COVID-19 antibody testing: Proceed with caution, plus other things you need to know - AZCentral

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Stephanie Innes Arizona Republic

Published 11:55 PM EDT May 6, 2020

Anyone who is wondering whether that February cold and cough might have been COVID-19 is not alone.

Curiosity about how many of us have already had COVID-19 is fueling interest in antibody testing. It's understandable: a test that would in theory tell someone whether or not they've developed immunity to COVID-19 has a lot of appeal.

In reality, many unknowns remain about antibody testing.

The unknowns are important for the public to understand, experts say, because antibody testing has generated a lot of buzz. 

The buzz in part comes from "bad actors" claiming to do antibody testing. This week, the U.S. Food and Drug Administration responded by tightening its policies on the testing.

As of Wednesday, nearly 12,000 antibody tests had been completed in Arizona, according to the Arizona Department of Health Services, which has started to report antibody testing statistics on its COVID-19 dashboard.

So far, not many people are testing positive for the antibodies that could confer immunity to COVID-19 — just 3.6%, the latest state data shows.

The number of antibody tests is expected to increase significantly in Arizona over the next few weeks, because availability of the tests is rising.

Tempe-based Sonora Quest Laboratories launched antibody testing April 24 and as of this week is offering antibody testing for COVID-19 without a doctor's order.

The University of Arizona has begun an initiative, with $3.5 million in state funding, to conduct antibody testing on 250,000 health care workers and front-line workers throughout Arizona.

Here are seven important facts about antibody testing that Arizonans need to know.

Not all antibody tests are alike

Some COVID-19 antibody tests on the market require a blood draw, others a finger prick, and the different tests don't measure the same antibodies.

New tests are hitting the market at a rapid pace and not all are approved by the FDA.

The test Sonora Quest is using has been authorized by the FDA under Emergency Use Authorization for use by authorized laboratories, which includes Sonora Quest, company officials said.

The UA says it has filed an emergency use authorization request with the FDA for its test, and is authorized to begin testing now.

"We anticipate formal FDA approval in the coming weeks," the UA's COVID-19 antibody testing website says.

Speaking May 5 on NBC's Today show, UA president Dr. Robert Robbins called the test being used by his university, the "most accurate antibody test in the country. ... We know it's very, very accurate."

The UA Health Sciences test is what's called a "total antibody test." It is an adaptation of the Mt. Sinai Hospital of New York test and was developed in the laboratories of UA scientists Deepta Bhattacharya and Dr. Janko Nikolich-Zugich. 

Separating these antibody classes into different tests is of little clinical value, UA spokesman Mark Lane wrote in an email to The Arizona Republic. "There are multiple classes of antibodies (IgM, IgG, IgA), all of which can provide protection. Quantifying total antibodies is the best way to estimate the level of protection."

It usually takes at least 10 days after symptom onset for antibody response to reach levels that are detectable on a test, UA officials say.

Sonora Quest Laboratories is using an antibody test developed by EuroImmun and it measures immunoglobulin G (IgG), said Brian Mochon Scientific medical director for Sonora Quest's infectious disease division.

Measuring IgG is more indicative of telling people they have been definitively exposed and that they may have protective immunity, Mochon said.

"Early on, there were a lot of these rapid antibody tests that are in my opinion, not as well-studied. The false positive rates were a lot higher and that's what we saw a lot in the media," Mochon said.

"Is this test the perfect test at this point? No. But I think it's as good as we're going to be at this very moment."

Antibody tests can't diagnose

Antibody tests are often referred to as serology tests. They test for antibodies, which are proteins that help fight off infections and can show whether someone likely has been previously infected with COVID-19.

The COVID-19 diagnostic tests are completed with a nasopharyngeal swab, and are often referred to as polymerase chain reaction or PCR tests. The PCR test looks for infection with new coronavirus, or SARS-CoV-2, the virus strain that causes COVID-19 disease.

The PCR test can give someone a diagnosis that they are positive or negative for COVID-19 infection. The serology or antibody test can tell someone whether they have developed antibodies against COVID-19.

