WASHINGTON – Make Grandma spill the beans: Uncovering all the diseases that lurk in your family tree can trump costly genetic testing in predicting what illnesses you and your children are likely to face.

It may sound old-fashioned, but a Cleveland Clinic study comparing which method best uncovered an increased risk of cancer helps confirm the value of what's called a family health history.

All it costs is a little time questioning your relatives, yet good family health trees are rare. A government survey estimated less than a third of families have one - and time-crunched doctors seldom push their patients to remedy that.

I view family health histories as back to the future," says Dr. Charis Eng, a cancer geneticist at the Cleveland Clinic's Genomic Medicine Institute. "It's the best-kept secret in health care.

Holiday gatherings can be a good chance to gather the information, as can reunions or even funerals. The U.S. Surgeon General operates a free website - https://familyhistory.hhs.gov - that helps people create a family health history and share it electronically with relatives and their doctor.

However you do it, get the scoop on both sides of the family, says another study of 2,500 women. Researchers found that women not only know less about the health of their paternal relatives, they tend to dismiss the threat of breast cancer if it's on Dad's side.

It's a risk no matter what," says Dr. Wendy Rubinstein of Chicago's NorthShore University Health System, who presented the research last week at a meeting of the American Society of Human Genetics.

Far too often, a family health history consists of whatever you happen to jot down on that clipboard in the doctor's waiting room. Mom hasn't confided her soaring cholesterol? Can't recall what killed Aunt Mary? Don't realize that the hunched back of Grandma and her sisters could foreshadow your osteoporosis? Or maybe Dad never mentioned that in his 40s he survived the prostate cancer that killed his own father.

Looking for patterns of familial illness can predict someone's brewing health risks, so gaps can be a problem.

How does a good history compare with those online genomic testing services - sold without a doctor's prescription for hundreds of dollars - that analyze DNA glitches and predict people's predisposition to various diseases?

To find out, the Cleveland Clinic's Eng recruited 44 people - 22 patients in her family cancer clinic and their spouses - for a family health history and a saliva test from one of those genomic services, Navigenics, to calculate their risk for colon cancer and breast or prostate cancer.

Both approaches classified about 40 percent of participants as having above-average risk - but they picked the same people only about half the time. For example, the genomic screening missed all nine people with a strong family risk of colon cancer, five of whom Eng's clinic gave extra scrutiny to prove they carried a specific gene mutation.

A patient might have done this testing and been very reassured and not come to medical care," she told last week's geneticists' meeting.

On the other hand, Navigenics listed eight men at risk for prostate cancer when their family history predicted a risk no higher than average.

Why the difference? No matter the brand, genomic screening takes a broad look at DNA variations, including some that scientists aren't sure play a big role. Yet it often doesn't include high-profile gene mutations that are linked for specific diseases and can require more specialized testing, Eng explains.

Navigenics didn't return a call for comment.

Family history remains the best genetic tool we have, but health care providers are not taking advantage" of it, says Dr. Maren Scheuner of the Veterans Affairs Healthcare System in Los Angeles, who is leading a pilot project to add family cancer histories to the VA's electronic medical records at two area clinics.

Popularity may be growing. The surgeon general's office counts nearly 30,000 new visitors a month to its "My Family Health Portrait" Web site since summer, about a third of whom return, suggesting they're compiling an electronic family health tree.

Chicago's Rubinstein, who is testing a next-generation tool, found that women's newly created family health histories include much more information than was in their regular medical charts - even if they did need a nudge about the paternal side.

It's not uncommon to think, 'I look like one parent, that affects the illness I'm going to get,'" Rubinstein says. "Generally that's not the case.

Because genes seldom are destiny, a family health tree also should reflect shared environmental or lifestyle factors that can further affect an inherited risk, says James O'Leary of the nonprofit Genetic Alliance, which just won government funding to help spread family health histories to community health centers that serve the poor.

Collecting your family health history isn't just about knowing, it's about making healthy choices," he says.

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EDITOR's NOTE - Lauran Neergaard covers health and medical issues for The Associated Press in Washington.v