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Interpreting the PPD

Find out how the cutoff for a "positive PPD" depends on clinical clues. 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

Voiceover: Let's say you plant a PPD and a person comes back in, and now they want to find out whether it's positive or negative. The way I kind of think about it is I break people down into their risk group. I think of 3 risk groups. I think high risk, medium risk, and low risk. If they're not high or medium, then they basically by default become low risk. That's how it works out. Going to the high risk, there's 2 ways to think about it. Either I'm really worried that they are going to have TB, and that could be someone who I suspect has it, maybe they have a very abnormal chest x-ray, or maybe they lots of fevers, cough, night sweats, things like that and I really suspect TB. They would fall in the high risk, right? Because I worry that they have it. In fact, even their family members, close contacts, let's say a grandchild of a person who has TB, or and uncle or an auntie of somebody who has TB, someone in the household, they are also at high risk of getting TB. These are folks I'm worried may have tuberculosis, but also in this high risk group I'm going to put a third group, or a third category which is immunocompromised people. This is people that do not have a normal immune system so these folks it's not that I'm worried that they have TB necessarily, but it's that if they did have TB, gosh, that would be really awful because tuberculosis requires that you have a good immune system, and if they don't have a good immune system then tuberculosis is going to be devasting for these folks especially folks with HIV. In this first group then, the high risk group, either I'm really worried that they have it, or I'm worried that if they did get it they would get really sick with it. For these folks, a positive test would be induration, remember in PPD you're measuring induration anything above 5 millimeters, anything above this range would be considered positive. For example, if someone with HIV comes and they have a PPD of, let's say, 6 millimeters then they would definitely be positive, but if it was 3, then they would be negative. This red stuff that I'm kind of filling in, this is the positive range, positive anywhere in here. Anything below that, I'm going to write it in white down here. This would be negative, anything below 5. If it's exactly 5, then that's still positive. Who's in the medium range? These folks. One group would be people coming from an endemic country. Let's say they recently came across from China or parts of Africa. If they're coming from a country where tuberculosis is very common or endemic, then they are also in that medium risk group. Also folks that live or work in large groups. Think about nursing homes, or let's say, a jail or an army barracks. Anyone that lives or works in these kinds of places in a large group setting, I'll say large group setting, is at risk for getting TB because, of course, it spreads from person to person very quickly. If you're around a lot of people, you're upping your chances. Similarly, people that are in health care so anyone working in a hospital setting because, of course, people with TB often come to health care settings to be take care of. If you're in a health care setting working as a nurse or a doctor, then you also are in the medium risk. I, myself, because I'm a physician, am in the medium risk for this reason. I'll put a few more up here; IV drug use. This also puts you in the medium risk. Then I'm going to actually switch gears, and just as before where I said I'm also worried, specifically worried about certain groups, here I'm very worried about children less than 4 years of age. If you're less than 4 years of age meaning if you're 3, or 2, or 1, those children they may not have any other reason for getting TB like the people I just mentioned like health care setting or endemic country where it's around, but if they did get it, it would be particularly problematic because young kids don't have great immune systems. Also, people that are actually fighting other diseases. We call those comorbidities. Let's say you have already gotten a disease like diabetes, or let's say your kidneys aren't working properly, if you already have other diseases then you're also at risk because if you did get TB, it would be even tougher to fight it off. For all of these folks, 10 millimeters or above is considered positive. I didn't make an exhaustive list. There actually are a few other ones, but I tried to highlight some of the most important ones on this list. For example, just to make sure we understand how to utilize this, let's say you have a person who is 3 years old and that person comes in with a PPD of 12 millimeters. That would be positive here, but if it was only 8 millimeters, it would be negative so that's how you would use this graph. So down here then we have anyone else, anyone that doesn't fit into the other categories fits into this final one so this is the low risk category. For these folks, 15 millimeters or greater are needed to be considered positive. If you have someone that is very healthy, and doesn't fit into any of the other things that we just talked about and their PPD is 20 millimeters, then they would be considered positive, but if it was only, let's say, 13 millimeters somewhere in this other range down here, then they would be considered negative. You basically measure the induration, and then you kind of think about what risk group a person falls into; high, medium, or low, and then you kind of organize it using these graphs, very easy, right? So above 5, or above 10, or above 15 tells you which is considered positive versus negative. A couple of questions people always ask is they think, "What if someone falls into 2 of these categories?" What if they're, let's say, below 4 years old, let's say we're talking about a 3 year old so they have this one, but you're also really worried that they have TB so they're in this one. If you ever have someone that falls into 2 categories, you have to go with the more conservative approach, and say if they meet the high risk category, I'm going to use that one to decide if they're positive or negative. In this case, a 3 year old who I suspect has TB automatically I would have to go with this top one because I suspect TB simple as that. The other question that often comes up is what about BCG vaccine? This is a vaccine that some countries offer to their citizens to help prevent disseminated or spreadout TB especially a problem among young babies. What if someone has had BCG vaccine? It's very clear. The Centers for Disease Control has been very clear on this point. They say, "You do not need to interpret the PPD "with this in mind. Ignore the BCG vaccine completely." In other words, when you're trying to figure out whether someone's PPD is positive or negative, just rely on the high, medium, and low risk criteria that we've talked about. Do not worry about the BCG vaccine. That should not affect your decision.