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by CrHn3 1383 days ago
Non-Invasive Prenatal Testing (NIPT) was made available in the US in 2011. It’s highly sensitive for detecting down’s syndrome. Self pay is now 100-300 USD but a few years ago I think it was as much as 3000. It’s fully covered and encouraged for geriatric pregnancies, which are much more likely to have an extra copy of chromosome 21.

The NIPT can screen for the sex of the baby earlier than it can be detected on a scan, so many women elect to have it done.

The first trimester ultrasound is around 96% accurate at detection of down’s syndrome.
1 comments
Waterluvian 1383 days ago
We got this for free in Canada. My first kid had the soft positive marker for it in his first ultrasound (Interesting you say 96% because our doctors said it meant a 1 in 300 chance. Maybe we are talking about different things). So they sent us to Hamilton a week later for genetic screening. The geneticist walked us through the results and said that the tests are effectively negative and now his odds are something like 1 in 5000.

This is all to say: indeed they have non-invasive tests and they’re very accessible.

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jpollock 1383 days ago
Without knowing the number of tests performed, you can both be correct.

Theirs would be the false positive rate (FPR) or false negative rate (FNR), and yours would be an probability, which takes the FPR (or FNR) rate, the number of tests and the base incidence rate to arrive at 1/300, 1/5000.

For example, in Canada, 1/750 of live born babies has Down's [2].

There are about 400,000 births every year in Canada [3].

So, there are 400,000 / 750 = 500 kids with Down's born every year.

Assume the test has a FNR rate of 0, and a FPR of 5%.

400,000 * 0.05 + 500 = 20,500 children test positive

Odds of positive test indicating Down's (5% FPR): 500/20,500 = 2.4%

Not quite the 1/300, but as you said "soft marker", so maybe the FPR is higher for that, vs all the markers?

To get to 1/300, the FPR rate on the marker would need to be 37.5%?

The 1/5000 is (I believe) a change from a FPR rate to the FNR rate, since the second test is indicating that the disease _isn't_ present.

1/5000 would indicate a negative genetic test will miss one Down's diagnosis in a decade in Canada.

Interestingly, it looks like the FRP increases with age, with it being 4% at age 30 (combined ultrasound/blood test)? [1]

[1] https://pubmed.ncbi.nlm.nih.gov/11702835/

[2] https://www.canada.ca/en/public-health/services/publications...

[3] https://www.statista.com/statistics/443051/number-of-births-...

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