Prevalence Drives Diagnosis: Pathophysiology, Risk Factors & Time Frames Explained

for every disease process you've faithfully memorized the list of risk factors and you have this sort of intuitive sense that you know smokers get lung disease smokers get cancer we get how that works but remember what our overarching theme is clinical medicine is built on a foundation of basic medical sciences so we know these risk factors work right lack of activity leads to atherosclerosis

smoking lung cancer we can do that but there is basic sciences underlying that and so if the purpose of the exam is not the what but the why then ask yourself why are these risk factors so powerful in the diagnostic process and it comes down to a single mathematical biostats word prevalence prevalence is this idea

that is so deep in the diagnostic process it has to become a way of thinking if you're new here I'm Doctor Philip Tisdale I practiced for 40 years I've been teaching well over a decade and I wrote a book and we're developing a community of people who are going to learn how to do medical school so that exams are not only unstressful they're gonna carry through

and make you a great clinical clerk so hit subscribe we've talked in previous videos of how all diagnostic medicine is a chief complaint from which we can form a differential diagnosis and we've talked about this idea that differential diagnosis is not the shotgun blast of a bunch of possible diagnoses and you hope for divine inspiration that one will light up like day glow doesn't work like that

you start with a differential that says which organ system could I be in shortness of breath is heart lung now stop for a sec I'm not saying every possible diagnosis that's just hopeless who can learn 400 things it's always 80 20 80% of all diseases come from a handful of diagnoses those are the ones we carry in our head

so in your mind when you're learning you're saying what am I gonna keep in my head what is the long list you go look that up so when I say chief complaint and you say differential you start with which organ systems shortness of breath is heart and lung from that we develop this idea of which family of diseases are we in but disease is a floppy term that we use lots and lots of different ways

what we mean here is a specific tight view of pathophysiology so when I say shortness of breath heart it's a very specific pathophysiology it is the pathophysiology of left ventricular congestive heart failure and I'm clearly saying that with some meaning and I can say it with some confidence cause I've got a really tight picture in my head when I say that

so now we come to this idea of path of physiology is a really tight way of looking with that disease with a really tight image and a really good story to tell through it and if that works well if it's a disease that lends itself to a good path of physiology you can do symptoms you can do the physical exam and all of your testing inevitably have to come out of that picture

so now you're not learning a bunch of lists now you've got a single picture with a story and when someone asks you about that you flip that up into your head and you say where am I in this picture that's what we mean by pathophysiology one of the surprising elements of this is not only what you do find positive findings of symptoms and signs and tests

it also tells you what you don't expect to find no better example here one of my favorite questions is acute cholangitis because when you say tell me the pathophysiology of acute cholangitis you know what I'm gonna say oh two phases not one phase 2 phases first phase is obstruction with cramping second phase is gonna be breakdown and inflammation

so when we say first phase just obstruction guess what there's no inflammation there's no physical findings so yes colic you know we've got this colicky epigastric pain but your physical exam is negative that's important if it's positive it's not first phase of a Q colic cystitis that doesn't have inflammation yet yes second phase when everything starts to break down and get inflammation now we see the pain move

so now it goes from epigastric in the center yeah now it's going to run up a quadrant I've got an inflamed peritoneum oh yeah everyone can tell you Murphy's sign now well now of a sudden we've gone from should not have physical findings to should have physical findings from a single simple picture of pathophysiology so once you have a good picture of pathophysiology now of a sudden you can say

what would I expect to find for symptoms signs and testing no better example than if we go for example to chest pain now if you ask me what my picture is no kidding I'm gonna use a transverse cat scan C5 of the thorax I really am if you've got that picture in your head and yes it's a standard picture to draw you're going to have from the outside in chest wall pleura pericardium

aorta and then what we're really looking for is angina ischemia of the heart okay so once I say inflamed chest wall inflamed pleura inflamed pericardium then you go what are you gonna find well you're gonna find something with the pain chest wall hurts when you push pleuritic pericardium give you rubs great what are you going to find from the aorta the answer is well

