Scientists have developed a simple formula that can help find colon cancer patients with certain inherited bad genes — information that can help determine the best course of treatment and identify family members at risk of developing the disease, too.

The formula combines such factors as a patient's age and sex, and cancer characteristics like the tumor's location, and gives a score for the likelihood that inherited mutations are responsible. Its inventors have posted it on a Web site that any doctor can use.

Click here to view the formula.

"It is quite simple — the authors couldn't have made it any easier," said Dr. David Weinberg, a gastroenterologist at Fox Chase Cancer Center in Philadelphia who had no role in the study.

The findings appear in Thursday's New England Journal of Medicine.

They come from a huge effort to analyze every case of colon cancer in Scotland from 1999 to mid-2003 involving people under 55. All cancers are caused by bad genes, but ones diagnosed earlier in life are more likely to be from inherited mutations.

The most common of these mutations cause the Lynch syndrome, a type of colon cancer that progresses very quickly. These mutations also raise the risk of uterine, ovarian and other forms of cancer, making it important to identify relatives who may be carriers.

But doing $3,000 genetic tests on every colon cancer patient to find the 4 percent who have such mutations is expensive and impractical, considering that 150,000 Americans and 500,000 people worldwide are diagnosed with the disease each year.

Researchers led by Dr. Malcolm Dunlop at the University of Edinburgh in Scotland tried to devise a simple way to screen patients and find which ones should get the expensive tests.

The researchers used the formula plus a relatively simple $300 lab analysis on the 870 participants in the study and successfully identified two-thirds of the patients who had the mutations.

"There's a big gray area in the middle of this, but it's a start," said Dr. C. Richard Boland, a gastroenterologist at Baylor University Medical Center in Dallas who wrote an accompanying editorial and specializes in assessing genetic risk in colon cancer.

Patients with such mutations may want to have their entire colons removed instead of just the cancerous parts, because of the high likelihood of cancer coming back. Women also may want to have a hysterectomy to avoid the chance of developing uterine cancer. Mutations also may affect the effectiveness of certain chemotherapy drugs.

"Just last week we had a case here, a woman 37 years old," Boland said. "She had the wrong surgery. They didn't discuss with her taking out her uterus, taking out her ovaries, taking out her whole colon. She came from a little town in Texas and her doctors didn't know how to deal with this."

Too few doctors are considering the possibility of inherited mutations fueling cancer and putting other family members at risk, Weinberg said.

"Nothing is more upsetting than meeting these families" and learning that there were many generations of people with colon cancer but no one thought to test them, he said. "Testing of this sort is not done as much as it should be."