Different paths to the same destination: screening for Covid-19
Medical Device Network takes a look at the differences between PCR, LFT and antibody tests for Covid-19.
Over the course of the Covid-19 crisis, the importance of reliable, accessible testing to screen for the disease has become increasingly apparent. Tests for Covid-19 can be divided into antigen or antibody tests, both of which use different kinds of samples to search for different hallmarks of the SARS-CoV-2 virus. Medical Device Network takes a closer look at the different types of Covid-19 test.
What are the different types of Covid-19 test?
- Polymerase chain reaction (PCR) tests are sent away to a lab to diagnose disease
- Lateral flow tests (LFTs) can diagnose Covid-19 on the spot, but aren’t as accurate as PCR tests
- Antibody (or serology) tests can’t diagnose active infection, but they can help to tell if a person has immunity to Covid-19
What is PCR testing?
PCR tests are used to directly screen for the presence of viral RNA, which will be detectable in the body before antibodies form or symptoms of the disease are present. This means the tests can tell whether or not someone has the virus very early on in their illness.
During Covid-19 PCR testing, substances known as reverse transcriptase or DNA polymerase are added to a nasopharyngeal sample in a lab. These substances work to make numerous copies of any viral RNA that may be present. This is so that enough copies of the RNA are present to signal a positive result, as specifically designed primers and probes attach themselves to sequences of the genetic code of the virus to signal that a pathogen has been found.
“PCR gives us a good indication of who is infected,” says University of Sussex senior lecturer in microbiology Dr Edward Wright. “They can be isolated and get in contact with people they’ve been in touch with so they can be quarantined too, just in case. That’s the true advantage of the current major diagnostic tests; you can break that transmission chain and get a clearer picture of what’s happening.”
By scaling PCR testing to screen vast swathes of nasopharyngeal swab samples from within a population, public health officials can get a clearer picture of the spread of a disease like Covid-19.
However, PCR still has its caveats. These types of Covid-19 test need to be sent away to a laboratory for analysis, meaning it can take days for people to find out their results.
False negatives can occur up to 30% of the time with different PCR tests, meaning they’re more useful for confirming the presence of an infection than giving a patient the all-clear. They can also provide false positive results, as they’re so sensitive they can potentially signal a positive result upon detecting dead, deactivated virus still present in the body of someone who has recovered from Covid-19.
Warwick Medical School honorary clinical lecturer Dr James Gill said: “During the course of the outbreak, the PCR testing has been refined from the initial testing procedures and with the addition of greater automation to reduce errors. As we are looking at swabs taken from people, who have lots of other organisms floating around, we are essentially dealing with the question of how ‘right’ the result we are looking at is.”
How about a lateral flow test?
LFTs are similar to PCR tests, in that they’re both types of antigen test, designed to pick up active Covid-19 infection rather than antibodies to the disease. With a Covid-19 LFT, a nasopharyngeal sample is placed on a small absorbent pad, which is then drawn along the pad via a capillary line to a strip coated in antibodies, which bind to SARS-Cov-2 proteins. If these proteins are present, this will show as a coloured line on the test, indicating infection.
The major benefit of LFTs over PCRs is that they do not need to be sent away for confirmation, and instead provide results within 15 to 30 minutes. However, what they gain in speed they sacrifice in accuracy.
A review of 64 studies from Europe and the US showed a wide variance in accuracy between different brands of LFT. The review also found that the tests were far better at identifying Covid-19 in people who had symptoms than those who did not. LFT sensitivity in symptomatic people ranged from 34% to 88%, with an average accuracy of 72%. In people without symptoms the LFTs correctly identified an average of 58% of those who were infected.
While the use of LFTs for mass asymptomatic screening has been encouraged in countries like the UK, experts have cast doubt on how useful these types of Covid-19 test really are in this context.
What is antibody testing?
Wright says: “An antibody test tells us what proportion of the population has been infected. It won’t tell you who is infected, because the antibodies are generated after a week or two, after which time the virus should have been cleared from the system. But it tells you who’s been infected and who should be immune to the virus.”
A study published in the journal Immunity has found that people who recover from even mild cases of Covid-19 produce antibodies for at least five to seven months, and could do so for much longer.
Historical studies have indicated that people who survived the sudden acute respiratory syndrome (SARS) outbreak in the early 2000s had antibodies in their blood for years after recovery. Both SARS and Covid-19 are caused by similar coronaviruses, so it’s not unreasonable to think that Covid-19 could have a similar effect.
“If there’s a high enough level of people in the population who have immunity, they will then stop this virus from circulating within the population, which is known as herd immunity,” says Wright. “If someone is infected, as long as the people around them have immunity the virus won’t be able to spread.”
Unlike PCR tests, which commonly use swabs to detect Covid-19, blood samples are usually used for antibody tests. This is because there will be a very small amount of Covid-19 circulating in the blood compared to the respiratory tract, but a significant and measurable antibody presence in the blood following infection.
Antibody tests are being used to evaluate the immune responses in people who have been vaccinated against Covid-19. Researchers don’t yet know how long vaccine-induced immunity will last or if booster shots will be needed. There has been some indication that Covid-19 variants are making certain vaccines less effective, but thus far they still appear to generally provide enough protection to guard against severe or fatal disease.
Source: Medical Device Network