Testing and travel: PCR, Lamp and TMA tests explained

From PCR to lateral flow – the tests that count for travellers

Distant dream: the Seychelles (Simon Calder)

Many countries now demand negative Covid-19 test results from incoming travellers.

But there is neither international agreement on the type of test nor the timing requirements. Even within the UK’s favourite foreign country, Spain, travellers are being given conflicting information on acceptable tests.

These are the key questions and answers.

What tests are there?

Covid tests look either for the presence of the virus in the body right now or evidence of a previous response to it by your immune system.

Those that look for current infection are broadly known as diagnostic tests. They can be molecular, such as PCR tests, which look for the virus’s genetic material; or antigen tests that detect specific proteins from the virus, usually the so-called “spike” protein. Molecular tests are certainly preferred; the World Health Organisation says negative antigen diagnostic test results “should not remove a contact from quarantine requirements.”

An antibody test looks for antibodies created by your immune system in response to a virus. They can take several days or weeks to develop after you have an infection, and stay in your blood after you recovery. It is not clear if the presence of antibodies can give immunity to Covid-19 in future.

Which is the best Covid test? 

Tests are never 100 per cent accurate, and vary in their “sensitivity” and “specificity”. Sensitivity means the proportion of carriers of the virus who are correctly identified; specificity refers to the proportion of non-carriers who are correctly identified. So 100 per cent sensitivity would mean no false negatives, while 100 per cent specificity means no false positives.

The most popular among government stipulating a particular test for travellers is the PCR (polymerase chain reaction) version. This is also the standard NHS test.

A swab is used for the back of the throat and/or the top of the nostrils. The sample is then processed to try to detect genetic material in the virus called RNA, which is evidence of the presence of Covid-19.

In a specialist laboratory, a solution known as a reagent is added to the sample. The resulting substance is then cooked: subjected to a series of alternating temperature steps using a “thermal cycler,” to create billions of copies of the RNA. This makes them detectable.

The analysis typically takes 12 hours. Since samples must be transported to the lab and are typically processed in bulk, getting a result can take much longer.

Are swab tests uncomfortable?

They are. Many people find the swab procedure unpleasant: a throat swab can make you want to gag, while a nasal swab feels intrusive and even threatening. But health professionals are trained to target material that may contain RNA. 

Self-testing kits are available – but there is concern that untrained individuals or their partners may not properly carry out the test. Some providers will arrange for a medical professional to talk you through the test on a video call.

How do Lamp tests compare with PCR?

“Lamp” (loop-mediated isothermal amplification) diagnostic tests are also molecular. They use similar swabs, or simply (and more comfortably) a saliva sample, and do not require that expensive and time-devouring thermal cycler.

They can be processed on site – for example at Heathrow airport, where Collinson offers the technique for its testing centres in Terminals 2 and 5.

In a wide-scale experiment carried out at NHS trusts and universities, Lamp tests had a sensitivity of 79 per cent and a specificity of 100 per cent – when compared with the PCR results for the same people. In other words, there were no false positives (which is good news for uninfected people) but one in five false negatives (bad news for countries wishing to keep out carriers of the virus), relative to the PCR result.

But the government reported: “In samples with a higher viral load, the sensitivity of the test increased to 94 per cent for saliva and 100 per cent for swabs.”

How do TMA tests compare with PCR?

This is a Transcription-Mediated Amplification test, another molecular variant, similar to the Lamp test. It is molecular, and still needs the sample to be heat-treated to replicate any RNA but is swifter, simpler and cheaper. They are acceptable in Spain, but are not generally available in the UK, annoyingly.

How do lateral flow tests compare with PCR?

These antigen tests are at the heart of the British government’s “moonshot” plan for widespread mass testing. They are cheap and quick, and do not require any cooking. During winter, when people often have symptoms of the common cold or flu, performing an antigen test can help check whether the condition is the coronavirus or not.

But the British Medical Journal cautions: “Their performance as a ‘test to enable’ is lacking.”

