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Re: MSTS RiskFrom: Allen Worrall (jworrall@alaska.net)Tue Feb 13 19:39:36 2001
Yes, a risk of 1:500 means there is one affected pregnancy in 500 pregnancies. The odds are 1:499, meaning one affected pregnancy in 499 normal pregnancies. See British Journal of OB and GYN, April 1999, Vol 106, pp.371-372 Calculating correct Down's syndrome risks by Howard Cuckle and Indera Sehmi. To calculate risk, multiple the odds by the likelihood ratio, which gives you a new odds. Lets say the likelihood ratio is 3. Then the odds become 3 x 1/499 or 3/499 or 1/166.333. That would then be a risk of 1:167.333. We would, of course, round that to an odds of 1/166, and a risk of 1:167. I think (but am not sure) there may be a convention in some places to express risk as 1:xx and odds as 1/xx, or maybe I have it backwards. In Dr. DeVore's recent paper on determining risk of Down by ultrasound, he has another method of applying the calculations, but probably it is basically the same thing.
-- Allen Worrall
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