Philadelphia ? Two to four of every 100 people unknowingly carry a combination of genes that renders them unusually vulnerable to Alzheimer’s disease as early as their 60s.

If you want to find out whether you’re one of them, the Philadelphia company Smart Genetics is about to offer a test you can order over the Web. For $399, customers receive a kit and send back a saliva sample. Within three weeks they learn whether they carry one or two copies of an Alzheimer’s-associated genetic variant known as APOE4.

But while the company’s chief executive officer expects to launch the test this month, some geneticists and Alzheimer’s researchers question its value. They argue that those test results can do nothing to stave off the fatal disease, which robs people of memory and mental abilities.

Others say information is power, and see a rising demand for genetic testing – whether or not it’s recommended by doctors or vetted by government agencies.

A study announced Monday in the Archives of Neurology may fuel further interest because it confirmed a gravely increased risk for people whose parents both developed the disease.