Still, says Dr. Robert Nussbaum, chief of the Division of Medical Genetics at UC San Francisco, no matter how well it's explained, some of the information people get simply doesn't mean much. Some DNA tests are highly predictive of disease -- for example, tests for mutations in the cystic fibrosis gene -- but others are based on flawed data or so weakly predictive that many who test negative will end up getting the disease, while many who test positive will not (an example would be tests for risk of Type 2 diabetes).
In such cases, Nussbaum says, "the information is basically useless." It might even be counterproductive, he adds, if someone who tests negative -- for, say, diabetes risk -- then has less incentive to adopt healthful life habits.
Even highly predictive tests can be unhelpful if they predict conditions that can't be treated or prevented, such as Huntington's disease, Nussbaum adds. They can simply make a client worried or scared.
In all, says Hank Greely, director of the Center for Law and the Biosciences at Stanford University, while some companies clearly provide useful information to their clients, "some companies provide nothing beyond a new way of wasting money."