Next time you’re a patient, ask whether your “health-care provider” is a doctor.

On Jan. 1, New York changed the standard for who can practice medicine, putting patients at risk. It became the 19th state to capitulate to aggressive lobbying by nursing groups to let some nurses play doctor without going to medical school.

In these states, nurse practitioners can do everything primary-care doctors do — diagnose, treat, prescribe and even open their own independent practices — once they’ve worked 20 months under a physician’s direction.

That is, “can” legally. That doesn’t mean they have the know-how. And therein lies the danger.

Don’t get me wrong: Nurses are the backbone of the health-care system, and generally they’re better than doctors at educating patients and providing many types of routine care.

But their training is different, and it doesn’t prepare them to do everything doctors do — especially diagnosing less common conditions.

Nurse practitioners are registered nurses who’ve earned an advanced degree. But they’ve never been to medical school; they have half the years of training a doctor gets (generally six years beyond high school, instead of 12), and they don’t take the same state licensing exam as doctors.

So you’ll be fine if you have a urinary-tract infection or a sprain. But don’t assume they have the in-depth knowledge to diagnose an uncommon illness or handle a complex problem.

Indeed, Health Maintenance Organizations cooked up the term “health-care provider” to blur the differences between physicians and less expensive caregivers.

Dr. Sandeep Jauhar, a cardiologist at Long Island Jewish Medical Center, criticized New York’s law when it was enacted last April.

Medical students with “two years of clinical training are not considered fit to practice medicine,” he warned, but in the Empire State “nurse practitioners with perhaps even less clinical education will be allowed to do so.”

Lower cost is why insurers, drugstore chains with walk-in clinics and federal health programs applaud replacing doctors with nurse practitioners. It’s also why the Veterans Administration is considering cutting its backlog by sending vets to nurse practitioners.

But they don’t save money in the long run, Dr. Jauhar suggested, because they tend to order more CAT scans and MRIs than physicians treating similar patients — probably as a crutch to try to get to the right diagnosis.

A 2013 analysis in the New England Journal of Medicine shows why physicians oppose the change, and it isn’t to keep business for themselves. (With the physician shortage, that’s not an issue.)

Doctors know that nurse practitioners get less education in how organs and bodily systems work; they’re trained to treat symptoms.

One doctor I spoke with recalled a patient with apparent signs of adult-onset diabetes. A nurse practitioner, he said, would’ve prescribed medicine to produce insulin.

But the patient mentioned having had a gallstone attack. Connecting the two events, the doctor realized that a gallstone had become lodged in her pancreatic duct, “burning out” her pancreas and keeping her from producing insulin. No insulin medication would undo that.

That detective work, he said, drew on what is taught in medical schools.

Dr. Jane Fitch, president of the American Society of Anesthesiologists, began as a nurse anesthetist but later earned a medical degree. Speaking out against nurses practicing independently, she looks back on being a nurse and says, “I didn’t know what I didn’t know.”

Nursing organizations suffer from that over-confidence.

These groups point to studies purporting to show that patients do as well with a nurse practitioner as with a primary-care doctor. But most of these studies are sponsored by nursing outfits or lack scientific rigor.

Take the oft-touted study by lead author Mary Mundinger: It only lasted six months, so most patients saw their “provider” only once, and there’s no way to tell who fared better in the longer term.

“Far from convincing,” a Journal of the American Medical Association editorial termed that study. Longer-term studies are needed.

One good outcome of last December’s federal budget deal is that doctors’ groups convinced Congress to delay the VA’s plan to substitute nurse practitioners for primary-care doctors until the risks could be assessed. If only New York lawmakers had shown the same concern for patient safety.

Betsy McCaughey is a senior fellow at the London Center for Policy Research.