thanks, PPV is exactly what I'm after.

The alternative to giving a doctor positive & negative predictive values for each maternal age is to give false positive & negative rates for the test plus the prevalence rate for each maternal age. Not much difference in terms of the information load.

One concern I didn't consider before is that many doctors would probably resist reporting PPV's to their patients because they are currently recommending tests that, if they actually admitted the PPV's, would look ridiculous! (e.g. breast cancer screening).