SAN FRANCISCO--Whether to administer genetic testing to a patient is
a complex issue, Patrice Watson, PhD, of Creighton University School
of Medicine, Omaha, said at the American Cancer Society Second
National Conference on Cancer Genetics.
"Certainly these tests have costs and benefits. Theyre
expensive, first of all, and they can be inconclusive. And with these
tests, patients receive knowledge of their transmission risk, which
can lead to discrimination and anxiety," she said.
That is why Dr. Watson believes that clinicians should consider not
only the potential health impact of the test--ie, that it will lead
to increased screening and prevention--but also the potential
emotional and economic impact.
"We also have to consider the test sensitivity in deciding
whether the patient should have it. Is it really worth giving?"
she said. When considering the BRCA1 and 2 tests for hereditary
breast and ovarian cancer, for example, the clinician needs to take
accuracy into account.
Researchers estimate that BRCA testing has an 80% sensitivity and a
99% specificity. "And the researchers are making an optimistic
estimate of sensitivity," Dr. Watson said. "In reality, the
tests may miss many people who have the carrier gene. We really
dont know the true sensitivity and specificity of this test,
and the results often depend on the nature of the mutation
search--and the type of mutation found."
Regarding breast and ovarian cancer, she said, the probability of a
positive BRCA1 or 2 gene mutation test are quite low even in those
with a family history of breast cancer. "Its only in the
small family subgroups with a striking family history that there is a
high proportion of positive results," she said. Because the
likelihood of a positive test result is so small, the value of this
test is reduced.
Even less is known about the hereditary nonpolyposis colorectal
cancer (HNPCC) test. Of those who take the test, about 50% get a
positive result, she said. So the sensitivity of the HNPCC test is
much less than that of tests for BRCA1 and BRCA2. "At the same
time, the probability of a positive result in someone with a striking
family history is probably high," she said.
For both tests, therefore, the best candidates are those with a
striking family history and personal history of disease, Dr. Watson
concluded. Because there is such a low probability of finding the
gene, clinicians should realize that the vast majority of those with
a family history of cancer will test negative.
Thats why taking a family history--and truly deducing whether
or not the family history is striking--is such an important issue in
genetic testing. "With such a history, you can give family
members the greatest amount of information and tell them how this
disease might affect them," she said.