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Re: MSAFP vs triple screenFrom: Andrew R. MacRae (prenatal@dis.on.ca)Thu Jul 10 16:43:20 1997
> If you screen all your patients (incl. <35) with double/triple test > your false positive rate must increase (the 60% pick-up rate > applies only to maternal age >35). NOT TRUE!! It is well documented that the Down syndrome prenatal detection rate of 60% for a false positive rate of 5% is a common finding when triple screening is offered to women of ALL ages, not just those over 35 (see Palomaki et al in J Med Screening, 1996, 3: 12-17). The false positive rate and the detection rate will vary, depending on the age distribution of the screened population. In women over 35, the detection rate is close to 90%, with a false positive rate of 25%. Therefore, 75% of patients over 35 will be informed that their risk is below the risk cut-off for consideration of amniocentesis (in North America, usually that of a 35.5 year old). If triple screening is available to women of all ages, and if there is close attention to communicating the results of the screen to patients such that the patients understand their own risk (instead of a general population risk like "women over 35"), the amniocentesis rate will diminish. Why? Because the false positive rate of the triple screen is always lower than the percentage of pregnant women who are over 35. Returning to the original question, is triple screening justified over MSAFP, the answer is "yes". In addition to the studies in the reference quoted above, we just reported our detection of Down syndrome in 10,540 screened women (median age 29.4 years) using a risk cut-off of 1:385 with a false positive rate of 8%. Of 21 cases at mid-trimester, maternal age detected 6 (29%), age+MSAFP detected 8 (38%), age+AFP+hCG (the "double" screen) detected 12 (57%) and age+AFP+uE3+hCG (the "triple" screen) detected 15 (71%). Approximately 11% of our population is over the age of 35 (possible 11% amniocentesis rate); however, the amniocentesis rate for ALL reasons was only 8%.
-- Andrew R. MacRae, Ph.D. FCACB Whitby Ontario, and Department of Clinical Biochemistry University of Toronto, Canada
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