Different paths to the same destination: detection of Covid-19

TIB Diagnostics examines the differences between PCR, LFT, and antibody tests for Covid-19.

During the Covid-19 crisis, the importance of reliable and accessible tests to detect the disease became increasingly evident. The Covid-19 test can be divided into antigen or antibody tests, which use different types of samples to look for different characteristics of the SARS-CoV-2 virus. TIB Diagnostics takes a closer look at the different types of Covid-19 tests.

What are the different types of Covid-19 tests?

Polymerase chain reaction (PCR) tests are sent to a lab to diagnose the disease

Lateral flow tests (LFT) can diagnose Covid-19 on the spot but are not as accurate as PCR tests

Antibody (or serology) tests can't diagnose an active infection, but they can help determine if a person is immune to Covid-19

What is the PCR test?

PCR tests are used to directly detect the presence of viral RNA, which will be detectable in the body before antibodies form or symptoms of the disease appear. This means that tests can determine if a person has the virus at a very early stage of the disease.

In the Covid-19 PCR test, substances called reverse transcriptase or DNA polymerase are added to a nasopharyngeal sample in the laboratory. These substances work to make many 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 specially designed primers and probes adhere to the virus's genetic code sequences to indicate that a pathogen has been found.

"PCR gives us a good indication of who is infected, “They can isolate themselves and come into contact with people they have been in contact with so that they can also be quarantined, just in case. This is the real benefit of today's leading diagnostic tests; you can break that chain of transmission and get a clearer picture of what's going on. "

By stepping up PCR testing to analyze large strips of nasopharyngeal swab samples from 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 tests need to be sent to a lab for analysis, which means it can take days for people to know the results.

False negatives can occur up to 30% of the time with different PCR tests, which means that they are more useful in confirming the presence of an infection than in giving the patient the green light. They can also provide false positive results, as they are so sensitive that they can potentially indicate a positive result by detecting killed and inactivated viruses that are still present in the body of a person who has recovered from Covid-19.

How about a lateral flow test?

LFTs are similar to PCR tests in that they are two types of antigenic tests, designed to detect 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 pulled along the pad through a capillary line to an antibody-coated band, which binds to SARS proteins - Cov -2. If these proteins are present, they will appear as a coloured line on the test, indicating infection.

The main advantage of LFTs over PCRs is that they do not need to be submitted for confirmation and instead provide results within 15 to 30 minutes. However, what they gain in speed, they sacrifice in precision.

A review of 64 studies in Europe and the United States showed a large variation in accuracy between different brands of LFT. The review also found that the tests were much better at identifying Covid-19 in people who had symptoms than in those who did not. The sensitivity of LFT in symptomatic individuals ranged from 34% to 88%, with an average accuracy of 72%. In people without symptoms, LFTs correctly identified an average of 58% of those infected.

While in countries like the UK the use of LFTs for massive asymptomatic screening has been encouraged, experts have questioned the usefulness of these types of Covid-19 tests in this context.

What is the antibody test?

Wright says, “An antibody test tells us what proportion of the population has been infected. It won't tell you who is infected, as the antibodies are generated after a week or two, after which the virus should be gone from the system. But it tells you who got infected and who should be immune to the virus. "

A study published in the journal Immunity found that people who recover from even mild cases of Covid-19 produce antibodies for at least five to seven months, and they could do so for much longer.

Historical studies have indicated that people who survived the outbreak of Sudden Acute Respiratory Syndrome (SARS) in the early 2000s had antibodies in their blood for years after their recovery. SARS and Covid-19 are both caused by similar coronaviruses, so it is not unreasonable to think that Covid-19 could have a similar effect.

"If there is a high enough level of immune people in the population, then they will stop the circulation of this virus in the population, which is known as herd immunity," Wright said. "If someone is infected, as long as the people around them are immune, the virus will not be able to spread."

Unlike PCR tests, which commonly use swabs to detect Covid-19, blood samples are typically used for antibody testing. Indeed, there will be a very small amount of Covid-19 circulating in the blood compared to the respiratory tract, but a significant and measurable presence of antibodies in the blood after infection.

Antibody tests are used to assess immune responses in people who have been vaccinated against Covid-19. Researchers do not yet know how long the vaccine-induced immunity will last or whether booster shots will be needed. There have been some indications that the Covid-19 variants make some vaccines less effective, but so far they still appear to offer enough protection to guard against serious or life-threatening illnesses.