Coronavirus antibody testing is now easy to get. But it’s hard to be sure what you’re getting.

Suddenly, getting a test to see if you’ve been exposed to the coronavirus and have antibodies to it is almost as easy as getting a flu test.

Vybe urgent care centers are the latest to jump on the immune-response testing bandwagon. All you need is a telemedicine consultation, a blood draw at one of the nine centers in Philadelphia or Delaware County, and several days of patience for results to come back.

But there are lots of caveats. The length of antibody protection — assuming there is some — is unknown. Scores of antibody tests, some totally unreliable, have been allowed on the market by regulators. And depending where and how you access an antibody test, you may be charged about $100, despite a new federal law designed to make coronavirus testing available with no cost-sharing.

Vybe CEO Peter Hotz readily acknowledged the uncertainties, and Vybe’s website explains some of them. “But we believe there’s a public health value in doing this to get at answers to some of those questions,” he said Friday.

To help you decide whether testing makes sense, let’s unpack the caveats.

Up to now, COVID-19, the disease caused by coronavirus, has been diagnosed by complex molecular tests that look for the virus’ genetic material in nasal swab samples.

On Saturday, the U.S. Food and Drug Administration authorized the first “antigen” test for rapid early diagnosis of COVID-19. Antigens are produced by the virus early in an infection and prompt an immune response. The test can detect the antigens in samples collected from swabs swiped inside the nasal cavity, the FDA said in a statement.

Antibodies are a key part of the immune response triggered by viral antigens. Antibody tests look for three types of types of immune proteins — abbreviated IgA, IgM, and IgG — that are produced more than a week after a coronavirus infection sets in. Experts believe that even people who have mild symptoms, or none, get some level of immune protection. Judging from coronaviruses that cause common colds, the protection could last a year or so.

The hopes are that widespread antibody testing could track the scope and spread of the epidemic, identify people who could safely go back to work, and help recruit people to donate their antibody-laden blood to treat severe COVID-19 cases. Although the research to support these hopes has barely begun, testing is proliferating because time is of the essence.

“We’re not telling people, ‘If your antibody test is positive, go out and mingle,’ ” said Hotz, who had a positive diagnostic test and is waiting for his antibody test results. “All of us have to continue to suppress this virus [with social distancing] because there’s a lot we don’t know.”

In mid-March, the FDA weighed the urgency of the coronavirus crisis against the stakes of imperfect antibody testing and decided to let companies market their tests without even a cursory review. That led to a flood of unreliable and fraudulent tests, which forced the FDA to backpedal.

The agency announced Monday that companies would have to submit their validation data within 10 days and earn the agency’s emergency authorization. Leading diagnostic companies — including Cellex, Ortho Clinical, and Autobio Diagnostics — had already gotten emergency authorizations.

But just because a test has been validated in a lab — using infected blood samples and uninfected control samples — doesn’t mean it will be reliable in real-world, clinical use. Test results can be thrown off by antibodies from common cold-causing coronaviruses (a problem called cross reactivity), or because the levels of IgA and IgM have fallen off. Some big companies, notably Abbott Laboratories and Roche Diagnostics, have opted to test only for IgG, the antibody that is produced later in infection and stays robust longer.

A Bloomberg News reporter who had symptoms of COVID-19 but no diagnostic test decided months later to find out if he had antibodies. The contradictory results — two positive tests, two negative tests — left him in doubt.

Another problem: Even tests that are near perfect in the lab are not very good if the prevalence of infection in the community is low, meaning only a small fraction of people — 1% to 5% — has been exposed to the virus. Most experts estimate that’s the range of coronavirus prevalence, except in hard-hit places. (A prevalence study in New York City recently found that a quarter of residents have antibodies to the virus.)

Why does prevalence matter? Because if few people have what a test is looking for to rule in or rule out a condition, then the chances for error are magnified. Based on the lab accuracy of a test and the prevalence of the condition, biostatisticians can calculate the probability that real-world results will be wrong, either by telling people who are negative that they’re positive (the “positive predictive value”), or telling people who are positive that they’re negative (the “negative predictive value").

AllCare, a chain of urgent care centers in Maryland and Virginia, took the unusual step of explaining these confusing statistical concepts and providing a comparative review of coronavirus antibody tests being sold by big manufacturers.

“Positive predictive value for the same exact test varies from virtually useless/dangerous to very helpful, depending on how many people have had the virus in the population being tested," the AllCare coronavirus webpage explains.

When the pandemic hit, the government wanted to make sure that people with insurance, as well as people who are poor, uninsured, or without primary care doctors, could get diagnostic and antibody testing with no out-of-pocket costs.

Measures designed to ensure that are part of a new federal law, the Families First Coronavirus Response Act, and part of rule changes made by the Centers for Medicare and Medicaid (CMS).

While supply shortages have limited coronavirus diagnostic testing, antibody testing is increasingly accessible. A big reason is that LabCorp and Quest Diagnostics — which each have thousands of storefront service centers across the United States — are fast becoming dominant players in this niche. LabCorp says it is using the Abbott and Roche antibody test platforms. Quest says it’s using Abbott and Euroimmun test platforms.

Vybe’s urgent care centers are among the customers.

"We’re working with LabCorp and a little bit with Quest,” Hotz said.

But consumers may still wind up being charged for antibody testing, either because they choose to — or don’t realize that free testing is mandated.

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Quest, for one, is marketing its test directly to consumers for $119 plus a $10.30 “service fee.” Sonora, a Quest partner in Arizona, is doing the same thing for $99. The consumer goes online, requests the test, then a Quest-contracted doctor orders it after making sure it’s appropriate. (Antibody testing can’t be done until at least 10 days after COVID-19 symptoms go away.)

Quest is touting this as “consumer-initiated.”

“We understand that individuals can have COVID-19 testing fully paid by their health plan or the government,” Quest spokesperson Kimberly Gorode emailed. “For some paying out of pocket, it may come down to convenience and privacy. Consumer-initiated testing ... provides convenient, online access to quality, physician-guided testing paid for by the consumer.”

Does charging consumers for testing skirt federal rules? An FDA media relations person said: Ask CMS and the Department of Health and Human Services. Media relations persons at CMS and HHS repeatedly emailed that they were “working on" getting answers.

After almost two weeks, they still had not succeeded.