The strengths and weaknesses of lateral flow tests
The debate about lateral flow tests continues to twist and turn. Reports about the tests appear in the news on a daily basis. Opinion about the accuracy, effectiveness and value of the tests amongst clinicians is deeply divided.
Lateral flow tests process saliva or fluid samples without the use of a laboratory. They are designed to show the presence or absence of a specific marker, like a virus or a hormone. The best known examples of lateral flow tests are home pregnancy tests.
Because they offer quick results, often in around 15 minutes, lateral flow tests could be significantly more helpful in testing for COVID-19 than laboratory based testing. Polymerase Chain Reaction (PCR) testing usually takes between 12 and 24 hours to process in a laboratory. The time taken to get the sample to a laboratory is in addition.
The government have committed to using lateral flow tests to mass test asymptomatic people in schools, workplaces and as part of ‘surge’ testing in areas where new COVID variants are found.
Alarmingly, studies (including analysis done by the government) have highlighted serious concerns about the accuracy of the tests. Some eminent clinicians have publicly called on the government to stop using them, because they believe they may cause serious harm.
However, another group has suggested the concerns are misplaced. They suggest that the risks are being inflated and misinterpreted, which could cause harm to public health in itself.
Around 1 in 3 people who have COVID-19 have no symptoms and may be spreading the virus without knowing it. A test that can rapidly identify such people may be very useful in fighting the virus. The tests are now being recommended by the government for use in the workplace.
However, deciding whether to use lateral flow tests in the workplace is not a straightforward decision.
The main reasons for using the tests
- The tests are very quick and easy to use, especially compared to PCR testing. Although they are licensed for use only under medical supervision, this can be done remotely.
- The tests are significantly cheaper than PCR laboratory tests (they are offered to us wholesale on a daily basis at between £2 and £4 per test).
- So far, the tests have found at least 14,800 cases of infection that would otherwise have gone unrecognised.
The main reasons not to use the tests
- The main criticism of lateral flow tests is that they are simply not accurate enough. Potentially missing between 30-60% of people (see below) who are infected is a big problem.
- The tests must be used under supervision, which is especially relevant for employers. Those who cannot afford to self-isolate may not use the tests properly otherwise.
- People given a potentially inaccurate negative result (a “false negative”) may unwittingly go on to infect others.
How accurate are the tests?
Manufacturers frequently claim the tests as being “100% accurate”, having sensitivity (true positive) rates of 97% or higher and specificity (true negative) rates of up to 99% too.
However, those results stem from testing in lab conditions, often on very limited numbers of people, all of whom had symptoms.
That matters greatly because you would expect to find more people with positive results amongst those who already have symptoms. In turn, this could make the success rate of the test appear better than it may be if it were used amongst people who did not have symptoms.
The government evaluated lateral flow tests via Public Health England and also during a mass trial in Liverpool. The “Liverpool” study showed that the tests missed about 30% of positive infections amongst people who had a high viral load (and are therefore more likely to be infectious to others).
Furthermore, the results also showed that the tests missed 60% of people who were positive via PCR testing. It is true to say that many of those people may not have been infectious, but it is equally difficult to prove that they could not still transmit the virus.
The Government also reports that the tests find 77% of positive cases, based on a study run by Oxford University and Public Health England.
However, that study has also been criticised as being unrealistic because it was focussed on people who already had symptoms. Additionally, samples were obtained by experienced clinicians, which could in turn improve the accuracy of the tests.
In mitigation, the Government are very clear that the tests are being used only in conjunction with PCR tests to support public health. They are not claimed to be diagnostic tests to establish that people who are ill have the virus.
It is currently very difficult to definitively prove the accuracy of the tests in ‘real life’ scenarios, amongst people who do not already have symptoms. However, the public health benefit for using the tests is still quite strong.
Whatever test is used, PCR or lateral flow, they are only useful if the results are acted upon. If compliance with self-isolation is not supported, financially or otherwise, the risk of ongoing transmission remains.
The balance of risks
The Government’s justification for using the tests rests on the need to protect public health. Doing something is better, on balance, than doing nothing.
There are, however, still notable risks for employers in using the tests. Protecting the health of a whole population involves different considerations than protecting the health of a workforce.
Whilst it may not factor too greatly in the need to protect overall public health, it is very unclear where liability for problems associated with using the tests may rest, especially as they have known weaknesses, if they are used in a workplace setting on a private basis.
Sadly, it is not beyond the realms of fantasy that an employer could be accused of being responsible if there are consequences arising from the use or misuse of the tests.
“I infected my Grandmother after the test you gave me said I was negative” and “I have long COVID after I was exposed at work, because the tests you used are not accurate” are both scenarios that merit consideration by employers before a testing programme begins.
Our current advice for employers
Whilst the tests offer a very rapid indication of the presence of the virus, the results are difficult to rely upon with certainty. They cannot currently accurately confirm the absence of the virus.
The tests will continue to develop and they should become more accurate quite quickly. We continue to monitor developments closely. Unfortunately, it is very difficult to ascertain the provenance of many of the kits being supplied at the moment.
For now the tests are probably best considered as ‘red light’ tests – if they come up positive you are potentially infectious to others and you must self-isolate.
However, they are not ‘green light’ tests. If the test is negative, you cannot be certain you are not infectious and you must continue to take the usual precautions. This should be made clear to all employees if testing is introduced. Testing is not a substitute for infection control measures.
Whilst we await more independent evidence of the benefits and accuracy of lateral flow tests, it is important to consider their use within their limitations.
Although lateral flow tests have many benefits, they are not a risk-free choice for employers.
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