Results of antibody testing are positive, negative or indeterminate.

A positive result may suggest an immune response to COVID-19 infection, but the relationship between positive antibodies and immunity to COVID-19 has not yet been firmly established, UA spokesman Mark Lane wrote in an email.

"Negative or indeterminate results do not rule out SARS-CoV-2 exposure," Lane wrote.

Most people are testing negative and you could get a false positive

One of the problems with the tests is that they could have a positive result from detecting one of the more common forms of coronavirus that causes the common cold and not the new coronavirus that causes COVID-19.

False positive rates vary for tests that are on the market.

The Sonora Quest antibody test has a 98.5% specificity, meaning for every 200 patients, three will get a false positive, Mochon said.

That may not sound like a lot but it's an important statistic to consider when using the test results to decide whether to return to school or work.

Mochon of Sonora Quest noted that the vast majority of people are testing negative.

And a lot of people who never got tested likely are curious about whether an illness they had recently could have been COVID-19. Now that a doctor's order isn't needed, Mochon expects to see more of those people seeking an antibody test.

"More likely it was just influenza. I could spend five minutes just listing all the respiratory viruses that are out there," he said. "There are lots of viruses that we see as common colds or deeper respiratory infections. ... From a scientist's perspective, I'll be interested to see the prevalence of positivity."

It's unclear how much protection antibodies provide, so you can never assume that you are immune

Experts do not yet know the amounts of antibodies that are required to fully prevent subsequent infections, but expect there might be some level of protection.

Someone with a positive result should never assume they are immune, Mochon said.

"We don't have enough information to say that definitively at this time," he said. "These results should not change our social distancing behavior."

No one knows how long any immunity to COVID-19 might last

"One of the hallmarks of the human's response to any pathogen is to develop antibodies to that particular pathogen," said Dr. David Engelthaler, co-director of the Pathogen and Microbiome Division of the Translational Genomics Research Institute in Arizona.

"We develop different types of antibodies. We develop some that work really quick and that helps with the initial state of the infection and helps to clear out the pathogen. Then there's also another type of antibody that usually starts to produce a bit later and those antibodies typically last a long time and provide what we've all been talking about, which is immunity."

But scientists still don't know a lot about the new coronavirus and the immune response, Engelthaler said.

"We are learning everyday," he said. "There's a lot of antibody testing happening right now. We don't truly understand what it means for protection and whether or not people are safe from being reinfected yet."

Most studies indicate that people who have recovered from COVID-19 infection have antibodies against the virus.

Still, an April 24 scientific brief from the World Health Organization cautions that "there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection."

You may be required to get an antibody test prior to elective surgery

Mochon of Sonora Quest said most of the people getting serology tests are health care workers and people who fit into the "essential worker" category, as well as people who think they've been exposed and people who think they had COVID-19 but were never tested.

Sonora Quest is seeing an increasing number of patients whose antibody tests were ordered by their doctors prior to elective surgery, along with a COVID-19 test.

Many hospital systems and health providers in Arizona are now requiring a COVID-19 diagnostic (PCR) test prior to elective surgery. Mochon said many health providers are asking for both the PCR and the serology tests for those patients.

Even with the unknowns, there is an upside to antibody testing

Antibody testing helps with what's needed most during the COVID-19 pandemic — research, including population data.

Such testing could be important for determining the reach of the pandemic and for developing treatments for COVID-19 patients, as well developing vaccines to guard against infection.

The testing possibly could be used to help determine who can donate what's known as "convalescent" plasma from people who have recovered from COVID-19 as a treatment for people who are seriously ill with the disease. However, the tests are not able to determine how effective someone might be as a donor.

The FDA's recommendation to health providers is to continue using antibody tests as appropriate, but be aware of their limitations.

"I think it is a step in the right direction. I think it is important from an economic perspective, to feel like we are moving in the right direction and it is most likely that these antibodies that we're testing are associated with immunity," Mochon said.

Reach health care reporter Stephanie Innes at Stephanie.Innes@gannett.com or at 602-444-8369. Follow her on Twitter @stephanieinnes

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