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?

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.

Types of Tests

There are different types of COVID-19 tests – diagnostic tests and antibody tests.

Diagnostic tests can show if you currently are infected with SARS-CoV-2, the virus that causes COVID-19. There are two types of COVID-19 diagnostic tests:

Samples for COVID-19 diagnostic tests are typically collected using an anterior nares (nasal) swab sample. Some diagnostic tests use mid-turbinate, nasopharyngeal, oropharyngeal, or saliva samples. COVID-19 diagnostic tests can be performed at a laboratory, a standalone testing site, a doctor’s office or health clinic, or at home. For some COVID-19 diagnostic tests, you go to a testing site to have your sample collected and for others you can collect your own sample at home using a home collection kit and mail it to a laboratory for testing. Other tests can be performed completely at home, giving you results within minutes, without needing to send your sample to a laboratory.

If you think you need a COVID-19 diagnostic test, you can find a community testing site in your state. You can also use an FDA-authorized at-home COVID-19 diagnostic test which gives you the option of self-testing where it is convenient for you. Be aware that COVID-19 diagnostic tests are authorized for specific uses. For example, some tests can be used by people with and without symptoms and other tests are only for people with symptoms. Also, laboratory-based tests, such as PCR tests, are generally more accurate than at-home tests.

For details about each authorized COVID-19 diagnostic test, see the lists of authorized Molecular Diagnostic Tests and Antigen Diagnostic Tests, as well as the At-Home COVID-19 Diagnostic Tests webpage. Using the search box in the EUA tables, you can use keywords to search and filter the type of test or collection kit you are looking for. As new tests are authorized for use, they are added to these tables so that anyone can access up-to-date information on all authorized tests and collection kits.

Antibody (or serology) tests look for antibodies in your blood that your immune system produced in response to SARS-CoV-2, the virus that causes COVID-19. Antibody tests should not be used to diagnose a current SARS-CoV-2 infection or COVID-19 and, at this time, should also not be used to check for immunity. More research is needed to determine what, if anything, antibody tests can tell us about a person’s immunity.

Samples for antibody tests are typically collected by a doctor or other medical professional by taking blood from a finger stick or your vein. For more information about antibody testing, visit Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers.

Types of Samples

Different tests are authorized to be used with different types of samples. The most common sample types are:

Swab samples use a swab (similar to a long Q-Tip) to collect a sample from the nose or throat. The types of samples include:

  • Anterior Nares (Nasal) – takes a sample from just inside the nostrils
  • Mid-turbinate – takes a sample from further up inside the nose
  • Nasopharyngeal – takes a sample from deep inside the nose, reaching the back of the throat, and should only be collected by a trained health care provider
  • Oropharyngeal – takes a sample from the middle part of the throat (pharynx) just beyond the mouth, and should only be collected by a trained health care provider

Saliva samples are collected by spitting into a tube rather than using a nose or throat swab.

Blood samples are only used to test for antibodies and not to diagnose COVID-19. Venous blood samples are typically collected at a doctor’s office or clinic. Some antibody tests use blood samples from a finger stick.

What are rapid point-of-care tests for COVID-19?

Rapid point-of-care tests aim to confirm or rule out COVID-19 infection in people with or without COVID-19 symptoms. They:

  • are portable, so they can be used wherever the patient is (at the point of care);
  • are easy to perform, with a minimum amount of extra equipment or complicated preparation steps;
  • are less expensive than standard laboratory tests;
  • do not require a specialist operator or setting; and
  • provide results ‘while you wait’.

We were interested in two types of commercially available, rapid point-of-care tests: antigen and molecular tests. Antigen tests identify proteins on the virus; they come in disposable plastic cassettes, similar to pregnancy tests. Rapid molecular tests detect the virus’s genetic material in a similar way to laboratory methods, but using smaller devices that are easy to transport or to set up outside of a specialist laboratory. Both test nose or throat samples.

Why is this question important?

People with suspected COVID-19 need to know quickly whether they are infected, so that they can self-isolate, receive treatment, and inform close contacts. Currently, COVID-19 infection is confirmed by a laboratory test called RT-PCR, which uses specialist equipment and often takes at least 24 hours to produce a result.

Rapid point-of-care tests could open access to testing for many more people, with and without symptoms, potentially in locations other than healthcare settings. If they are accurate, faster diagnosis could allow people to take appropriate action more quickly, with the potential to reduce the spread of COVID-19.

What did we want to find out?

We wanted to know whether commercially available, rapid point-of-care antigen and molecular tests are accurate enough to diagnose COVID-19 infection reliably, and to find out if accuracy differs in people with and without symptoms.

What did we do?

We looked for studies that measured the accuracy of any commercially produced, rapid antigen or molecular point-of-care test, in people tested for COVID-19 using RT-PCR. People could be tested in hospital or the community. Studies could test people with or without symptoms.

Tests had to use minimal equipment, be performed safely without risking infection from the sample, and have results available within two hours of the sample being collected.

Authors' conclusions:

Antigen tests vary in sensitivity. In people with signs and symptoms of COVID-19, sensitivities are highest in the first week of illness when viral loads are higher. The assays shown to meet appropriate criteria, such as WHO's priority target product profiles for COVID-19 diagnostics (‘acceptable’ sensitivity ≥ 80% and specificity ≥ 97%), can be considered as a replacement for laboratory-based RT-PCR when immediate decisions about patient care must be made, or where RT-PCR cannot be delivered in a timely manner. Positive predictive values suggest that confirmatory testing of those with positive results may be considered in low prevalence settings. Due to the variable sensitivity of antigen tests, people who test negative may still be infected.

Evidence for testing in asymptomatic cohorts was limited. Test accuracy studies cannot adequately assess the ability of antigen tests to differentiate those who are infectious and require isolation from those who pose no risk, as there is no reference standard for infectiousness. A small number of molecular tests showed high accuracy and may be suitable alternatives to RT-PCR. However, further evaluations of the tests in settings as they are intended to be used are required to fully establish performance in practice.

Several important studies in asymptomatic individuals have been reported since the close of our search and will be incorporated at the next update of this review. Comparative studies of antigen tests in their intended use settings and according to test operator (including self-testing) are required.

Sources:

https://www.medicaldevice-network.com/features/types-of-covid-19-test-antibody-pcr-antigen/

https://www.fda.gov/consumers/consumer-updates/covid-19-test-basics

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