Randy Neil Biostats 04: USMLE Step 1 Biostatistics - Prevention, The Box, Logic

Prevention before disease occurs. Classic examples include vaccination, counseling, and risk-factor reduction.

Screening or early detection of asymptomatic disease.

Treatment, rehabilitation, or complication reduction after disease is established.

PPV = TP / (TP + FP)

Answer: A) Primary

Vaccination prevents disease before onset, which makes it primary prevention.

Answer: B) Secondary

Screening tests detect early or asymptomatic disease.

Answer: C) PPV

PPV asks: given a positive test, what is the probability of true disease?

Alright guys, we get about four questions here. These were sent in by some people who just wanted to kind of have this reviewed, so I have no problem at all. The first one, essentially remember when you see a long problem, make sure you go down to the very last sentence because that's where the question is at, and that'll let you allow you to focus on what's important at the top. So, which of the following tests best describes this method of disease prevention. And of course, you're asking about this whole primary, secondary tertiary and the important area types. So, long story short, I mean, you have to know these basically for all the step exams. And we got into that primary and kind of a shout out to one of the guy who looked at the comments section for this. Primary prevents secondary, as I say, screens, and then tertiary, these are the main ones, tertiary treats. And so, an example of a primary prevention that's preventing the disease from occurring of course, that's going to be your vaccination, right? Disease is not existing. We're trying to make sure that that does not happen. The second year prevention is going to be your screening test, right? And an example of that one would be like a colonoscopy, right? Now, the key with that one is understanding that that can either prevent the disease or it will kind of decrease the morbidity of the disease, right? So, like a colonoscopy might have polyps and stuff, but you're just going to prevent it from getting to the point where there's actual symptoms. All right? So, primary prevention, there is nothing. It's going to prevent it from occurring. Secondary is going to screen for the test, and either prevent the disease or decrease more morbidity. And then tertiary is obviously going to treat and that's just going to be your basic medication. So, the disease is present, okay? And then this whole coordinate thing. You

can think of that as when they say that it's not spending money or any type of resources, but it wants something that really doesn't have that doesn't work per se. So, you have to know, these are your big three, okay? Primary, secondary and tertiary, primary prevents secondary screens, tertiary treats. Now, this question. Fitty-year-old may all presents to his primary care physician for routine work up. Your report says he's doing well overall without mother-sheem symptoms. His pastoral history is distinct for hypertension, which has been well controlled with a certain vital signs, did a, did a, did a, did a, there's respiratory pulse octomy. Everything looks good. So exam is not revealing any concern after maladies. The physician recommends a fecal occult blood test at this visit to screening, of course, are given you a nice little word there for the presence of any blood in the patient's stool. That might be suggestive of an underlying colorectal cancer, in which the following best describes this. So just the fact that they gave you, you know, they essentially gave you the key word for screening, and so you're going to jump all over secondary in that sense. But again, primary prevents secondary screens, tertiary treats. Okay? And now, this one says the incidence of chronic disease and a population may be decreased by which of the following. Now incidents, right? We talked about that in one of the longer, longer videos, lectures. That incidence has an end in it. So you always think incidence is going to mean the number of new cases. So which of these is going to prevent the number of new cases? Is it prolonging the lives of people with disease? No, because that's not going to, that's not going to help anything as far as the number of new cases. So it's definitely not that guy. Decreasing case fatality. No, at least people already have disease. We're trying to

prevent new cases, improving treatment of the disease. Once there's diagnosed, that's just going to prolong, right? It's going to increase like prevalence. And then, you know, excuse me, it's going to increase from improving the treatment. Improvement treatment. So technically people, I guess, in theory, could live longer because it could increase prevalence. Primary prevention, right? That's going to primary. That's going, like the example is you're vaccine, right? So that is going to prevent. So I kind of like that one. And then secondary, essentially that could possibly have the disease as already present. So the answer to choice in this to prevent the new is going to be primary. Again, primary prevents secondary screens, tertiary treats. Now this one says, what is the positive predictive value of the physician's exam? Now you know you're going to get an equation on this positive value, negative value, sensitivity specificity. When you see this, we talked about it. You always draw the box. And this is more of a review. But it's important because you're going to get that question hands down. So you always put reality on top. And you always put the test on the side. And it's positive, negative, positive, and negative. And then what does this question say? So we want the positive predictive value. So yeah, that's top left going to the right, correct? And so whatever we, and if you want, we can just kind of label these boxes. Whatever we circle goes on top. And whatever thing we connect with the arrow goes on the bottom. OK, so that is technically going to be our positive predictive value. But we got to fill in the box first, right? So where did they give us? This says a physician examined a population of 1,000 patients in an attempt to detect heart disease. They prevalence of heart disease and this population is going to be 15%. The sensitivity of physicians exam is

60% specificity is a patient's who test positive physician for the physician are sent by examination, sent for examination by the cardiologist, blah, blah, blah, blah. All right. So we got to, we can solve anything as long as we can fill in these boxes, right? So where did they give us? They say the prevalence of heart disease is known to be 15%. So that means how many people actually have it is 15. So in reality, how many people actually have it is going to be 15%. Now they say there's a thousand people in this thing. So if we wanted to go 15%, essentially say 150 people. All right. So we can say that and then, you know, n equals 1,000. So then how many people in reality don't have the disease? Well, we just have to subtract that, right? So that's going to be 850. Don't have it. So everybody in this column is going to equal 150. All right. So one step closer. Now it says the sensitivity of the physician's exam is 60%. So we know that sensitivity is top left going down. So in theory, 60% or remember in math, we don't like percent, we like to use desmos. So point six is equal to a, right? Top left going down. So it's a, it's what we circled over a plus c. The two boxes that we connected. And it says the specificity is 80%. Now we know a specificity is the bottom right going up. So the 80% or point eight equals d, because that's the box we circled. And then the boxes we connected with the line are going to be d plus b. But so what do we know so far? We've got to fill in these boxes, right? We said that this whole column equals 150 and that's a plus c. So a plus c equals 150. So in theory, I can just go ahead and, you know, I can just go ahead and put that guy right there and that's a point six. And that'll give me a and that's what I want, right? I want a. So I just got to solve this right here. So in theory, I can go point six times 150. And you know, I can, I guess I

