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Current time:0:00Total duration:8:37

Video transcript

I wanted to talk about the Mantoux test and this is spelled ma n tou X kind of an interesting spelling after a French doctor who popularized it and another way you might hear this referred to as a PPD or a TST and what these things stand for is the 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 OPA that in parentheses and the the location of the test is actually also going to be kind of a clue here with TST this is a tuberculoma again referring to tuberculosis tuberculous this tells you where we're going to put all that protein right we're going to put it in this skin and you may have seen this and this is kind of a picture right here someone doing the test sometimes this refers to as kind of the bubble test a 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 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 later 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 on the right there and that needle is full of some TB protein so this purified protein derivative is in that needle is that you're 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 1/10 of a milliliter so 0.1 milliliters and you're putting it intradermally intradermally 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 going to notice is that be as you put a little volume in there a little 0.1 MLS that is going to bubble up right 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 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 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 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 nose 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 going to happen is you're going to have some macrophages these macrophages are going to 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 going to come and they're going to pick up some of this TB protein so they're going to take it in side of themselves and they're going to present that TB protein to another cell this is our t cell so they're going to present this T V 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 kind of 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 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 thing 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 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 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 going to get picked up as before by the macrophage so the macrophage is going to come by pick up some of this TB protein and it's going to just as before it's going to find a T cell and these T cells are also kind of moving around and 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 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 aha 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 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 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 4 type 4 and the reason that they're there typed out differently is that for 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 right 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 in duration 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 going to be reading these PPD's 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 because that gives enough time to either go flat like this or to actually get indurated like that and one key point I want to make is that 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 in duration so just keep that in mind in duration not redness not redness because they're very very easily confused for one another but it makes a big difference not redness