Expectant parents have gotten an ugly exposure to a rapacious aspect of modern medicine: Over testing, over diagnosis, and over treatment, specifically with a new, fast-growing high-tech twist.

The grownups — whether over-reaching to safeguard the unborn or in a simply silly way to determine the gender of their hoped-for bundle of joy — are ordering unnecessary, expensive, and too often alarming prenatal genetic blood tests. These rapid exams purport to tell whether a fetus may have the rarest of congenital diseases, the New York Times reported in some admirable digging, triggered by a stack of patients’ surprise medical bills.

Reporters Sarah Kliff and Aatish Bhatia found a big problem with the high-tech prenatal screens: The tests too often are dead flat wrong.

Although hyped by their Silicon Valley makers as being capable of detecting missing snippets of chromosomes (so-called microdeletions) and forecasting that this will results in dire consequences — babies born with rare syndromes affecting their development, cognition, and leaving them with significant disabilities — the tests may produce false positives or results that experts have found are erroneous way too often, the newspaper reported:

“To evaluate the newer tests, The Times interviewed researchers and then combined data from multiple studies to produce the best estimates available of how well the five most common microdeletion tests perform. The analysis showed that positive results on those tests are incorrect about 85% of the time. Experts say there is no single threshold for how often a test needs to get positive results right to be worth offering. They note that when the tests do accurately identify an abnormality, it can give expectant parents time to learn about and prepare for challenges to come. Some said one common microdeletion screening, for a condition called DiGeorge syndrome, has the most potential to do good.

“But there are hundreds of microdeletion syndromes, and the most expansive tests look for between five and seven, meaning women shouldn’t take a negative result as proof their baby doesn’t have a genetic disorder. For patients who are especially worried, obstetricians who study these screenings currently recommend other types of testing, which come with a small risk of miscarriage but are more reliable. Some said the blood screenings that look for the rarest conditions are good for little more than bolstering testing companies’ bottom lines …”

Any screening test for rare conditions will generate lots of false-positive results. That is a well known statistical phenomenon stemming from Bayes theorem. Bayes was an 18th-century British minister whose theorem was published after he died. It’s best stated by way of example. Let’s say a test has what looks like a very high rate of accuracy: 99% of the time, a positive result on the test will be correct, and only 1% of the time will it be a false positive. The Rev. Bayes realized that this was only the starting place and that the real accuracy of the test would depend on how common the condition being tested for was.

So, if we have a rare genetic condition that afflicts only one in 100,000 people, what happens when we run the test 100,000 times? For the one person who has the condition, the 99% accuracy rate tells us that that person’s test will come out positive 99 times out of 100.

But what about the other 99,999 people who were tested? One percent of those will also test positive according to our hypothetical scenario of a test that has true positives 99% of the time and false positives only 1% of the time. One percent of 99,999 is 999 false positives. That means that in our group of 100,000 people tested, there will be 1,000 positive test results, and 999 of them will be wrong, even with a test that on its face is right 99% of the time! Medical scientists have known this statistical law for a long time, and that is why any test for a rare condition requires confirmation with more specific testing even after the screening test comes back positive.

To be sure, doctors told the newspaper that the microdeletion testing has become a useful tool to detect Down syndrome. But the tests’ makers have promoted them aggressively, notably by contending they have broader capacity and urging on parents prenatal tests for diseases so rare that a researcher at a National Institutes of Health lab said she had not seen case in her decades career.

The companies, of course, are reaping a windfall from expanded testing, a market that one leading maker estimates could add hundreds of millions of dollars to its bottom line annually.

As is the case, critics say, with the huge problem of medical over testing, over diagnosis, and over treatment, patients suffer. If they heed the tiny print in the microdeletion test materials, they could learn that they should not regard the quick blood test as definitive. Instead, they should consult with their doctors, with the likelihood that further, costly, and invasive procedures may be required to determine whether the blood tests generated a false positive or other inaccurate information.

Here is a grim part of the newspaper report: Because some women take the blood test early and they said in interviews that they did not discuss the results disclosed by the company to them with their doctors in detail, they decided, instead, to terminate their pregnancies. Only later have they learned about the testing shortfalls.

In my practice, I see not only the harms that patients suffer while seeking medical services, but also the damage that can be inflicted on them and their loved ones by defensive and excessive over testing, over diagnosis, misdiagnosis, and over-treatment. Patients struggle too much already to afford needed medical services much less those that are invasive, painful, wasteful, and unnecessary.

Up to a third of medical spending goes for over-treatment and over-testing, with an estimated $200 billion in the U.S. expended on medical services with little benefit to patients. But getting doctors and hospitals to stop this excess isn’t easy, nor is it a snap to get patients to understand what this problem’s all about so they’ll push their health care providers to do something about it.

The ABIM Foundation, created by the American Board of Internal Medicine and dedicated to the improvement of the practice of medicine, and Consumer Reports years ago launched a great initiative called “Choosing Wisely.” The two organizations worked closely with an array of colleagues and medical specialists, as well as medical educators, to raise awareness about and to campaign against patient-harming over treatment and over testing, including by providing detailed information about procedures to avoid.

With medical tests, like the dubious rare-disease prenatal blood screen, serious questions must be asked about the dearth of federal oversight. It has been opposed by those who argue that patients benefit best by government staying out of the way of innovation. As the New York Times reported, the federal Food and Drug Administration simply decided against exercising its authority over these medical items:

“There are few restrictions on what test makers can offer. The Food and Drug Administration often requires evaluations of how frequently other consequential medical tests are right and whether shortfalls are clearly explained to patients and doctors. But the FDA does not regulate this type of test. Alberto Gutierrez, the former director of the FDA office that oversees many medical tests, reviewed marketing materials from three testing companies and described them as ‘problematic’ [noting] ‘I think the information they provide is misleading…’”

Caveat emptor, consumers. Patients must insist that their doctors explain fully any test or procedure. This is patients’ fundamental right to informed consent. This means they are told clearly and fully all the important facts they need to make an intelligent decision about what treatments to have, where to get them, and from whom. Patients, in turn, must put in the effort to safeguard themselves by improving their knowledge and comprehension about their care — a constant learning process that is crucial for those who wish to bring new lives into the world and to nurture them for decades after.

We have much work to do to rein in costly, wasteful, and harmful testing, especially as patients may be victimized by the hype at which Silicon Valley has become so expert. We’ve learned from nightmarish experiences with high-tech forays into information services like social media, where critics argue with considerable justification that gee-whiz, get-rich-quick entrepreneurs have imperiled our very democracy with their ethos of “move fast and break things.” Did regulators and Congress also not get a powerful rebuke about the willingness of Valley hucksters to play the system for every penny they could squeeze from it with the debacle of the sham blood testing company called Theranos — and its just convicted fraudster guru Elizabeth Holmes?