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Calculation of Risk for DownFrom: Allen Worrall (jworrall@alaska.net)Thu Feb 22 23:10:48 2001
I am aware that a negative ultrasound can reduce the risk assigned by a triple screen, by about 50%. What about a patient who has a very low risk Down Triple screen result, but one soft marker is found? Can the soft marker likelihood ratio be applied to alter the triple screen risk? Case in point: 35 year old with 4 previous C Sections, triple screen risk 1:7600. I found hyperechoic bowel (it seemed to me it was as echogenic as bone, but I find this a little subjective), 19weeks gestation, nuchal fold 5.4 mm, and a scan was not requested at 11-14 weeks. On today's scan I found no other anomalies or "soft" markers. (I think Dr. Nyberg would consider a nuchal fold of 5 mm or greater as significant, Dr. Benacerraf 6 mm or greater). If we say the nuchal fold was normal (not yet 6 mm or greater): hyperechoic bowel has a likelihood ratio of 5.5. If we consider her age related a priori risk, it is ~ 1:302 at twenty weeks. Apply the likelihood ratio for the hyperechoic bowel and her risk is now ~ 1:56, clearly an indication for amniocentesis. Also if we use Dr. Benacerraf's scoring system, age 35 and score 1, amniocentesis should be recommended. Or should the 5.5 likelihood ratio be applied to her triple screen risk of 1:7600, which would increase her risk to 1:1382,and thus amniocentesis not indicated? I suspect the answer is that the patient should be offered amniocentesis, but I would like to see what others think about this.
-- Joseph A Worrall MD RDMS Fairbanks, Alaska
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