Mantoux test (aka. PPD or TST) | Infectious diseases | NCLEX-RN | Khan Academy


Male Voiceover: I wanted to
talk to about the Mantoux test. This is spelled Mantoux, kind
of an interesting spelling. after a French doctor who popularized it. And another way you might hear
this referred to is a PPD or a TST. And what these things stand for is PPD stands for Purified
Protein Derivative, and actually gives us a clue as
to what we’re using in this test, which is that we’re using specifically
TB protein. I’ll put that in parenthesis. and the location of the test
is actually also going to be
kind of a clue here with TST. This is a tuberculin. Again,
referring to tuberculosis. Tuberculin Skin Test. This tells you where we’re
going to put all that protein. We’re going to put it in the skin. You may have seen this, and
this is a picture right here
of someone doing the test. Sometimes it’s referred
to as the bubble test. A lot of people say, “Oh
yeah, my doctor injected “some liquid in my forearm
and it bubbled up.” This is how people usually
think about this test. They remember that because
of a very obvious visual. And so what I wanted to
do was give you an example or a diagram of what’s actually
happening when you get this test done. Let’s imagine this is your skin layer. This is also referred to as the dermis. Below the dermis is some subcutaneous. Below the skin, subcutaneous layer. Usually not layer. One of the
most common things is fat. I’m just going to draw that in here. This is the subcutaneous fat. So the idea here is that you’re basically
putting a little needle in here, which is what you’re seeing in
that picture on the right there. That needle is full of some TB protein. So this Purified Protein
Derivative is in that needle. It’s actually going to be injected in. So you’ve got all these
little TB proteins in here. And the volume you’re
putting in is a small volume. It’s about one-tenth of a milliliter. So 0.1 mL and you’re
putting it intradermally. This is actually an important point. It’s going into that dermis layer. So it’s intradermal injection. What happens is that if you
then let’s say moments later you remove the needle, you throw it away. And now what you’re going
to notice is because you put a little volume
in there, a little 0.1 mL, that is going to bubble
up because that volume is going to make the skin
puff out a little bit. But if I came back over some time,
this protein is diffused over, this liquid has been
absorbed into the skin, and that bubble will disappear. If you come back, you might
see a little bit of redness, because of course poking the skin causes
a little bit of redness and irritation. But the bubble will disappear over time. So what are we hoping to
accomplish with this test exactly? Let me bring up a couple
of more pictures for us. Well this test is going to
help us answer the question of has the person had prior TB exposure? Just remember that. That’s the question
we’re trying to answer. Have they had prior TB
exposure? Yes or no. Let’s think about what would
happen in either scenario. So this would be that they
have not had prior TB exposure. And on this side let’s talk
about what would happen if they
have had prior TB exposure. So two scenarios. Let’s
start on the no side. Let’s draw our skin again. So this is just as before. Let’s draw some TB protein in here. What’s going to happen is you’re
going to have some macrophages. These macrophages are going to come around and they’re always patrolling the area. They’re making sure
that almost like police officers making sure
that there’s no problem. They’re going to come
and they’re going to pick up some of this TB protein. So they’re going to take
it inside of themselves. They’re going to present that
TB protein to another cell. This is our T-cell. They’re going to present this
TB protein to the T-cell. The T-cell is going to
say, “You know, I have had “no prior TB exposure. I don’t
recognize this TB protein. and it’s going to go on its merry way. It’s not going to make a big
deal about what’s going on. So the T-cell kind of meanders away. It leaves the area. It
leaves proteins over time. It starts to get chewed up
and digested by macrophages. Eventually all of it is gone. If you look on the
outside you see flatness. The skin looks nice and flat. This might seem very
obvious from this picture. Of course it looks flat
but that’s essentially what we’re looking at here is flatness. Looking at this picture, you can
see a little bit of redness here. If you were to feel it with
your finger, it would be flat. So it’s red but it’s flat. And that’s the key. In this person we would say
if there is no bulge or bump, we would say this person
has a negative PPD. This person right here has a negative PPD. So that’s basically how we
would read this flat PPD. Now what happens on the yes side. Let’s say the person has
had prior TB exposure. So the same setup as before. Let’s draw the skin. There we’ve got some TB protein. Let me draw that in here. And this TB protein is
going to get picked up as before by the macrophage. The macrophage is going to come by
and pick up some of this TB protein. And just as before it’s
going to find a T-cell. These T-cells are also
kind of moving around. This T-cell this time is going to say, “You know what? I have seen
this TB protein before.” And this T-cell is going to
start getting very excited. And this is the key difference,
right? It’s going to get excited. Before it didn’t get excited. It just left the area unexcited. Now it gets excited and it
starts releasing chemokines,
little chemical messages. You know what that does? That attracts lots and lots
of other cells to the area. Lots of macrophages
start coming to the area. They say, “Aha, interesting.” The T-cell tells us that
we’ve seen this stuff before, and this layer, this intradermal layer, starts to swell up with cells. So it’s getting full
of cells because of all these new macrophages that are
being attracted to the area. So it’s actually going to
start looking like this. Full of cells, right, on both sides. It becomes a nice big bulge
and this is loaded with cells. Macrophages in here. Maybe a few
more T-cells in here as well. You get the idea. Lots and lots of cells. We call this a hypersensitivity reaction. When you see all these
cells coming into this area, this is a classic
hypersensitivity reaction. In fact, there are different types
of hypersensitivity reactions. We would call this a class 4, type 4. The reason that they are
typed out differently is that type 4 in particular
involves lots of cells. So this is a very cellular
reaction meaning lots of your
immune cells are involved. You can see that in the drawing. You can see lots of macrophages
and T-cells in that area. So this is a type 4 hypersensitivity
reaction happening here. And if you were to feel with
your finger from here to here, it would not feel flat, right? This is not flat at all.
This is actually bulging out. We call this indurated.
Meaning it feels very firm. It does not feel flat. This is what you’re seeing
in the picture here. You actually can see from here
to here there is induration. They’re actually
measuring it with a ruler. So this is the induration. You can also see that they’re doing
it perpendicular to the long axis. So in other words, if this
is the long axis this way, they are kind of choosing
a 90 degree angle to that, something like that, to
measure the induration. So that’s how you would
measure induration of a PPD. So this looks like a positive PPD
on this second picture over here. We’ll get into in just a
moment how we actually decide if it’s negative or positive. Now one thing I forgot to mention is
you’re going to be reading these PPDs 48 to 72 hours after you
initially injected the protein. So 48 to 72 hours later. This is when you actually read the PPD. That’s very important because that gives
enough time to either go flat like this or to actually get indurated like that. One key point I want to make
is let’s say you’ve got redness all the way around here. Do you actually
want to measure the redness? No. You want to measure the induration. Just keep that in mind.
Induration not redness. They are very, very easily
confused for one another, but it makes a big
difference. Not redness.


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