Navigenics, of Redwood Shores, CA, is developing a genetic test to help customers assess and respond to their risks of contracting numerous illnesses, including Alzheimer’s, type 2 diabetes, rheumatoid arthritis, and breast cancer. The genome-wide scan targets 20 diseases so far.

The company says that this year it will begin offering the test directly to consumers for $2,500, joining competitors deCode Genetics of Iceland and 23andMe of Mountain View, CA, both of which recently began offering personal genetic tests for under $1,000. Navigenics also says that it plans to follow up customers’ test results with extensive interpretation from on-staff genetic counselors.

A client will send a saliva sample to Navigenics, which will use a genetic array from Affymetrix, capable of detecting 1 million specific genetic variations, to scour the DNA in the sample for genetic misspellings. Proprietary software will then spit out a report that compares the client’s disease risks with those of the general population; explains the extent to which environmental and genetic factors contribute to the diseases; and suggests further screening measures and early interventions, ranked by the level of clinical evidence supporting them.

“If you knew 50 years in advance that you were at increased risk for Alzheimer’s disease, you could lower your cholesterol and keep physically and mentally fit,” says Navigenics cofounder ­Dietrich Stephan, the company’s chief scientific officer. “When early signs of Alzheimer’s start, you may be better equipped to start on cutting-edge compounds that might push the onset out by decades.”

Greg Feero, chief of genomic health care at the National Human Genome Research Institute, is less enthusiastic, warning that such tests have not proved their usefulness and could burden patients with worry and uncertainty. He says it’s far from clear what probabilities mean for a given individual or whether proposed interventions would be effective. He notes, too, that some patients might accept bad test results fatalistically instead of pursuing such interventions.

Still, Navigenics CEO Mari Baker sees a ready market, pointing out that “millions and millions” of U.S. residents already spend thousands of dollars every year on discretionary health purchases. But she admits that nobody knows how many people will order genetic tests. “It’s a brand-new industry,” she says.