What is the test used to diagnose COVID-19 infection? What are PCR and antibody tests all about?

“When your doctor sends you to a pathology lab for a COVID-19 test, the test you will have is called a polymerase chain reaction (PCR) molecular test,” says Dr Noluthando Nematswerani Head of the Centre for Clinical Excellence at Discovery Health. “PCR tests are used to diagnose people who have COVID-19 symptoms or those who have recently come into contact someone confirmed to have COVID-19.”

“However, since the start of the pandemic, there’s also been much discussion around the use of antibody tests for COVID-19. People are under the impression that an antibody test will work the same way, or even better than the currently available PCR tests to diagnose COVID-19 infection. These tests have often been marketed as ‘10 to 15 minute tests’ (offering quicker results). However, their limitations are not fully explained.”

It’s very important that we understand when to have PCR or antibody tests, and how to use them in COVID-19 testing.

Why has it now become important to understand when each of these tests is relevant?

On 25 August 2020 the South African Health Products Regulatory Authority (SAHPRA) issued authorisation for the following SARS CoV 2 antibody tests:

• One rapid point-of-care test kit
• Five lab-based serology (antibody) tests

“As a result, some of South Africa’s national pathology labs will now offer antibody testing for COVID-19. However, these tests are not recommended for the diagnosis of COVID-19 and its very important that we understand why,” says Dr Nematswerani.

SAHPRA and our National Department of Health make it clear that PCR testing remains the best way to diagnose cases of COVID-19. SAHPRA also makes it clear that no COVID-19 test kits are to be advertised or sold directly to the public as they are Class D medical devices.”

Why are antibody tests not recommended for diagnosing COVID-19 infection?

Let’s explore this in more detail. First, what are antibodies?

Antibodies are proteins that the immune system produces when it encounters a pathogen in the body, like the SARS CoV 2 virus that causes COVID-19. Antibodies can attack the parts of the virus that cause damage to the body. One type of antibody (the neutralising antibody) can attach to a virus and disable its harmful effects.

We can think of antibodies as custom-made weapons your body develops through a complex process that takes time. That’s why when a virus first infects us, it’s possible that we won’t have antibodies for at least one to three weeks.

What antibody tests do:

• Antibody tests will show you that you have COVID-19 far too late after you get the disease to be useful for diagnosis. It can take between one and three weeks after infection for your body to make antibodies to fight off COVID-19. To see if you currently have COVID-19, you need a PCR test. This test is the only way to diagnose COVID-19 early on in the disease within seven days of infection even if they don’t have any symptoms, or when people start showing symptoms.
  Therefore, early in the infection antibody test results would falsely show you don’t have COVID-19 even if you are have the disease. And, if people who have COVID-19 think they don’t have it, they won’t self-isolate and so will unknowingly continue spreading the infection.
• Antibody tests can tell if a person has had a past infection with COVID-19, even though these tests may not always be 100% accurate.

What do positive and negative antibody test results mean?

“A positive antibody test result shows you may have antibodies that developed after a COVID-19 infection,” says Dr Nematswerani. “The problem is that there is a chance that the antibodies the test is picking up come from your having had previous infections with other coronaviruses that cause a common cold. Some antibody tests are not sensitive enough to tell the difference between different coronaviruses.”

What you need to know about antibody test results:

• A negative antibody test can mean you have never had COVID-19.
• A negative test can also mean you had the test too soon to detect antibodies after COVID-19 infection. That is, your body is still busy making antibodies to COVID-19.
• An antibody test will only be positive if there is a high enough level of antibodies in the blood. The amount of antibodies we produce when we contract COVID-19 depends on our individual immune systems and also on how severe our illness is. People who have no symptoms or experience mild illness may not produce enough antibodies for a test result to return as positive. So while their antibody test is negative, these people will still have had COVID-19.

So when are antibody tests valid?

“Antibody tests play an important role in helping us to understand whether the vaccines that South Africa and other countries are working on against COVID-19, work,” explains Dr Nematswerani. “We hope that having the vaccine will trigger the production of neutralising antibodies. We can then use antibody tests to look for an antibody response at various points in time after vaccination. This tells us whether the vaccine gives us protective immunity and for how long.”

Antibody tests also play a role in research and in epidemiological (looking at disease in a population) as well as serosurveillance studies (measuring levels of immunity in a population).

The National Department of Health’s recommendations on using antibody tests (both laboratory-based and rapid diagnostic tests) are:

• To diagnose COVID-19 retrospectively in patients who have recovered from an illness that their doctor diagnosed as likely to be COVID-19 but who tested negative when they had their PCR test
• To diagnose COVID-19 in patients in who are admitted with a suspected COVID-19 infection but who test negative, such as children with suspected multisystem inflammatory syndrome who may test negative on a PCR test for COVID-19
• As part of scientific research studies and clinical trials

The National Department of Health also advises that:

• All the results of all antibody tests conducted must be recorded and reported to the National Health Laboratory Service.
• Rapid diagnostic tests must only be administered by suitably qualified and trained healthcare professionals.
• Laboratory-based antibody testing should only be conducted in accredited facilities.

If I have had COVID-19 and have antibodies to the disease, am I immune?

Antibodies linger in the blood after a virus is gone and may be present for a short or longer time. In the case of diseases like measles and smallpox, antibodies can stick around and provide lifelong immunity. When it comes to the common cold (which is caused by other, seasonal coronaviruses), we don’t acquire long-term immunity.

“We are still unsure how long the immunity lasts once people have developed antibodies against COVID-19. We also do not know the amount of antibodies required to sustain immunity and how long they stick around. Current research shows that antibodies to COVID-19 remain in the blood for at least three months. Ongoing studies will eventually reveal more on this,” says Dr Nematswerani. “The detection of antibodies may not correlate with immunity so any positive antibody tests can’t be regarded as proof of immunity or that we can reduce or abandon preventive measures for COVID-19.”

• Read the WHO’s guidance on antibodies and immunity to COVID-19.
• Read our National Department of Health’s guidelines on antibody testing.

“If a healthcare provider recommends that a member of Discovery Health Medical Scheme undergoes an antibody test for COVID-19, this will be funded from the patient’s available Medical Savings Account funds,” says Dr Nematswerani.

• Keep in mind: If doctors refer Discovery Health Medical Scheme members for PCR molecular testing for COVID-19, the scheme provides cover for COVID-19 from the World Health Organization Global Outbreak Benefit. This benefit complements existing Discovery Health Medical Scheme benefits and is available on all Discovery Health Medical Scheme health plans. In combination with the existing Discovery Health Medical Scheme plan benefits, members are covered for:
  • Screening by a healthcare professional
  • Diagnostic testing (PCR testing)
  • Consultations with healthcare professionals
  • Defined supportive treatment and medicine
    • Read more on PCR testing for COVID-19

You don’t need a PCR test for COVID-19 if:

• You have no symptoms of COVID-19.
• You have not seen your doctor and are just looking for reassurance that you do not have COVID-19. At all stages of the COVID-19 pandemic it’s very important that tests are available to those who really need them.

Always contact your doctor first to see if you qualify for testing and ensure that your doctor refers you to a pathology lab.

• Did you know that members of Discovery Health Medical Scheme can book a virtual consultation by logging in to the Discovery app and contacting one of the healthcare providers in the Discovery network?