you can't see it and you can't hear it so I guess you gotta image it okay well once you have that picture that's pretty obvious tell me about the angina well here's what's important it's negative physical exam it's not chest wall pain it's not a rub can you see it or feel it well no you can't see or feel the heart so I guess we've got to have testing well

is it going to be lab or imaging the answer is well not imaging it's got to be lab so I need two I need the EKG for ST depression ischemia and I need evidence plus or minus of necrosis so I guess I'm gonna need troponins so look how fast the story unravels I've got chest pain I expect you to tell me the physical exam I need negatives on the physical exam

it's not chest wall rubs I'm gonna have to in certain circumstances have imaging to look for the aneurysm of my of my aorta but if it's hard I gotta have an EKG and troponins any questions gotta give you that because it's gotta be part of the diagnostic processes all from a nice tight mental image of the differential diagnosis of chest pain so when we say chief complaint gives you a differential diagnosis

for each possibility in that differential diagnosis you've got to have a really good pathophysiology because that tells you what you can expect from the history and the physical and if testing's gonna be needed and it includes the negative findings so if I say to you edema what are your organ systems look what we're gonna come up with we're gonna come up with liver cirrhosis

we're gonna come up with heart right ventricular congestive heart failure we're gonna come up with kidney chronic kidney disease stage 4 and we're gonna come up with kidney nephrotic syndrome do you think when I say those words I have a little picture in my mind to run a story through I'm just waiting at that point for you to tell me which are gonna come from history which are gonna come from physical

but if it's an erotic system I guess I got a dip for the protein don't I all from a differential of four each with its own patho phys okay so we've developed this idea of patho physiology if I've got late cholecystitis I have to have a positive physical exam when the fundus of the peritoneum rubs under my fingertip it's got to be there

do I have to have a stone sure 95% are obstructing so you tell me I got a gallbladder with a stone and a positive Murphy sign I'm done I've got acute cholecystitis that's really different than saying I've got a female with multiple children who's overweight and she's 44 years old right because if you take a look at all 44 year old multiparous women in United States

15% of them have gallstones yeah that hasn't given them a cholecystitis yet has it it just tells you they might have it so look at the difference now I've got a stone impacted in the cystic duct I've got obstruction that's patho physiological so let's look at the difference between pathophysiology and risk factors if I have a gallstone that's only a risk for obstruction

if that gallstone is impacted in the cystic duct I do have obstruction so a stone impacting is patho physiology a stone in the gallbladder is only a risk factor quite a big difference so when we're telling our story and we say what is the likelihood of gallbladder disease you say well

the likelihood goes up if they are over 40 if they are female if they've had multiple children right and if they are overweight you know the cheeky 4 FS but none of those make your patient have acute cholecystitis that epigastric pain could be heart right but the second I tell you I have an ultrasound with an impacted stone and edema of the gallbladder wall

now I'm into pathophysiology not risk factors one is gonna be diagnostic the other only changes the likelihood of where you're gonna go next to prove what you suspect so a risk factor is not truly pathophysiologic it only increases the likelihood and there's a mathematical basis for that it's this infinitely important idea

predictive value positive is proportional to prevalence times sensitivity that's the biostats underlying why we talk about risk factors so let's talk about each of those it's it's so important we could talk about this every day and it would never get old predictive value positive says what

is the likelihood that this finding represents disease and the likelihood depends on prevalence and the sensitivity of the test yeah I can't remember the true definition I don't even try to remember the true definition when you say what is prevalence honest to God 40 years later this is what I put in my head if you have 100 normal people how many have the disease I I actually do say 100 I mean

obviously a larger number would be better I can handle that it served me well it'll always work prevalence equals from this population of 100 how many have disease so if I pulled 100 people off the sidewalk and said how many will I find gallstones in the answer is if they're middle aged 7% of males and 15% of females

sensitivity says if I use this test how many people with the disease will test positive so for example in Lupus Ana is 99% positive it's really sensitive if you don't know write in 95% most of the tests that we want to use in routine medicine are that good so you sort of start how do I think of sensitivity start at 95%

