"I would rather not lay out the $200 myself," said Mary, who requested that her last name be withheld for the same reason she paid for her own test. "But it seemed safer."

Kathy, a financial analyst in Houston who would like to know if she, like her two sisters, has a genetic predisposition to breast cancer, said she was not even going to take an anonymous test. "Then," she said, "I'm just in a position of having to lie."

The culture of secrecy around genetic information is stronger in the United States, some experts say, than in countries where people are guaranteed health care. Among Americans at risk for Huntington's disease, an incurable brain disorder, only 5 percent take the DNA test to determine if they will develop it, compared with 20 percent of Canadians in the same position, according to Michael R. Hayden, a professor of human genetics at the University of British Columbia in Vancouver.

Doctors often feel obligated to inform patients of the potential financial downside.

"I always warn them," said Stephen Moll, director of the Thrombophilia Program at the University of North Carolina, who uses a genetic test to determine the best treatment for patients with blood clots. "Especially if they are self-employed, I don't want it to be a surprise if their health insurance premium goes up."

Unknown risks

After receiving a similar warning from her doctor, Katherine Anderson's parents did not allow her to be tested for Factor V Leiden, a genetic condition she might have inherited from her father that increases the risk of blood clots.

But last year, with nothing in Katherine's record to indicate reason for concern, a gynecologist prescribed a birth control pill to regulate her uneven periods. Six weeks later, Katherine, then 16, developed a clot that stretched from her knee to her abdomen. The pill, combined with the gene she had indeed inherited, had increased her clotting risk by 30-fold.

Now largely recovered, her primary concern is whether she will be viewed as a health insurance liability for the future.

"I don't want to have to work for a big business just to get insurance," she said. "This could be determining what I can do for my whole life."

For Judith Berman Carlisle, the price of privacy was forgoing the DNA test that would have convinced her not to have surgery. Carlisle, 48, who was setting up her own therapy practice, was afraid testing positive for the high-risk breast and ovarian cancer gene that runs in her family would prevent her from buying health insurance.

But her sister had developed ovarian cancer the year before, an aunt had died of it, and Carlisle was desperate not to get it herself. Her doctor agreed to remove her ovaries based on her family history — the way such decisions were commonly made before a genetic test was available.

Carlisle was convinced the surgery would be less damning than proof that she carried a defective BRCA1 gene, which also confers a very high chance of developing breast cancer.

"There's a big difference between someone saying, 'I have a strong family history,"' Carlisle said, "and saying, 'I only have a 13 percent chance of not getting breast cancer during the time you're insuring me."'

Last fall, after the surgery to remove her ovaries, she began to consider a double mastectomy to remove any chance of breast cancer, the disease her grandmother and another aunt had died of. Having secured health insurance, she took the test for the BRCA1 mutation. It came back negative.

"The first thing they said to me," Carlisle said, "is that I have no higher risk than anyone on the street."