can be lazy and go six tenths and start dividing. Divide by five. I get two divided by five. I get 30. I can go down 15. And that's going to give me a 90. Okay, a equals 90. And that's fine. And so now I'm left with this one. So I know that b plus d equals 850. Okay. And that's over d and that equals point eight. So it's kind of the same scenario over here. So I can go 850. I can go times point eight. So of course on the exam, you're actually going to use your calculator. But since we don't have that handy, I'm going to say 0.880 or 8.10. Okay. And I can divide by 10 and I get one and then I get 85. And then I have to kind of multiply that 85. So it's a 608. So 680. So 680 actually equals d, right? But I need b, right? I need b to make the positive predictive value. So how do I figure that out? Well this whole column equals 850. And this is 680. So then 850 minus 680. And I'm going to get 107, right? 170. So now, in theory, I can get every box in here. If I wanted this box right here, what would I do? I take 150 minus 60. I mean, 150 minus 90 and give me the 60. So once you have all the numbers for the boxes, I don't care if they gave you sensitivity, specificity, positive value or negative value. You'll be able to solve any of these and guarantee it on all your stepping exams. You're going to get questions like this. You just have to kind of see it enough and kind of play with it to understand how to solve these things. So in this situation, positive value is top left going to the right. So we're going to have top left is 90 divided by 90 plus 170. And that's going to give us what 260 or 90. And if we do all our math, it should come up to round about 34 or 346. Right. And that's going to give us answer choice. Okay. Now, that may seem like a lot. But at the end of the day, if we were to, if we would have just zeroed our focus and on these two boxes and did a little

bit quicker, I think we would have saved us a lot of time. But the good part about this problem is I don't care if you get any of these things, you're going to get them right. So it's better to take that extra 30 seconds, do the math. This is one of the few pieces of the step one or step two or step three exam where you know when you leave, you're going to get this question right. So make sure you study these. Now the last one is one, again, sent in by one of your comrades here. It says, it's always looked long like this, always go down the last two to the actual question and it says the recurrence, the recurrence rate on Santa Therapy is found to be 8% in order for the FDA to approve KM 28. What is the maximum incidence of the recurrent is acceptable for women treated with KM plus standard therapy R. So pretty wordy there. The key with this one, the reason I like this question is because at some point in this exam, if you don't know the formulas or for some reason it's escaping you just go with logic. Now if you were to get all technical on this problem, you know, AR, tribunal risk, reduction, minus the relative risk, okay, and that'll put everybody to sleep. But ask, what are their estimates here? They studies design to evaluate the efficacy of the new drug KM 28. The study will compare KM 28 plus standard care versus standard care alone, okay. So it's like if I add the struggle to standard care, is it any better than standard care alone? With regard to decreasing the incidence, number of new cases, of recurrent breast cancer. The FDA for the next generation will approve the new drug if KM 28 plus standard care decreases the rate of breast cancer recurrence by at least 40% compared to standard therapy alone. The recurrence rate of standard therapy is found to be 8%. Okay, so this appears to be a target. We just got to beat that target by 40%. In order for FDA to

approve it, okay. So blah, blah, blah, blah. So here's baseline, right? It's 8% and again in math, we don't like percents. We like to work with the decimals. The new thing has to do this plus 40% of that, right? So very simple. What's 40% of 0.08? Now if you don't know, you just kind of break. Yeah, times 0.4 times 0.08 or however you like to put on top of bottom, use the calculator. But at the end of the day, 32 and your three decimal spots. So it's going to 0.02 or 3.2%. But now that's not what the answer is, right? That's just the 40% of the 0.8, which we have to meet anyways. So now where I say where does this logic come in at? Is do you think that this new drug is going to increase that point? Is it going to be 0.08 plus the 0.032 or is it going to be less than? Because what they're saying is, if they just did nothing and use the standard of care, you got 0.08%. So if you're going to have a new drug, you sure is better have that thing go down, right? If better go less than 0.08, so you're going to take that point 0.03, which is 40% of it, and actually subtract it. Okay. You're going to subtract it from that and you're going to get 0.740. Right? There's our decimal. We termed Dessposed into 0%. Like that, and you should get 0.8, which is right there. But the whole key to this problem right here is just to understand. Sometimes if you can't remember formula is instead, try to use some basic logic. Here was our basic target. This thing, the new guy had to beat it by at least 40%, which gave us this, and we would have decreased it on down. So again, logic for that. So these are just a few of the questions that were sent in. Feel free to send some more. If I can at least get maybe 3 or 4, then I don't mind doing a quick video on it. But if you have any questions on these other topics, now that I'm kind of out of residency, I get plenty of time to work on some of this

stuff. But these are just some of the, you know, I think the sheets that I always had, kind of when I was studying for step one, and even in residency, I used them, but you can see there's first pharmacology, there's plenty of psychiatry, obviously. And then, of course, there's a nice, like the genetics one that's always kind of curing too. But these are the sheets. And what I did was I took them, you know, I kind of made these sheets, and so I could take them to a, I was at that panera wherever I was at, studying, they had a laminated, so if they got, they would never get wet or torn up or anything like that. But pretty much got every subject through here. So there's a subject you like, just put it in the comment box, maybe we can kind of review that for you. But hope this was helpful, and we'll see you in the next video.