NEW YORK (NYTIMES) - Before Ms Juhi Singh, 46, who owns a high-end wellness centre on New York City's Upper East Side, jetted off to the Amalfi Coast last month, she packed her bathing suits and left her 10-year-old son with his grandmother.

Her driver took her to Sollis Healthcare, a medical concierge service in Manhattan, to measure her antibodies for the coronavirus. She had received the Johnson & Johnson vaccine in February and wanted to see if her immunity was still robust before joining friends at a five-star resort overlooking the Tyrrhenian Sea.

"I wouldn't go on a trip without my antibodies," Ms Singh said. "It's nerve-racking, but my numbers have been good."

An e-mail arrived 24 hours later with her results: 14.8 arbitrary units per millilitre. Although medical experts warn that an antibody count cannot tell if somebody is protected against the virus, patients have been reading into the numbers anyway.

"Mine have dipped a little bit, but I know my vaccine is still working and I am still protected," Ms Singh said.

Antibody testing on a monthly or regular basis has become a common practice among certain members of the nervous affluent class.

"A lot of my patients and some of my friends are counting their antibodies," Ms Singh said. "It's the Upper East Side, the Hamptons circles. It's like dinner conversation at this point. It almost feels like counting calories."

Medical concierge services, including Sollis, have started offering antibody tests as a perk to clients. "I check them for people daily," said Dr Scott Braunstein, the medical director of Sollis' office in Los Angeles.

My Concierge MD, an upscale healthcare practice in Beverly Hills, California, has set up a drive-through service where clients, including celebrities and Hollywood executives, can be tested without getting out of their G-Wagons and Teslas.

"We do it with a finger prick," said Dr David Nazarian, who runs the practice. "Let's just say with the Delta variant, the test sites are busy right now."

But Professor Arthur Caplan, a bioethics expert at New York University's (NYU) Grossman School of Medicine, is concerned about the overuse of antibody tests.

"The doctors who are promoting them are not promoting good science," he said. "I think they are putting their patient at risk because there are no agreed-upon antibody levels."

Some people check their antibodies because they are immunocompromised or living with high-risk individuals. (Ms Singh is the caretaker for her 91-year-old grandmother.) Others do it for peace of mind before taking an international flight or attending a gala.

And there are those who just want to be armed with more information about their medical status, something that has become normalised in the era of health monitoring apps.

"Our patients are very analytical," said Dr Alan Viglione, who runs Montecito Concierge Medicine, a private healthcare provider in Montecito, California. "We have a lot of patients who want to know their numbers. It's become a new trend to know what your antibodies are."

Patients who get a low antibody count may decide to change their behaviours or "life choices", Dr Braunstein said. "They might decide to skip that wedding. They might take extra precautions."

Some may opt to get a booster shot. Although the United States' Food and Drug Administration (FDA) has authorised booster shots only for those who are immunocompromised, there is some evidence that more than a million Americans have already received unauthorised third doses.

One medical concierge service has even encouraged high-risk patients to check their antibody levels before getting a booster shot - something that no public health agency has recommended.

Getting an antibody test - also known as a serology test - is a relatively easy procedure. Blood, drawn through a finger prick or vein, is screened for antibody proteins created by the immune system to fight infection or after vaccination. Antibody tests do not check for the virus itself and cannot be used to diagnose whether someone has Covid-19.

"It's a simple blood test and we see the results the next day because there are a number of large labs across the country doing it," Dr Braunstein said. "It isn't overly expensive. Most insurances will cover it, but if they don't, it's about US$100 (S$135) to $200."

But the results offer limited information. Current tests look only for antibodies for Sars-CoV-2, the virus that causes Covid-19, and not for T cells, which play an important role in the body's immune response.

It is also not clear what the antibody count means. For starters, different tests measure the strength of the antibodies differently. A person tested by Sollis, for example, may get a count of 20 or higher, while a different laboratory may give a result of 270 or higher. A person without antibodies would get a negative result. However, a higher number may not necessarily confer great immunity.

"We don't have exact data on what a four means versus a 15," Dr Braunstein said. "You can't specifically say that you are a nine and I am an eight, so you are more protected than me."

Indeed, the FDA does not recommend that people use antibody tests to gauge immunity and, in fact, warns of its potential risk. People who receive a high antibody count may take fewer precautions, such as mask wearing, that could result in infection or spread. (Dr Braunstein said "all of our patients are informed of this recommendation and advised to follow all CDC recommended safety measures, regardless of the test result".)

"It can give you a false sense of security," said Prof Caplan, who also leads NYU's division of medical ethics. "They might say, 'I got my antibody test, so I am not going to wear a mask or I am going to that concert, because I know I'm immune' which, in fact, they don't know."

He is also concerned that people might use their antibody count as an excuse to skip a booster shot when the time comes. "Even if you have antibodies, it doesn't mean you have enough to fight off new variants," he said.

Still, antibody counting has become a practice among the wealthy who regard their health as a full-time undertaking, where no medical test is too insignificant and no medical resource too expensive.

"People read articles and get on the Internet and do research and want to do tests," Prof Caplan said. "But it is up to the doctor to filter that and calm me down, so I don't spend money on useless or harmful things."

Medical concierge services, he argues, often do the opposite - that is, cater to their patients' wishes. "The problem is when you are a concierge practice, you have to honour what they want because they are paying you money to do what they want."

The poor, he said, often avoid medical tests because they are scared they will lose their health insurance or job if they get a bad result. "If it turns out you have low antibodies, all of a sudden, you might think, 'I can't tell anybody about this, because my boss won't let me come into work'," Prof Caplan said. "The penalties for knowing more about bad health are not problems the rich face."

As fears mount, concierge doctors are offering other ways to gauge immunity. Sollis now offers a commercially available test for US$200 that checks for T cells.

"The test is much more difficult because there are only a few labs in the country doing this," Dr Braunstein said. "There is a seven-day turnaround with results, but we think it's worth it."

"I have had two or three clients ask for T cell tests," added Dr Viglione of Montecito. "Right now, only speciality labs will do it, but in a month or so, I think it will be much more common. It's trendy to have a lot of personal data."