GP and to do so urgently if your symptoms demand it.

What do our public-health officials say about private tests done for reassurance?

They're not in favour. "We would not advise that as a protective measure," assistant chief medical officer Dr Ronan Glynn said last week, adding that "a test is only as good as the context in which it is taken".

Dr Glynn told a National Public Health Emergency Team (Nphet) briefing that if people want to protect their families or meet people with underlying conditions there are “many other avenues” available to them.

By this he meant limiting their contacts, staying at home if unwell and deploying the other public-health measures such as social distancing and mask-wearing.

Dr Philip Nolan, chair of the Nphet epidemiological modelling advisory group, said at the weekend that getting a private test "to reassure yourself you don't have Covid is pretty much useless as a way of reducing the risk" and he strongly discouraged it. He said if a person does not have symptoms and gets one of these tests "the risk of a false negative test is too high and if you're reassured by a false negative test – in other words you have the virus but it's not fully expressed in your system – you are then assured that you're healthy but in fact you're infected and that's a pretty dangerous position to be in".

What about travel?

The Government says passengers arriving from a region classified orange under the European Union’s traffic-light system should restrict their movement for 14 days unless they have a negative result from a PCR test within three days of their day of departure. Passengers arriving from a red region are asked to restrict their movements for 14 days but this can end early if they have a negative result from a PCR test taken at least five days after arrival in the State. You have to wait for your negative test result to be returned before ending the period of restriction.