Why are COVID-19 tests not always accurate? | #coronavirus | #kids. | #children | #schools


The test was positive.

And then just days later, it was negative.

For one of Gov. Phil Murphy’s top aides, the quick reversal did not keep Dan Bryan, senior advisor for strategic communications, out of quarantine — even with two successive tests showing no sign of COVID-19. But it underscored a problem of coronavirus testing, especially in those without any symptoms of the highly contagious virus.

The tests sometimes provide conflicting results.

Bryan’s initial diagnosis came from Abbott Lab’s so-called rapid response BinaxNOW antigen test that detects the presence of proteins from SARS-CoV-2, the virus that causes COVID-19. He later took a series of follow-up molecular tests that instead looks for molecular particles of the virus. Both turned out to be negative.

The governor has repeatedly tested negative for the virus.

No one is classifying Bryan’s initial testing as a “false positive,” since it might have been possible that his antigen test picked up traces of a viral infection that had come and gone without notice. But the issue shows how difficult it can be for health officials to accurately track the coronavirus infections. At the same time, experts say testing those without symptoms, or not yet infectious, can sometimes be a shot in the dark.

How is COVID-19 detected?

There are basically two major tests in play to fight the pandemic, each looking for the virus in different ways.

The most reliable test for COVID-19 utilizes what is known as polymerase chain reaction, or PCR, to detect the nucleic acid of the virus. Currently the basis for most coronavirus testing, PCR amplifies tiny bits of genetic material to spot even traces of the virus. It is extremely sensitive because it can see much fewer copies of the nucleic acid of the virus, explained Maria Gennaro, a professor of medicine at the Rutgers New Jersey Medical School.

But while highly accurate, the tests require lab analysis and often takes days or more to get results.

A faster and cheaper way to look at the virus are antigen tests, as was used by the governor’s office. Antigen tests do not require a lab and can be done anywhere. However, the proteins targeted by antigen tests may not be present in sufficient amounts to detect in individuals without symptoms and it is not known how many viral particles it takes to make someone infectious.

Which test is more accurate?

Antigen tests, while offering a quick turnaround, are known to give “false-negatives.” That means people who may have the virus present in low levels may test negative for COVID-19, giving them an unwarranted sense of security as they spread the virus to others around them.

Or they could give false-positive results, as when Ohio Gov. Mike DeWine, who had no symptoms, tested positive for COVID-19 with an antigen test earlier this year. Hours later, the Republican governor got a PCR test and was found to be free of the coronavirus.

PCR tests are considered far more reliable. Still, they are not infallible, and can be affected by contamination or mishandling. Earlier this month, Alabama football coach Nick Saban announced that he had tested positive for the corona virus and entered isolation at his home in Tuscaloosa, Ala. A school official said the result came from a polymerase chain reaction test.

But in subsequent follow-up screenings, Saban, who was asymptomatic, tested negative for the virus three times in the days following his initial positive result and was ultimately cleared to return to the sidelines to lead the Crimson Tide a 41-24 win over Georgia.

Why use rapid response tests if they are not as accurate?

Point-of-care antigen tests, such as Abbott’s BinaxNOW system, offers a rapid turnaround time and easy access, do not require a lab, and can be done anywhere. That is an advantage when you have to screen a lot of people, such as visitors to the White House, families seeing their loved ones in nursing homes, or even players on professional and college sports teams.

But people with the virus present in low levels may actually test falsely negative for COVID-19 on antigen screening, or conversely can give a false positive result. Experts say for those who are asymptomatic, antigen testing may be no better than a coin toss. Until recently, New Jersey officials were reluctant to allow the use of antigen tests in nursing homes.

Proponents of the tests, though, say repeated use of antigen tests may be better for overall infection control compared to less frequent, highly sensitive tests, because they do not carry the prolonged turnaround times of PCR screenings.

However, as was seen at the White House, antigen screening can sometimes completely miss the mark. The Trump administration had been using another rapid screening tool from Abbott known as ID NOW — a rapid PCR test —to check out staff members and visitors who came into close contact with the president. At the end of the summer, though, the White House switched to the BinaxNOW antigen tests, which was not meant to check for the virus in asymptomatic individuals.

Both the president and the first lady subsequently were found to have been stricken by the virus.

In a statement, Abbott Labs said more than 11 million Americans have taken the ID NOW test, helping to stop the spread of the virus, which said has demonstrates performance of 95% sensitivity and 98% specificity in people within 7 days of post-symptom onset.

What about antibody tests?

Antibody tests do not look for traces of the virus, but rather for antibodies. It is a blood test that looks for the body’s response to SARS-CoV-2. Its purpose is to determine if someone had the disease, but is not so good for determining if an individual currently is infected.

Experts say antibody tests should not be used to diagnose the virus.

When should you be tested?

The New Jersey Department of Health said those experiencing symptoms — including fever, cough, shortness of breath, chills, sore throat, muscle pain, shivering, headache, or new loss of taste or smell — should seek testing.

It also recommends tests for those who have been in close contact with someone who tested positive for COVID-19, were recently in a large crowd where social distancing was hard to maintain, or recently travelled to an area or a state with high COVID-19 infection rates.

For most people who do not have symptoms but believe they may have been exposed, the department said the PCR test is the better choice because it can find small amounts of the virus that might be seen before symptoms start.

Our journalism needs your support. Please subscribe today to NJ.com.

Ted Sherman may be reached at tsherman@njadvancemedia.com. Follow him on Twitter @TedShermanSL.





Source link