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Testing for the flu | Infectious diseases | Health & Medicine | Khan Academy

November 23, 2019

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.


  • Reply rhoadess January 18, 2013 at 2:32 pm

    Here is a great video on the probabilities of having something given a positive test result, etc … using Bayes' Theorem. The probabilities are actually counter intuitive. It is titled Bayes' Theorem and Cancer Screening /watch?v=j2tNxIaGpR4

  • Reply Eileen Virrey July 17, 2017 at 5:21 pm

    Thank you for this video. I recommend this video for NPs.

  • Reply Hannah January 29, 2019 at 5:04 pm

    When ever the doc puts that in my nose I pull it out and scream

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