The Down's test is definitive, testing for low incidence issues isn't a problem when you can be 99.999....98% confident in your test. Or, more importantly, test

The Down's test is definitive, testing for low incidence issues isn't a problem when you can be 99.999....98% confident in your test. Or, more importantly, tests for low incidence occurrences are fine as long as your rate of incidence is substantially higher than your rate of false positive.