95% of the people with disease will test positive let's use my favorite example let's go to prostatic specific antigen PSA why not let's take two different people I'm gonna start with an 80 year old with a palpable nodule on his digital rectal exam and back pain he has a PSA of 400 normal

less than 5 nanograms per milliliter now I'm gonna have a 22 year old who has had unprotected sex with multiple females comes in with a fever says I feel like I'm sitting on a walnut and he has a soft tender body prosty he has a PSA of 402 exact same test isn't it exact same number let's just see what prevalence does

well over 80 right degenerative malignant somewhere in the order of 70 or 80% of males of that age will have adenocarcinoma he's got a palpable nodule sure cancers are mass forming give me another cause of a mass in the prostate and he's got back pain back pain is the first and commonest metastasis of prostate

and other than screening the commonest way it presents yeah what do you think old guy with a palpable nodule and back pain what do you think his PSA is from it's diagnostic of cancer yes we'll still biopsy to prove it belts and suspenders but I promise you if that biopsy is negative you just missed the cancer let's go over to our 22 year old who's in the absolute peak period for venereal disease

do you think gonococcal prostatitis causes acute inflammation and destructive tissue changes in the prostate oh I'll bet so do you think the that leaks into the blood and his PSA goes up of course it does if you came to me and said I'm gonna do a biopsy I'd say overruled you're not because the prevalence of his age and his lifestyle says that is an acute venereal prostatitis

notice same PSA all I've changed is prevalence prevalence drives diagnosis OK so we've established prevalence drives everything you should think of prevalence in a couple of different ways there's prevalences that are so common so overridingly important you build them into every case whether they're going to help you or not because it's just how we see it and that's your age gender

time frame so when you talk about age as a prevalence over 65 degenerative or malignant can they fall off a curb and break their leg of course they can but we're talking about how you think about it when you approach a chief complaint when we talk about middle aged women we talk about endocrin and autoimmune and when you're talking about someone 25 and under you're talking about hereditary congenital does it have to be nope

but we've sure moved the prevalences gender very straight forward right Gender's kind of a path of this isn't it male gets male reproductive female gets female reproductive but atherosclerosis is so prevalent in our society and females get that 20 year hiatus from their estrogen production so we always include male in anything that could be related to that because males are plus 1 prevalence for atherosclerosis

females are minus 1 prevalence for atherosclerosis so age gender obviously pretty important time frame okay you can argue time frame is pathophysiological I get it that's perfectly fair acute and chronic defines pathophysiological processes it's just so overwhelming important and it just kind of trips off the tongue when you say age gender time frame I include it with prevalence you want to argue it's path of his that's perfectly okay

but you always start every discussion with age gender time frame the other prevalence I kind of always include is where is the caregiver you know that's not a hard prevalence changer but it certainly gives you a feeling for how serious and urgent is this if a patient's been brought in an ambulance to an er very different than if they've made an appointment and they're seeing the physician in her office

so I always include where we do the care it's a good colorizer different than when we talk about disease specific so if you're talking about something like chest pain and we're talking about angina we know what the obvious risk factors for atherosclerosis are because that's gonna be cardiovascular disease they're gonna be diabetes hypertension uh hyperlipidemia

and then some other rare things like sedentary lifestyle so you're always going to include those as risk factors if you're talking about chest pain but there's also some important negatives remember who said how chest wall can give you chest pain well it's usually acute stress so you would mention have you done any new physical activity like go shovel that spring snow out in the walk and you're a clerk

and that's way beyond your fitness level well that would be an important positive or negative so when we talk about constitutional or ever present risk factors age gender time frame place of service when we talk about disease specific you gotta have a differential diagnosis and you gotta do the risk factors for every one of those entities positive or negative

