The Economist explains
SIDE FLOW TESTS, reportedly a cheap and quick way to identify the covid-19, hit badly. In September, the World Health Organization said “very few” had undergone “strict” regulatory procedures. In a mass testing pilot in late 2020 in Liverpool, in the north-west of England, antigen tests performed by the American company Innova Medical Group missed 60% of asymptomatic cases – more complex polymerase chain reaction (PCR) tests were detected. Later, some nursing homes in England refused to use the tests due to concerns about their accuracy. But lateral flow tests remain at the heart of the British government’s strategy to stop the virus from spreading. From April 9, everyone in the country will be offered free test kits twice a week. How do lateral flow tests work and are they reliable?
Lateral flow tests were already widely used before SARS-CoV-2, the virus that causes covid-19, appeared. Their most common use is in pregnancy tests, where they detect the hormone present in the urine of pregnant women. They can analyze other body fluids such as blood and saliva, as well as other substances such as food. As for the detection of covid-19, the lateral flow test takes a swab of a sample of mucus from a person’s nose or throat. This is dipped into a test tube containing the sample dilution solution and then placed at one end of the porous strip in the test cartridge. As the sample is drawn along the strip by capillary action, it encounters a line of antibodies (proteins also found in the body’s immune system) designed to recognize SARS-CoV-2 antigens (specific proteins found only on the surface of the virus) and bind to them. As with a positive pregnancy test, if a colored band appears on the bar, it indicates a covid-19 infection.
Two major advantages of lateral flow tests are speed and simplicity. They give results within ten to 30 minutes, and can be easily performed outside the lab, unlike PCR tests that look for the entire genetic sequence of a virus, not its antigens. They are also cheaper than PCR tests. According to the analysis Which?, a consumer publication, a private PCR test in Britain costs an average of £ 120 ($ 165), compared to about half the price for a private side-flow test. But there are questions about the accuracy of the lateral flow test. To judge this, scientists consider two elements: specificity (the proportion of tests that correctly give negative results for people who do not have covid-19) and sensitivity (the proportion of tests that correctly give positive results for people who do so). Data collected through the British Testing and Monitoring System show that lateral flow tests have a specificity of at least 99.9%. This is high, but still means that on average one in every 1,000 tests will give a false positive result.
And lateral flow tests are less sensitive than PCR tests. If an infected person does not produce enough antigen, which is usually the case at the beginning or end of the infection, the test may give a false negative result. Another problem is that the sample probe must work deep enough in the throat or nose. Research conducted by Oxford University and Public Health England shows that the sensitivity of tests to PCR tests dropped from 79.2% in the laboratory to 57.5% when performed by self-trained members of the public.
Lateral flow tests are best in identifying highly contagious individuals. This makes them useful in the fight against covid-19, even if they are not perfect. The sooner cases are detected, the greater the chances of reducing the spread of infections. But rapid testing alone is not enough to control a pandemic. This must go hand in hand with other testing methods and measures such as social exclusion and vaccination.
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