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Mixed findings on disease prediction from genetic tests

The usefulness of genetic testing for making predictions about a person's risk for a particular disease varies widely, ranging from highly useful to potentially harmful, according to a new report in the British Medical Journal.

"Predictive genetic testing is still very much in its infancy," said cancer geneticist James P. Evans of the University's School of Medicine and the Lineberger Comprehensive Cancer Center. "It needs to be tailored to the disease one is trying to predict and to the individual and their family."

In their report, published in the April 28 issue, Evans, University colleague Cecile Skrzynia and Wylie Burke of the University of Washington point out that predictive genetic testing differs from conventional diagnostic testing in a number of ways.

Conventional testing, such as a blood count or imaging study, says something about the patient's present condition.

"While such information may have implications for the future, its overwhelming utility lies in the information it provides about the here and now," the authors said.

In contrast, a genetic test "informs us only about a future condition that may develop." Thus the risk is sometimes high, but always contains "a significant component of uncertainty regarding not only whether a specific condition will actually develop, but when it may appear and how severe it will be," the report said.

Evans noted another difference between predictive genetic testing and conventional diagnostic tests.

"Predictive genetic testing has a profound impact on additional people besides your patient. It invariably tells you important things about family members who may or may not be interested in that information, but who nevertheless could be impacted profoundly by that information."

According to Evans, predictive genetic testing can be viewed in terms of a spectrum of usefulness.

"There are diseases for which it makes tremendous sense," he said. "And all the way down on the far end of the spectrum are genetic tests that offer very little besides harm to patients."

One example of a disease for which genetic testing is highly useful is multiple endocrine neoplasia type 2, a rare disorder leading to thyroid cancer. Identifying those with this disorder makes preventive surgery an option.

On the other hand, Alzheimer's disease illustrates the potential for predictive genetic testing to cause harm. A positive test is an imprecise measure of risk and could result in anxiety, social stigma or discrimination.

"And since we can do nothing to interfere with that disease process, it seems to most of us essentially unethical to do Alzheimer's disease genetic testing," Evans said. "That situation would change radically if, for example, some intervention arose and we found out this or that drug taken early could prevent the disease or delay it. In that case it would make tremendous sense to identify who's actually at risk."

He added: "It's the physicians' role to educate the public and to try to determine when this new technology is going to be useful for their patients. I would also emphasize that this is an incredibly rapidly moving field. Changes in our ability to prevent or treat diseases such as Alzheimer's could have tremendous implications for predictive genetic testing."

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