It reports: “Testing by Boots test centre employees (following written instructions) achieved sensitivity of just 58 per cent.” In other words, two out of five subjects who were infected were not picked up by the tests.

Antigen tests often become positive only after the individual has had symptoms for a couple of days. 

They have been suggested for use ahead of students returning to university in January. Students will need to be tested twice, once on arrival and again three days later. If the first test comes back positive then a PCR confirmation test will be required and the subject will need to self-isolate until the results come back negative.

When do I have to get the test?

Each country stipulates its own timeframe, usually in the number of hours that are allowed to elapse between the time of the test and your departure to, or arrival at, the destination.

Typically, you are expected to turn up with a negative test taken within 72 hours (three days) of arrival, though some insist on 48 hours while others  allow four, five or even seven days before departure – which can add another 12 hours to the time frame.

There is no need to self-isolate between taking the test and departure, though of course the less you venture out the lower the chance you will be infected.

So what do I need to travel?

That all depends on the country you are visiting – as well as your nationality and/or departure point.

For example, Portugal requires a PCR test conducted within 72 hours of departure for arrivals from most countries except UK, the EU/Schengen Area and a few other low-infection countries.

The leading destination for UK travellers, Spain, requires a test to be performed 72 hours before your arrival. It allows either PCR or TMA tests (though not for travellers under six). But to confuse matters the Canary Islands say they will also accept an antigen test such as lateral flow.

Amid the disarray, airlines are erring on the side of caution. For example easyJet is telling passengers: “The current requirement for Canaries is a PCR test, local governments have said they will now accept an antigen test, but the borders are controlled by the national government.

“It’s been confirmed by the country manager that we must still accept a PCR tests and customers will be refused if not.”

While there is still much confusion, travellers are urged to err on the side of caution and arrive with a PCR test to Canary Islands and anywhere else in Spain.

What about other key destinations?

The Caribbean islands that are open to UK visitors almost all stipulate a PCR test in advance – usually 72 hours before arrival.

Dubai allows a test on arrival for passengers travelling from the UK (as well as Germany and several Gulf states), but neighbouring Abu Dhabi requires a test within 96 hours of departure to the UAE – and a further test no more than 96 hours before your flight home.

For the Seychelles, you need a PCR test no more than 48 hours before departure. On arrival you stay in a designated hotel for six days, take another test and are then free to leave if it is negative.

Can I get a free test through the NHS?

No. Tests provided free for the public who are concerned that they may be infected must not be used for the purposes of travel. Conversely, if you fear you may have contracted coronavirus, you should seek an NHS test rather than one that is offered to travellers who are presumed to be negative.

So how do I find one?

If family or friends can recommend a contact, then go for that. You could ask a long-established travel health provider, such as the Fleet Street Clinic in central London, Nomad (with locations in London, Bristol, Cardiff and Manchester) and Masta (which has a presence in dozens of pharmacies).

An established, doctor-led practice is likely to be both more professional and more expensive than a new venture that is leading on price. They use the highest grade of medical test available with the best possible accuracy ratings.

Will I need a test on arrival? 

Quite possibly. Many countries stipulate one or more tests on arrival, usually at the traveller’s expense.

For journeys to Cuba, no advance test is required, but all air tickets now have have a “sanitary fee” of $30 (£23) included, which covers a test on arrival. Package holidaymakers are then transferred to their hotels, to be briefed on further medical checks or testing, while independent travellers must self-isolate and have a further PCR test on their fifth day in Cuba.

Iceland has a similar policy: either 14 days of self-isolation or a test on arrival and another five days later. “Children born in 2005 or later will be exempt from both testing and self-isolation,” says the Foreign Office.

What about test-to-release on return to the UK? 

A Lamp test should be acceptable, but the test-to-release scheme to halve the length of quarantine has begun with such disarray – with two of the providers already pulling out and others saying no tests are currently available – that it is too early to say.