so when we talk about prevalence the ones you always give and the ones you kind of got to know the differential to give if there's any single thing I can do to help you today that will change how you perform on tests and how medicine goes for you always and for the rest of your life it's understanding the difference between acute and chronic clearly I just can't overstate it so let's talk about it first off

there's this nasty little fact it has two different definitions and you have to know the setting there's clinical time frame and there's pathological time frame let's just get the pathology out of the way pathology defines acute as a process that has neutrophils and it defines chronic as processes that have the repair functions

of lymphocytes macrophages and going into fibroblasts so there's a pathological definition that correlates pretty well with clinical definition boy clinical definition actually has to have definitions and you've got to know it for each organ system so when I say liver when does that hepatitis B infection become chronic the answer is 6 months well OK let's go to kidney

when does that glomerular nephritis become chronic and the answer is 3 months well every organ's different isn't it okay so now you've got someone with joint pain and inflammation when are you allowed to mention the words rheumatoid arthritis yeah a joint isn't chronic until 6 weeks so this ability to go acute chronic matters heart long or what you'd expect right there about one week this is how tricky it can get

let's do diarrhea acute diarrhea is less than two weeks chronic more than four well apparently there's a little gap in there you kind of have to use judgment so when you start every disease process in your mind you're gonna have two definitions clinical and pathological but when you're in clinical remember it varies with every organ and if you don't know that you're gonna just be

shouting out the wrong diseases in your head right when we talk about diabetic kidney disease I promise you it's never acute you don't even allowed to think of it for five years and you don't start to think it's probable till 10 so understanding this time frame and the rhythm of diseases if there's any single thing you get from this today get your time frame right okay so let's summarize where we are in this process

we've talked about the absolute centrality of chief complaint and differential we've talked about how every differential has to have a nice tight pathophysiology with a storyline and now I said yeah and then after you do all those now we have to say show me the prevalences for each of those entities in the differential diagnosis they don't prove the diagnosis

like a patho phys finding but they set the stage for likelihood and we've talked about how if you have a good set of prevalences for your differential entity remember it includes negatives because you got this picture of pathosis in your head don't you okay so once you've done that and you've got your prevalences then don't forget you're going to break them into the ones you always give age gender time frame

and the ones that are actually differential specific which are going to be your risk factors and you sort of go okay well I get it how does medicine talk about them and really that's what they mean whenever they do epidemiology and frankly you know when I was writing the book you'll see me flip flop around you know risk factors epidemiology sometimes I put them with the history but sometimes I put them with the pathophysiology

you know these aren't bright lines you go oh it belongs here medicine is always about what's working in this setting so get your prevalences right they go in the second half of your history of present illness and you'll use them in the exam it's your examiner telling you what she thinks you want to be thinking about for the diagnosis so they're very revealing by what you include and don't include

so epidemiology risk factors prevalence is one of the fundamental ideas of diagnosis so it's this kind of thinking that is being asked of you when you get a good clinical exam it's being how do you think about do you recognize when I'm talking about atherosclerosis you'd better be talking about diabetes hypertension lipids and and life activity

because once you read those in the history you know where the person's going so learn this and the suddenly all those exam questions you can see the mind of the examiner it's how every doctor thinks so you can appreciate I come out of decades of practice and and I'm over a decade of teaching and I'm really trying to build an online community where students can come in and get this sort of

you feel like you wanna study after you hear this lecture don't you well after you study come on back and we'll talk about it because you'll be surprised how talking about something really settles the ideas in your head the other day I was talking to some students and we went to female reproductive hypothalamus and we got into pulsatile signaling that eventually LED us over to pulsatile PTH

and over to how I think of the drugs of osteoporosis well I promise you that wasn't expected when you came to learn female physio but boy all of a sudden you're suddenly pulling out osteoporosis out of the blue and giving it a good shake and a look see that's how you learn and make things sticky is not just learning them and not just doing multiple choice questions come to a community that we're building

that's going to include this ability to talk medicine if you found this video helpful come and watch this previous video and when I introduce deeply the subject of chief complaint and differential diagnosis it's clearly a perfect companion for today's topic