Mantoux Test (aka. PPD or TST)
- I wanted to talk about the Mantoux test, and this is spelled M-A-N-T-O-U-X, 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 ... The PPD stands for Purified Protein Derivative, and actually it 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 parentheses, and the location of the test is actually also gonna be kind of a clue here with TST. This is a Tuberculin, again referring to tuberculosis, Tuberculin Skin Test. So this tells you where we're gonna put all that protein, right? We're gonna put it in the skin. And you may have seen this, and this is kind of a picture right here of someone doing the test.
Sometimes this is referred to as kind of the Bubble test. Lot of people say, "Oh, yeah, you know, my doctor, "you know, injected some liquid in my forearm, "and it kind of bubbled up," and this is how people usually think about this test. They remember that because it's a very obvious visual.
And so what I wanted to do is kind of give you an example, or a diagram, of what's actually happening when you get this test done. So, let's imagine this is your skin layer. This is also referred to as the dermis.
And below the dermis is some subcutaneous, so, below the skin, subcutaneous, layer, and usually not layer. One of the most common things is fat, so 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, and 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 milliliters, and you're putting it intradermally. Intradermally, and this is actually an important point. It's going into that dermis layer. So it's intradermal injection, and 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 gonna notice, is that because you put a little volume in there, a little 0.1 mLs, that is gonna bubble up, because that volume is gonna make the skin puff out a little bit, but if I came back over some time, this protein is kind of diffused over ..
This liquid has been absorbed into the skin, and that bubble will disappear. So 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 gonna 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? And 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 whether they ... What would happen if they have had prior TB exposure. So, two scenarios.
Let's start on the No side. So let's draw our skin again, so this is just as before. Let's draw some TB protein in here. And what's gonna happen is you're gonna have some macrophages.
These macrophages are gonna come around, and they're always kind of patrolling the area. They're making sure that, almost like police officers, making sure that there's no problem. And they're gonna come, and they're gonna pick up some of this TB protein, so they're gonna take it inside of themselves, and they're gonna present that TB protein to another cell. This is our T-cell.
So they're gonna present this TB protein to the T-cell, and the T-cell's gonna say, "You know, I have had no prior TB exposure. "I don't recognize this TB protein," and it's gonna kind of go on its merry way. It's not gonna make a big deal about what's going on. So the T-cell kind of meanders away, it leaves the area. These proteins, over time, start to get kind of chewed up and digested by macrophages, and eventually all of it is gone, and if you look on the outside, you see flatness. The skin looks nice and flat, and this might seem very obvious from this picture that, of course, it looks flat, but that's essentially what we're looking at here, is flatness. And, looking at this picture, you can see a little bit of redness here, but 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. And so this person we would say, if there was 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, same setup as before. Let's draw this skin, and we've got some TB protein. Let me draw that in here, and this TB protein is gonna get picked up, as before, by the macrophage.
The macrophage is gonna come by, pick up some of this TB protein, and it's gonna, just as before, it's gonna find a T-cell. And these T-cells are also kind of moving around and this T-cell, this time, is gonna say, "You know what? I have seen this TB protein before." And this T-cell is gonna start getting very excited. And this is the key difference, right? It's gonna get excited, before it didn't get excited. It just kind of left the area, unexcited, but 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, "Ah-ha! "Interesting that 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, this is actually gonna start looking like this. Full of cells, right? On both sides, so 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, but you get the idea. Lots and lots of cells. And we call this a Hypersensitivity Reaction. So 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 four, Type 4.
And 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. And 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.
This is not flat at all. This is actually bulging out, and we call this Indurated. Indurated, meaning it feels very firm, and it does not feel flat. So this is what you're seeing in the picture here is that you actually can see from here to here, there's induration. So, that's the induration. They're actually measuring it with a ruler.
So this is the Induration. And 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're 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. And 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 that you're gonna be reading these PPDs 48 to 72 hours after you initially injected the protein.
So 42 to 72 hours later, this is when you actually read the PPD, and that's very important cause that gives enough time to either go flat like this, or to actually get indurated like that. And one key point I wanna make is let's say you've got redness all the way around here. Do you actually wanna measure the redness? No! You wanna measure the induration. So just keep that in mind: induration, not redness.
Not redness, because they're very, very easily confused for one another, but it makes a big difference. Not redness.
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