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Mantoux test (aka. PPD or TST)

Learn how the Mantoux test is done and how it works. 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 Stanford School of Medicine.

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Video transcript

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.