Sens/spec, +/-LRs, PPV/NPVs

David Meyers dm0015 at COMCAST.NET
Mon Sep 10 11:26:28 UTC 2018

Dr Yuen writes again:

“Not sure if this was posted-repost:

Thanks for the suggestion-including Dr. Brush's direct reply,

However, I had a copy of the book from a few years ago and read it several times before posting.  In particular I combed through chapter 4 ("Decision Making: Making Choices) - extremely well written and helpful - just not for the specific questions I (and my residents) had. In the book John makes the excellent point of how pre-test probably affects our interpretation of test results and the role of sensitivity and specificity and introduces likelihood ratios and Bayesian reasoning-all concepts I taught during my lecture (citing his book of course!)

However, he doesn't specifically address the following questions and I can't seem to find an answer to [them].  Again I understand the mathematical differences, but I am just questioning the practical differences.  For example, specifically:

1. Are there any tests for which specificity is NOT directly related to a positive likelihood ratio (+LR) and positive predictive value (PPV)?  If not, then a very specific test ROUGHLY means a test with a high PPV, ROUGHLY means high +LR.  In my residents mind (and my own actually) then, there really isn't any difference in knowing the subtle nuances between those numbers-a positive test would mean that the patient very likely has the condition tested for regardless of which statistic you looked up. And yes, if you use exact numbers there certainly are numeric differences in the actual probabilities, but for a busy clinician it all "means about the same thing".

2. Are there instances when using Sensitivity/specificity vs LR vs PPV/NPV results in an incorrect conclusion?  In other words, are there specific cases when interpreting a test result using a test's published sensitivity would lead you to a different conclusion than if you looked up the -LR? Again if not, for the typical learner- I understand why it seems like a "waste of time" trying to figure out the differences between all these terms if they roughly lead one to the same conclusion.

3. When would you preferentially use specificity vs. LR vs PPV?  If the resident has a patient with a moderate pretest clinical probability of acute myocardial infarction (AMI) and the high sensitivity troponin is negative- does it really matter if they refer to the test's LR vs NPV vs sensitivity?  They all seem to lead to the same clinical conclusion?

I apologize if I am misunderstanding an elementary concept and these seem like silly questions - but as I am asked these questions I really didn't have any good answer and I don't feel any closer to a satisfactory answer than 2 days ago. Again I can point out the statistical and mathematical differences in calculating these values- but to me they all seem to "tell the same story"

Your thoughts appreciated

Moderator: David Meyers, Board Member, Society to Improve Diagnosis in Medicine

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