In Alpha the scan update policy controls the reinterpretation and reclassification of screening tests from "screen positive for increased risk of Down's syndrome" to "screen negative" after the addition of ultrasound scan information. You can choose to reinterpret the test always, or to do so only if the new scan estimate of gestation differs from the 'dates' estimate by at least a specified number of days (between 1 and 28 days). It is desirable to set a limit (say 7 days) so converting a true-positive to a false-negative will be very rare. Tests that are not reinterpreted (because the difference in gestation is too small) are reported with the same screening result ("screen positive") as the original report, with the addition of a message indicating that the original interpretation remains unchanged.
Restricting the reclassification of screening results in this way helps to avoid giving false reassurance to women with affected pregnancies who have been given a positive screening result based on dates (1). Such revision of the test result who had initially been told they were screen positive is particularly distressing. The remedy is to perform a scan on all women at the time of screening, thereby avoiding the need to revise the gestational age estimate after a woman has been screened and found to be positive.