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September 22, 2014

September 22, 2014

Posted by **jessie** on Sunday, May 16, 2010 at 10:05am.

The commonly used test to assess Down's syndrome risk in the unborn foetus is the 'triple test', which combines maternal age with other markers of foetal and maternal health from a single blood test. The test has a 65% detection rate, and a 5% false positive rate. Incidence of Down's syndrome in births from 35 yr old mothers is 1 in 250. The triple test from one 35 yr old mother returns a positive result. What is the probability that her foetus has Down's syndrome?

- math -
**drwls**, Sunday, May 16, 2010 at 11:45amOut of a random sample of 35 year old mothers:

Probability of false positive = (249/250)*0.05 = 0.0498

Probability of correct positive = (1/250)*0.65 = 0.0026

Probability of false negative (undetected Down's) = (1/250)*0.35 = 0.0014

Probability of correct negative = (249/250)*0.95 = 0.9462

For the woman in question, she is either a false positive or a correct positive.

Probability of Down's = (.0026)/(.0026+.0498) = 0.04962

That equals a probability of 1 in 20.2

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