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Re: R: Calculation of Risk for DownFrom: Richard M. Roberts (gene@vol.com)Wed Feb 28 20:28:05 2001
For those in the US, and at risk of malpractice, please be aware of one very important point, which is supposed to be known by all certified genetic counselors; once you have a documented increased risk for Down syndrome, you can not repeat the test, no matter what. The next risk may be normal, but due to the statistics and nature of he screening test, it is more likely to be normal. Hence, in court for a wrongful birth suit of a Down syndrome infant, any Board Certified clinical genetisist would, unfortunately, have to testify that that constituted malpractice, if you placed any value on the repeat test, with regard to Down syndrome risk. You simply are not allowed to try again for Down risk, no matter how bad the laboratory or the computer package they are using. Once an increased risk is calculated, it stands. The acural value doesn't stand (like 1 in 233), but that woman in in a cohort whose risk is at least that of a 35-year old. That does not mean you can't counsel the women that this is an inaccurate test, and may have been improperly done (the usual triple screen has to include a serum dilution before analysis, and I bet that on Mondays, more abnormal tests occur, due to tech's hangovers, with inappropriate dilution.) The free-beta subunit is more sensitive and specific than total HCG, and I would go with that--it doesn't require any laboratory personnel to dilute the sample. You CAN, however, repeat the AFP. One abnormal AFP does not an NTD make. If the second AFP is normal, I tell the patient to forget it--just a statistical fluke (although there may be some evidence to the contrary). The difference here is that if there is a skin defect, with exudation of fetal serum into the amniotic fluid, ie an abnormal physical finding, or infection, then the next time you do an AFP, it should remain elevated. But I am a believer in ultrasound being much more sensitive and specific than AFP for detection of NTDs, and evidence will be publish supporting this. I was very gratified to hear from Dr. Corda--thank you. But those cases are not unusual-I would have expected them. Please don't get sued.
-- Richard M. Roberts, PhD, MD, FACMG
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