# Physical Exam Likelihood Ratios

by Stephen Holt, MD, MS

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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.

The lecture Physical Exam Likelihood Ratios by Stephen Holt, MD, MS is from the course Introduction to Physical Examination.

### Included Quiz Questions

1. Pre-test probability, likelihood ratio, and post-test probability
2. Pre-test odds, likelihood ratio, and post-test odds
3. Pre-test odds, likelihood ratio, and post-test probability
4. Pre-test probability, prevalence, and post-test odds
5. Pre-test probability, prevalence, post-test probability
1. Pre-test probability and likelihood ratio
2. Incidence of the disease in the population
3. Likelihood ratio
4. Sensitivity of the test
5. Specificity of the test
1. (Pretest probability)/ (1 – pretest probability)
2. Divide the prevalence by 30%.
3. This parameter cannot be calculated.
4. Divide the % likelihood of them not having the disease by the % likelihood that they DO have the disease.
5. Multiply the prevalence by the pre-test odds.
1. 30%
2. 10%
3. 20%
4. 40%
5. 50%

### Author of lecture Physical Exam Likelihood Ratios ### Customer reviews

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This is a must to learn before taking any physical examination
By Quang Linh V. on 15. December 2021 for Physical Exam Likelihood Ratios

The lecture shows me the roots of understanding how to apply evidence based to physical examination