Physical Exam Likelihood Ratios

00:00 For the next part of this talk, I want to talk about putting it altogether. Throughout this lecture series, we're going to be doing a lot of likelihood ratios and incorporating them into the physical exam that we're performing. So we'll talk a little bit about likelihood ratios now, how to combine different exam maneuvers, and how to hone your skills. So let's do that now. I will highlight here in this table referring back to the patient that started off our conversation who we were thinking about whether the patient had an ACL tear. That's our 2 particular exam maneuvers that we're going to show later in this course called the Lachman maneuver and the anterior drawer tests. And I'll highlight, circled here, the negative likelihood ratios for those tests are 0.2 and 0.5. So, what does that mean? So again, this 49-year-old man, left-sided anterior knee pain that injured it during a basketball game. If you just look at the historical information from that little case, in general we can guess that their pre test probability that this person has an ACL tear is around 20%. And that's, of course, going to be based on your own experience and also you can actually look at things like the general rational clinical exam series which will give you, based on historical details, what the probability is that somebody has a particular condition. So let's say 20% likelihood they have the disease. Well that means there is an 80% chance they don't have the disease. So the odds of having the disease is 1 in 4, 20/80. That's your pre test probability. You'll then take that pre test probability and just multiply the likelihood ratio that we had from the prior slide.

01:46 In that case, the negative likelihood ratio of the test is negative was 0.2. So 1/4 x 0.2 is 0.05. That's your post test odds and then you can reconvert that to your post test probability by simply taking your post test odds over 1 plus your post test odds and now while our pre test probability was 20% with 1 single test, we reduce the likelihood that this patient has an ACL tear to 4.8%. Now that was a relatively complicated series of steps that you'll get comfortable with once you've done it a few times. But if you don't want to go through that process every time, there are some cheat sheets, some quick ways to do this without having to do all the calculations. One would be you could carry around this likelihood ratio nomogram and maybe you don't want to carry around a likelihood ratio nomogram so there's even a faster way of doing this. But first on the nomogram let me highlight on the line. The vertical line on the left is your pre test probability, again in this case 20%. In the middle are a range of likelihood ratios and you would draw your line on the left from the pre test probability through the likelihood ratio of the particular test and you would end up getting a post test probability on the far right line as a result. But if you don't want to carry around a nomogram, you can just learn these few quick tips. So, a likelihood ratio of either 2, 5, or 10 correlates with an increase in the post test probability of about 15, 30, and 45%. Likewise, a negative likelihood ratio of 0.5, 0.2, or 0.1 correlates with a reduction in your pre test probability by about 15, 30, or 45. So that's a fairly easy straightforward way to remember just by grossly looking at a likelihood ratio how much the pre test probability is going to be modified up or down. So how are we going to combine these different maneuvers? I already suggested that for an ACL tear we have a Lachman's maneuver and we have an anterior drawer sign.

03:54 So, how are we going to put those things together? Well sometimes I like to harken back to this old metaphor of the elephant and the blind men. And maybe you've heard this before, but basically 3 blind men are told that there is a new animal that has been found and they're invited to go and examine this animal. So, the first blind man is around the elephant and all he feels is the trunk of the elephant and he says "Well, this animal is like a large snake." And the other blind man is just feeling the body of the elephant and he says "Well, this creature is basically like a wall and very difficult to move." And then the third blind man feels the tail and says "This animal is like a rope." And of course none of them are correct. The point here is that individual physical exam maneuvers rarely will give you the final diagnosis. It's important to put all of the exam findings together like a constellation of information that can actually help to flush out what the diagnosis is that you're looking at. Another way of thinking about this is a piano. A single likelihood ratio or single physical exam test is like one key on a piano, we're playing one note. And playing a single note on a piano doesn't really tell much of a story, but if you can play a full D sharp major 7 and all the notes that are involved in that, that tells a story, it tells a lot more, it flushes out what's going on. So, let's say for example you had a patient for whom you know they have advanced liver disease and you're not sure if they have ascites or not, let's say you set their pre test probability of 30%. Well if then you'll perform several exam maneuvers, say shifting dullness, you're looking for bulging flanks, peripheral edema and a fluid wave. If all those tests are negative, your pre test probability when you combine those maneuvers falls from 30% down to 2.5% which essentially excludes ascites. So, make sure that when you're doing the physical exam, you're not playing notes, you're playing chords. So how do we actually combine exam maneuvers? We simply multiply the likelihood ratios together. It's very easy. So for the Lachman's test of 0.2 and the anterior drawer of 0.5, we take those pre test odds, we multiply them by both of those likelihood ratios and end up with a new post test odds of 0.025.

06:08 We can tell our patient "You don't have an ACL tear and there's no reason to get an MRI at this point." Alright, so for the last section here I wanted to talk about how to hone your skills. First of all, complete this course. You've already begun, congratulations. Secondly, you're going to want to examine patients before and after utilizing technology. I'm not suggesting we abandon diagnostic imaging, it is an invaluable skill that has only gotten better with time. But when you examine the patient at the bedside and you think that their spleen is normal in size and then you get a CT scan for whatever reason and they have an 11 cm spleen, go back, re-examine that patient. This is the perfect person to really fine tune your splenomegaly examination skills on. And, I also encourage you as you're going through these lectures to sit next to a colleague or a peer and go through the course with a classmate. Watching me do these exam maneuvers is typically not sufficient. You need to actually go through the motor memory, moving your muscles around in space and examining your colleagues at least, if not patients, to really learn how to get these skills locked in to your memory. And I'll just wrap up by recommending a few sources that helped me become the clinician that I am today, not to mention my mentors who I learned from at the bedside, these are some textbooks and resources that I found to be invaluable. Joseph Sapira's Art and Science of Bedside Diagnosis, Steven McGee's Evidence-Based Physical Diagnosis which I draw from largely throughout this course, and then Gemma's Rational Clinical Exam series which has been running for decades and is an invaluable resource for this kind of up-to-date content. So in summary, the physical exam is invaluable and it's timeless. Once you learn it, you've got it forever. When examining, please remember to put your patient's comfort first. And lastly, know and apply the likelihood ratios so that you're practicing an evidence-based physical exam.