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Detection Rates


Okay, remember how I said we were going to talk about detection rates? Okay, here’s this little table, a little bit complicated but it talks about the detection rates of the various types of testing that we’re going to offer. Okay. So, if we were to just draw blood in the first trimester, if the baby had Down syndrome, the blood test would say the risk is high 68 percent of the time. That’s its detection rate. So if the baby has it, that’s the chances that the test would actually detect it. Okay? So in that first example, if we just drew your blood and we measured things in the blood the first trimester, if the baby had Down syndrome, 68 percent of the time the test would say, oops, bad, high-risk result. Okay. So that’s not perfect is it? If you add ultrasound to it, well if you do ultrasound alone, it only has a 62 percent detection rate in the first trimester. If you take the blood and the ultrasound together, we jump all the way up to an 84 percent detection rate. So we are going to detect most babies with chromosome problems if we do blood and ultrasound. That’s why I said if you’re going to do a test in the first trimester, it tends to be blood and ultrasound. Okay.

Now if we do a second trimester blood test like the quad screen, the detection rate is about 80 percent — somewhere between high 70s and low 80s percent. Similar to, but not quite as good as, the first trimester screen, but in that range.

Now if you take a second trimester screening ultrasound, just go have a routine screening ultrasound. If the baby had Down syndrome, we’d probably find something wrong 50 to 70 percent of the time. So it’s not a perfect test either but it’s going to look for the structural problems. Okay. So we could also do high-level ultrasounds where we’re looking in great detail at the things like I mentioned — the little fingers turned in, or the big toe is poked out, or the thickness of the skin at the back of the baby’s neck, and all kinds of things, the length of the arm bone, detailed look at the heart. If we do all of that stuff in a comprehensive genetic ultrasound, then our detection rate goes up. It depends on who is doing it. That’s a very personal driven thing, but it can be as high as 95+ percent in some people’s hands so a comprehensive screening ultrasound may be a very useful test.

All right, so we’ve got that, plus we could combine a screening ultrasound and then have a quad screen and those two together are going to have a high sensitivity. They’re also in the 90 percent range okay.

And then, finally, we could do a blood test in the first trimester, a blood test in the second trimester, ultrasound in the first trimester and add all those things up and that has the highest detection rate. Okay. But it’s also the most complicated test to do because you’re doing all that, and then you have to kind of have to wait. It doesn’t really give you a formal answer at the end of the first trimester. So there are tests where you get an answer if you’re really high risk in the first trimester, or if you’re really low risk and you’re done. And then it only does the second part of the test if you’re intermediate risk, and some people offer that test. Some people only do the first trimester test and some people just do the second trimester test. So depending on our individual institutions, in the military and out of the military, the basic idea is you could have a screening test that gives you a result in the first trimester, blood and ultrasound, a screening test that gives you a second trimester result, and then you have the diagnostic tests. Okay.

Now back to the diagnostic tests for a second — their detection rates. So if a baby had a chromosome problem, what are the chances that we would find it on a CVS? Pretty high rates, almost 98 percent. Okay. That’s why we say it’s pretty much a yes or no test. Okay. Now why wouldn’t CVS tell you for certain? Well, what if you couldn’t get the cells to grow or they didn’t grow well… that kind of thing. Okay, most of the time cells grow well and we’re able to get it, but it’s a little bit harder to get placental tissue than it is to get amniotic fluid a little later, so that’s one of the things about CVS. Okay.

Now finally, the amniocentesis again has a very high detection rate, 99+ percent because the cells almost always grow and we’re almost always able to get fluid. Okay. So that’s pretty much the yes or no tests. All right.

So now we better talk about the options. All right, one last thing before we do that, and I’ve said this a whole bunch of times but just think about this big question. How would the tests affect what I would do? Would I be willing to have an invasive test? Would I consider pregnancy termination? And those help you decide what screening strategies you should do.