Covid-19: Who needs a test and which one?

Efforts to ramp up Covid-19 testing have triggered a rush on labs in several countries and spread confusion about who should get tested and when. There are two types of tests available – diagnostic tests and antibody tests – and health authorities’ current guidelines on testing.

As they braced themselves for the long-anticipated “second wave” of the coronavirus pandemic, countries around the world rushed to expand their testing capacities, taking their cue from those – like Taiwan, Germany and New Zealand – that appeared to do better in weathering the first wave.

In many places, including France, the call for enhanced testing has resulted in long queues forming outside medical practices and pop-up labs amid an influx of anxious holidaymakers returning from their summer vacations.

Health workers have complained of tempers fraying due to the long wait. They have also reported widespread confusion concerning who should go for a test, which kind of test, and when.

In late August, US health officials sparked criticism after posting guidelines on testing from the White House coronavirus task force that ran counter to what scientists say is necessary to control the pandemic.

Seeking to clarify the recommendations, the head of the Centers for Disease Control and Prevention (CDC) said “testing may be considered for all close contacts of confirmed or probably Covid-19 patients” – only for critics to point out that “may be” was not helpful for a confused general public.

Who should get tested?

Guidelines for testing can depend on the stage of the outbreak in a given country or area and the local authorities’ capacity to test.

In cases of limited capacity, the authorities may decide to test only subgroups of suspected cases based on the availability of necessary equipment for testing or the level of community transmission of Covid-19, says the European Centre for Disease Prevention and Control (ECDC), an EU agency.

Using a “resource-conscious approach”, the ECDC has suggested prioritising testing for a number of groups including hospitalised patients with severe respiratory infections, symptomatic healthcare staff and elderly people with underlying chronic medical conditions.

As with face masks, shortages of which put governments on the spot around the world, health authorities’ guidance on testing has evolved in line with the availability of tests.

Several countries that have built up large capacities now encourage all members of the public to seek a test, even when they don’t have symptoms.

In France, which has a generous public health insurance system, all tests are reimbursed by the state. However, priority is given to people who have reported symptoms and obtained a doctor’s prescription, or those who have been in contact with someone infected.

To avoid a rush on labs and medical practices, Britain’s National Health Service says people should only get a test if they have symptoms or have been asked to get tested. “This will help make sure people who need a test can get one,” it explains.

Which test to seek?

Tests for Covid-19 include the polymerase chain reaction (PCR) diagnostic test, which involves a nasal swab, and the antibody test, a blood test that can tell whether the person was infected in the past.

Antibody tests are useful to determine whether people have had the virus before, including those who experienced no symptoms.

On the other hand, the antibody test is of little use to people who have had symptoms for less than a week, because it takes on average a week or two to develop antibodies. It should not be used to diagnose an active coronavirus infection since antibodies can stay in your blood for several weeks after recovery.

At this time, researchers do not know whether the presence of antibodies means that you are immune to the coronavirus in the future; or if you are immune, how long it will last.

It is also unknown if antibody levels decline over time to undetectable levels.

According to research by Britain’s Cochrane, antibody tests carried out one week after the first symptoms only detected 30% of people who had Covid-19. But accuracy increased in the second week with 70% detected, and was highest in week three (more than 90% detected), before declining again in the following weeks.

Because of this time lag, people who get tested within 8 days of symptom onset are advised to take a PCR test, which detects the virus rather than the antibodies, and can therefore tell whether or not someone has been infected very early on.

However, some studies have suggested that up to one in five tests may result in a “false negative”, indicating that the person does not have the virus when in fact he or she does.

This can be due either to the timing of the test or human error during testing, which is why many health experts advise repeat PCR tests, particularly if symptoms persist.

When to get tested or retested?

People who report symptoms are advised to take a PCR test as soon as possible and self-isolate before and after the test, pending the result.

The same applies to those who share a home with someone infected.

In cases of more limited exposure, such as having a drink with someone who later tests positive, the “contact” person must wait seven days before going for a test, to allow for the virus’s incubation period.

Health experts have warned that people who get tested too early after exposure can end up with a false-negative test, meaning they may later test positive.

Those who have a negative PCR test and then go on to be exposed or develop symptoms should be retested.

Anyone who tests positive is required to self-isolate at home for a minimum of 10 days and until all symptoms vanish.

Health officials in most countries currently require two consecutive negative PCR tests, taken 24 hours apart, before a patient is moved out of isolation.

Both the World Health Organization and the CDC also say people can be considered recovered and non-infectious 10 days after their symptoms began, as long as they have been symptom-free for three days.