Blood Tests

Transcript

So let’s talk about how these blood tests work. All right. Now the first blood test that we did, this is the first one that was offered to us when we had our first baby 24 years ago, I guess. It’s called maternal serum alpha-fetoprotein or alpha-fetoprotein (AFP). And the way we figure this test out, it wasn’t me, but the scientists took a look at this protein because it’s something that’s unique to moms who are pregnant. And they figured, well how much is supposed to be there? Is it supposed to be a little bit or a lot? And so what we did is draw mom’s blood and we made a graph here and we said, “Okay, no alpha-fetoprotein on this side, and a whole bunch over here.” And we started drawing mom’s blood and plotting where she landed compared to everybody else. Okay? So, if I kept doing this for long time — don’t worry. I won’t. I’d put you to sleep. But if we kept doing that, and then we drew a graph over the top of that of where people land … how much alpha-fetoprotein is usually in the mom’s bloodstream … we come up with something like that. Remember the statistics. A Bell Curve. Yeah, that’s a Bell Curve. Now what’s down the middle of a Bell Curve? The average. It’s kind of like the average right. Some other terms for that sometimes, but it’s kind of like the average. Let’s say that the average amount of alpha-fetoprotein in a mother’s bloodstream is one. Okay, just to make it easy to remember.

So if we drew a line down there in the middle at average and a line up here at two and half times the average, and then a line down here at one half the average. If we were to count the squares between that point five and that two point five we would find that 95 percent of people land between those two points, okay? So most moms, if you draw your blood, you would find that much alpha-fetoprotein in there … somewhere in that range. Okay. Now, how did we figure out that the alpha-fetoprotein was associated with birth defects? Well, when we found moms with spina bifida and drew their alpha-fetoprotein, we found that they ended up having a lot more alpha-fetoprotein on average then moms who have normal babies. Okay. And in doing that, then we also realized that moms who have babies with Down syndrome have less alpha-fetoprotein than average. So it’s a different curve. Normal … Down syndrome, down here, open spina bifida, up here.

Now what happens if we draw mom’s blood and she lands right down there? Does that mean the baby has Down syndrome? Not necessarily. Not necessarily, that’s right. Because she’s really part of the normal curve and a part of the Down syndrome curve. So which is she? You can’t tell. Now remember there’s like a 1000 people in the normal curve for every one person with Down syndrome, so you’re probably part of the normal curve. Okay. But a lower alpha-fetoprotein is … back to that breast cancer example … is a little bit like finding a breast lump. Okay? So it raises the risk but it doesn’t necessarily mean the baby has it. So the problem is this is a screening test. It doesn’t say yes or no. It just says higher or lower. Okay? So what we usually do, if we’re going to use the alpha-fetoprotein, is we draw a line down there at the point five and we say everybody below that is high risk for Down syndrome. Okay? Now what does that mean for the moms who have a baby with Down syndrome they may not know about? But they’re above the point five, because there are some moms with Down syndrome who have more than the point five? What’s the test going to tell them? Yes or no. It would be nice if it did, but this test is going … if they’re up there above the point five, the test would say to them, you’re at low risk. Yeah, but yet the baby still could have Down syndrome. That’s like not finding a breast lump but there’s still breast cancer. It’s a false negative test. Okay, so the alpha-fetoprotein isn’t a yes or no test. It’s a higher or lower test. Okay?

Now maybe we could look at that and say well, gosh, we’re going to falsely reassure people. Maybe we should draw the line up here. What’s wrong with that? Well, then almost half of us would think we’re at risk. Yeah, I mean that could scare a lot of people, right? And if you’re all nervous, how are you going to know. That’s the question. Then we start talking about invasive testing. So we don’t draw the line up there but that means that we’re going to miss some people. Okay, so it’s not a perfect test.

Now I want to say something here that I will summarize in a bit, but it’s an important point. If we draw mom’s blood and let’s say she lands below that point five, like the test is planned to do, and we tell her that she’s at high risk, what portion of those moms do you think actually have a baby with Down syndrome? Even if they land clear down there below the point five and they get the phone call. “Hey, it’s Dr. Fausett. You had your serum screen done and it came back where you had an increased risk for Down syndrome.” It turns out that somewhere in the range of three to five percent of moms would actually have a baby with Down syndrome then. So that means if we say you’re at high risk, 95 percent of people are still going to have a normal baby. Do you see why there’s a challenge with the screening test? Right. That’s why we get false positives and, in fact, most abnormal screening tests are a false positive. It’s the benign breast lump, but people are worried about it. And how are you going to find out? How are you going to know yes or no? It goes back to diagnostic testing. So you have to say, “Well, would I be willing to have an amnio?” You see where that goes, right? Right. You’ve got to think all the way through, “What am I going to do with the results of this test?”