Genetic testing is becoming a cornerstone of modern medicine—used to measure the likelihood of developing diseases from cancer to mental illness.

But as health-care providers move quickly to use the information, employers increasingly are running up against a federal law that governs how genetic information can be used.

The Genetic Information Nondiscrimination Act bars companies from asking about family medical history or genetic testing—limiting the information that some employers have used to gauge the fitness of their workers and encourage healthier lifestyles.

Asking for the information—for example, during a mandatory physical or an exam that was part of a job offer—is enough to constitute a violation. GINA also prohibits companies from using such information to hire, fire or promote employees.

"This is beginning to create a pretty direct conflict because in order to prevent the misuse of this information, you're stopping good medicine," says Larry Lorber, co-head of the Washington, D.C., labor-and-employment group at corporate law firm Proskauer Rose LLP.