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Testing for the flu

Learn when testing for influenza makes sense, and the most common test that is used. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

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Video transcript

So every winter a lot of kids come in and see me because they've got the flu, and I've got to make a decision. For each one of them, I've got to decide, am I going to get the flu test in this particular child? So when I see a kid that's sick, here's what I'm thinking. I'm thinking, well, the first thing I've got to decide is, do I really suspect the flu? And this is when I ask a lot of questions of the person who's sick or their parents. I say, well, when did it start? Was it abrupt? Because that's my first clue that it really is the flu. The second clue is how long did it last? And they might say, well, we're in the middle of it. It's been a few days. But if they tell me it's been weeks and weeks or months and months, then of course, I'm thinking it's something else. Right? It's not the flu, then. And finally, if they tell me that they're having respiratory symptoms, then that's, of course, really important. That sounds like the flu. But they also have to have some constitutional symptoms. Right? They must have some sort of fever, or fatigue to kind of convince me that this sounds like the flu. So let's say that the person has these symptoms. They have me suspecting that they have the flu. Is that enough for me to get a test? Well, not really. I've also got to think to myself, is it going to change my plan? So if the flu test results are going to change my plan, then I'm much, much more likely to get it. Right? But let's say that someone comes in with a fever and they're having cold symptoms, like runny nose, congestion, and it turns out to be the flu. And I tell them, well, you look pretty well. Why don't you go home and get fluids and rest, and you'll feel better in a week. Well, in that same scenario, if the flu test was negative, then I would say, well, maybe they have one of those other viruses, one of those copycat viruses, like rhinovirus, or RSV, or something else. And I would say, well, why don't you go home and get plenty of rest and drink plenty of fluids, and you'll feel better in a week. So you notice I'm saying the same thing. My plan is basically the same. And so in that scenario, it doesn't really make a lot of sense to me to get a flu test, because it's not going to change my plan. My plan is basically rest and fluids. But sometimes, every once in a while, the flu test actually does change my plan. And let me tell you what those circumstances would be. So let's say, for example, I've got a patient. And they're the hospital, and they're sick. And we don't know what they're sick from. We know they're having fevers, and they have a bad pneumonia. And we don't know what's going on, and I get a flu test and it's positive. Then that's really helpful in guiding my thought process. Right? I might not even need other testing. I don't have to look for other bugs. So it might get rid of the need for further testing. And it might help guide what I do next. And so in some patients where you're confused about what's going on, it really is quite helpful to have a positive flu test. Another instance would be if I have a patient and I want to treat them with a medication, knowing if it's flu A or flu B might affect my medication choice. So it could affect the medication that I choose to offer them. I want to be very clear. If I have a patient that's sick, I'm not going to wait to find out whether the flu results are positive or negative. I'm going to probably start treating them for the flu if I really suspect it. So I wouldn't wait on the results of a flu test to treat. But it might help me change my medication choice later, or it might help me figure out how long I want to treat for. There are lots of issues that you can resolve by getting a flu test in that situation. And finally, what about other choices that I have to make, things like, where do I place a child in the hospital? And I want to make sure, if he doesn't have flu, I don't put him with kids that do, because then he could get another illness. And so it could affect hospital infection control choices. So I'll write that as a third option, infection control. There are many other instances, too, you know. You could think of outbreak situations where you really want to know how many people are sick or what they're sick with exactly where you might want to get a flu test. So many, many places where a flu test is useful, but not always. Many times, I also don't get a flu test. So let me draw up a little bit of canvas. Maybe I'll just leave this up for the time being. Let me draw a little face here for you. So this is a person with their head tilted back. This is their eye. And a lot of times what you'll do is, you'll do what's called a rapid test. That's one of the most common tests that's done. And the rapid test, you can kind of get a sample by sticking a nasal swab, almost like a giant Q-tip, kind of at the back of the throat. And you go through the nose. The key with the nasal swab is you want to make sure you go back far enough so that you get cells, not mucus. You want to make sure you get cells, because that's where a lot of the virus is really hanging out. And you don't want to get just mucus, because you may not actually find the virus that way. And another way to do this rapid test, instead of the swab approach, is you'll see sometimes people actually wash-- they'll squeeze a little bit of salt water back there, and then they'll suction it back out. So they'll actually squeeze some salt water, suction it back out. It's usually a deep suction, because again, you want to make sure you get those cells. You don't want to get mucus. And they'll send that off to the lab. So this rapid test-- this rapid test, that's what it's called-- as I said before, it's one of the most common tests that's done. It's not the only one, but it is one of the most common ones. And so I wanted to talk about some of the pros and cons of this thing. So why would people do it, and why should you be careful about doing it? So pros and cons of this rapid test. And the pro, the first pro, is actually kind of in the name-- rapid. So it's actually a really, really quick test with a fast turnaround time. So I can actually get the results usually within 15 to 30 minutes. So the first advantage is that it's rapid. And I'll say, actually, you can do the test itself in about less than 15 minutes, usually. So that's a huge advantage. People don't have to wait around forever to find out if they have the flu. Now, the results is actually another advantage. It tells you whether they had flu A or B. So it usually tells you flu A and/or B. And I say and/or because a lot of them can actually detect both A and B. But a few of them only do flu A or flu B, in which case you would need to do a couple of tests to get the full results. Now, a couple of disadvantages to using these rapid tests, is that they don't tell you the precise H or N protein that is on that virus. Remember, this is relevant for the type A viruses, we said. But these rapid tests, they don't actually tell you which H and N virus you picked up. So you wouldn't actually know which one you got. And you also wouldn't know whether or not it was in the vaccine, whether it was a vaccine strain that actually made you sick. A lot of people like to know that, because they think, well, would I have gotten sick if I had gotten the vaccine? And unfortunately, most of these rapid tests don't tell you that. Now finally-- and this is probably the most relevant one-- is that these tests are not perfect. We like to think that if it's a flu test, it'll tell you with absolute certainty whether or not you have the flu. But actually, they are not perfect. And they sometimes make some mistakes, and I'm going to talk through two of the mistakes that they sometimes make. The first one-- I'm going to write them down here-- let's put down here the flu test results. So under flu test, I'll tell you what the result was for my rapid test. Let's say it was positive. So you're thinking, as you walk out of the doctor's office, if your flu test was positive, that you have the flu. But let's say in reality you actually don't. Maybe you have something else. You have some other virus or some other illness that's causing your symptoms. So that's a mistake, obviously, right? The flu test obviously made a mistake there. So we would call that false. And this is kind of alluding to the fact that it's a mistake-- a false positive. And the positive is because that was the result that it gave you. So if you have a false positive, then this is your situation. It's telling you, the flu test is telling you, that you're positive. But in reality, you actually have a negative. So that's the first type of mistake. The second type of mistake is actually kind of the opposite. It's called a false negative. And you can kind of guess what that might mean, right? So a false negative, again, it's a mistake. So that's what the false alludes to. That's what the false part means, that it's a mistake. And if it's negative, that means that over here, the flu test is telling you that you don't have the flu. But in reality, you do. So these flu tests, they're very good. They're very helpful. They're very fast. They tell you if you have flu A and/or B. But you've got to remember that they're not perfect. And every once in a while, they do make mistakes. So just be aware of that fact.