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I have an extra vein into my heart that, in most people, disappears during embryonic development. My eyeballs are shaped more like lemons than like baseballs. And I once woke up after a foot surgery to hear the doctor say, “I’ve never seen anything like that before.”

I’m not the only one who is loaded with physiological quirks. A survey of friends revealed tonsils that kept growing back after tonsillectomies; a missing kidney that was discovered missing only after age 40; and blood that tests positive for syphilis, even though there is no syphilis.

I heard about extra toes, fingers and nipples. One friend never grew wisdom teeth. Another grew seven of them.

It can be unsettling to get unusual health or anatomical news. But it’s a common experience, says internist Ashish Jha, director of the Harvard Global Health Institute at the Harvard T.H. Chan School of Public Health. He remembers learning in medical school that everyone has an average of three to five birth “defects.”

These abnormal findings are often medically irrelevant, even as they may provoke anxiety, along with a slew of follow-up tests and procedures.

“We are all full of abnormalities because we are people, and human beings are incredibly variable,” Jha says. “None of us is normal.”

Normal is a squishy concept in medicine. For some routine tests, including blood counts, standard ranges were determined decades ago based on studies of soldiers, Jha says. But it has become increasingly clear that a measurement that is normal for a fit 20-year-old man will not necessarily be normal for a 60-year-old woman.

And even though reference ranges for some tests now vary by gender and other categories, those scales are rarely specific enough to correlate to a person’s exact age, race and other circumstances. Different labs, meanwhile, can set their normal range at different levels.

The way normal is defined inevitably leaves some people out, adds Danielle Ofri, an internist at Bellevue Hospital and the New York University School of Medicine, and author of “What Patients Say, What Doctors Hear.”

Ranges are generally set to correspond with 95 percent of healthy people in a large study group. But some people will always be on the margins. And sometimes bodies can acclimate to unusual internal circumstances. Ofri had one patient whose iron levels were close to zero — a condition that could cause heart attack or death in some people but in this case caused no harm and no symptoms.

Some tests are more informative or meaningful than others. Blood levels of potassium and calcium, for example, have narrow ranges of normal, and falling outside those ranges can be catastrophic. Levels of chloride and iron, on the other hand, can vary more widely without as many consequences.

Doctors are used to seeing abnormal results, and they generally look at the context to determine whether there is something to be concerned about. But online patient portals that allow people to see results before talking to their doctors may end up scaring people, even if atypical results are clinically irrelevant.

One problem, Ofri says, is a tendency in our society to view medical tests as infallible. In reality, technologies are imperfect, as are the people who read results and images. False positives and inaccurate results are always a risk. “Lab standards are not the perfect black and white that patients and society expect,” she says. “You have to draw the line somewhere. But very few things are yes or no.”

Increasingly sensitive imaging tools can also cause trouble. Many people learn about their own medical anomalies when they undergo testing for something unrelated. These findings, dubbed incidentalomas, show up in CT scans and MRI images as much as 20 percent of the time, Jha says.

Some types of accidental findings are particularly common. Liver lesions, for example, appear in 10 to 14 percent of CT scans. According to other research, asymptomatic and unexpected thyroid tumors show up in 67 percent of people who get ultrasound imaging and in 15 percent of people who get CTs or MRIs of the neck. Pituitary incidentalomas get picked up in as many as 38 percent of people who get head and neck MRIs.

Findings like these can be a conundrum in the clinic, Jha says. When patients — and often also their doctors — learn that something might be wrong, they want to follow up. But in many cases, there’s no evidence that intervention will do any good, while extra procedures and tests carry risks of their own, including anxiety and exposure to radiation.

“It’s hard for people not to worry once they start thinking, ‘What does it mean?’ ” says Jha, adding that symptoms are often better than test results as indicators of illness. “Literally, it means that there is a lot of human variation, and most of it is not clinically meaningful at all.”

Occasionally, the definition of normal changes to accommodate new evidence. Imaging for low back pain is one notorious example, as studies now show that bulging disks and other abnormalities are extremely common in people who have no back problems at all. In one study from the mid-1990s, researchers took MRI scans of 98 adults with no back pain and found that 64 percent had some kind of disk abnormality. Thirty-eight percent had more than one abnormality.

Since then, research has built a convincing case that people who undergo imaging for back pain don’t end up with better pain management or faster healing rates. Instead, according to one analysis, CT scans for back pain cause as many as 1,200 extra cases of cancer from radiation exposure.

Healing might even be slower when patients find out about abnormal imaging results, even if those results are clinically irrelevant. In the case of back pain, thinking something is wrong can make people worry more, fixate on the pain and avoid exercise for fear of making the problem worse. Physician guidelines now recommend imaging only in certain cases, though doctors continue to order these tests at increasing rates.

One way to reduce the unintentional harm that can come from accidental findings is to push for a shift in the way doctors and patients talk about health issues, Ofri says. She teaches her medical students to consider whether a test’s results might change the treatment plan. If the answer is no, she tells them to think again about ordering the test. For patients, she advises viewing a clinic visit as a conversation about the risks and benefits of various tests, not as an opportunity to collect endless streams of data.

It may also help to accept that we are all full of weird quirks and that those anomalies help make us who we are.

“There are very few things in medicine that aren’t ambiguous,” Ofri says. “Being comfortable with uncertainty is a necessity.”