WOMENS-HEALTH Messages for December, 1998: Re: AFP and 40% miss on Downs

Re: AFP and 40% miss on Downs

From: William D. McIntosh, MD (anonymous@obgyn.net)
Sat, 5 Dec 1998 15:24:37 -0600 (CST)

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In reply to: AC: "AFP and 40% miss on Downs"

At Sat, 5 Dec 1998, AC wrote: >
>I was told by a genetic counselor that the AFP will miss 40% of actual
>Downs cases. My AFP result (taken at 18 weeks) is 1 in 10,000. Does
>this number take into account the 40% miss rate? I would like to know if
>I can use 1 in 10,000 as my risk factor or do I need to make adjustments
>to it.
>
>Thanks.

AFP has little predictive value for Down's syndrome. The triple screen (much more common these days) is better, but is still very weak for predicting Down's syndrome. Your counselor was right, at least 40% of acual Down's cases will be missed by the triple screen. This does not mean that you need to adjust your odds, just so you know that this SCREENING test (as opposed to DIAGNOSTIC tests like amniocentisis) is not all that great for this condition.

You can use the 1/10,000. This simply means that if there were 10,000 patients just like you, with your exact values on the 3 components of the triple screen, at just your gestational age, your weight, your race, and so on, only only one of you would deliver a child with this affliction. Those are pretty good odds. Carried to an illogical extreme (and understand that this is not really statistically valid) you could add the 40%, and come up with 1.4/10,000. Those are still great odds.

-- William D. McIntosh, MD Clarksville, TN

This is for educational purposes only. It is not intended to replace consultation and examination by your physician or other health care provider.

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