The truth about antibody testing

In the early weeks of the COVID-19 pandemic, people were desperate to better understand the disease and slow its spread. Many believed that if enough people had the illness, developed antibodies to fight it and were immune to catching it again, we could more quickly return to a sense of normalcy in our daily lives. In response, there were calls for widespread antibody testing.

Antibody testing — also known as serology testing — screens for past infection. The test determines the presence of antibodies, which are proteins that fight off infections and can often provide immunity against catching the same infection again.

However, according to Dr. Omid Bakhtar, a board-certified pathologist affiliated with Sharp, the early flux of commercial, quick-to-market COVID-19 antibody tests were highly error prone. And even after reputable manufacturers took the time to develop reliable antibody tests, it became clear that widespread antibody testing is not the silver bullet we had hoped for.

Here, Dr. Bakhtar answers questions about antibody tests and explains what information they can — and cannot — provide:

What do antibody test results mean?
First, it’s important to understand that COVID-19 antibody tests do not screen for active infection, so they cannot be used to diagnose a current illness. Furthermore, results are affected by the commonality of a disease within a chosen population. If a disease is common, the test results will be more reliable. However, the fewer people who have a disease such as COVID-19 in a community, the more likely there will be a high rate of false positives — a person is negative, but tests positive — and false negatives — a person is positive, but tests negative.

And so, while the COVID-19 antibody tests now being used have a high reliability rating, they can still provide inaccurate results — especially false positives — due to the low percentage of people in the population who have COVID-19, which is currently estimated to be less than 2% in San Diego County. In addition, recent data shows that COVID antibodies don't always last in people’s bodies. Some who test positive on an antibody test may later test negative.

• Negative test result
  If you receive a negative result on your COVID-19 antibody test, you can feel fairly confident that your result is correct and you have not had and recovered from COVID-19. However, it’s important to remember that you could have recently been exposed to the virus and have an active infection, which will not be detected by an antibody test.

• Positive test result
  If you receive a positive result, the test has detected the presence of antibodies, which develop one to three weeks after an infection. However, due to testing inaccuracies, there is a chance that you have received a false-positive test result and have not ever been infected. You cannot safely assume that you have had COVID-19.

If someone receives a positive antibody test result – and knows the results are correct because they were formerly diagnosed with COVID-19 – are they now immune from catching it again?
Immunity is determined by whether “neutralizing” antibodies are present in your body. Neutralizing antibodies find the virus, prevent it from infecting your cells and inhibit its ability to cause illness. The current tests on the market make no claims regarding their ability to identify neutralizing antibodies.

Your immune system may have launched a response to the presence of the virus in your body, but the testing cannot guarantee that the response included neutralizing antibodies. Therefore, such tests cannot determine immunity. What’s more, we still have much to learn about COVID-19 and cannot be certain that people who have recovered from COVID-19 are immune from becoming infected again.

Regardless of whether you receive a positive or negative result, you must continue to take precautions to protect yourself and those around you. This includes practicing social distancing, wearing a mask outside the home, washing your hands often and self-isolating if you become exposed to someone with COVID-19 or are experiencing symptoms.

If antibody tests cannot diagnose COVID-19 or determine if someone has immunity to COVID-19, what role do they play?
Antibody testing may be performed when people who have fully recovered from a diagnosed case of COVID-19 wish to donate convalescent plasma, which can be used to treat others with serious or life-threatening COVID-19.

Sharp is also providing antibody testing to a small number of patients with specific criteria. This includes young patients being ruled out for multisystem inflammatory syndrome-children (MIS-C), a rare, life-threatening condition that follows COVID-19 infection. People who have had two prior negative viral COVID-19 tests, but are considered highly likely to have had the disease, may also benefit from antibody testing.

In general, COVID-19 antibody testing is best used by epidemiologists, who study how a disease spreads and the risk of illness or death in certain populations. The testing can only tell us an approximate number of how many people have been infected by the virus, not who is immune or unable to spread the disease.

Talk to your doctor if you have been exposed to COVID-19 or are exhibiting symptoms — fever, cough, shortness of breath, chills, muscle pain, new loss of taste or smell, vomiting, diarrhea or sore throat — to determine if you should be tested for COVID-19. A viral test, not antibody test, will be used to screen for active infection. However, not everyone needs to be tested. Most people will have mild illness and recover at home without the need for testing or medical care.