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

Ted E Palen Ted.E.Palen at KP.ORG
Wed Sep 12 15:43:24 UTC 2018

Hello

In discussing sensitivity, specificity, NPV, and PPV you must remember that the equation for each of these values only includes part of the population being tested while LR includes the results of all members of the population being tested.

Do demonstrate:

[cid:f117e422-3bd8-434a-9bfb-83ab0024cef6]

Notice the Sensitivity calculation only includes the population with the disease/condition while Specificity only includes those without the disease/condition.

The PPV only includes patients testing positive while NPV only includes patients testing negative.

However, the LR+ and the LR- use the entire population, both those with and without the disease/condition and who test positive and test negative, as seen by the equations:

[LR+={\frac  {{\text{sensitivity}}}{1-{\text{specificity}}}}]

[LR-={\frac  {1-{\text{sensitivity}}}{{\text{specificity}}}}]

That is why the LR is a more powerful tool, it takes into account the whole population being tested and those with and without disease/condition.

[1498684070639_Ted]
Ted E. Palen, PhD, MD, MSPH
Physician Reviewer Utilization Management Department
Physician Investigator
Institute for Health Research

10065 E. Harvard Ave., Suite 300
Denver, CO  80231

Office: 303-614-1215
Cell: 303-514-8126
Email: Ted.E.Palen at kp.org<mailto:Ted.E.Palen at kp.org>

Assistant: Hilda Medina
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Hilda.Medina at kp.org<mailto:Hilda.Medina at kp.org>

________________________________
From: Tom Yuen <0000001243181998-dmarc-request at LIST.IMPROVEDIAGNOSIS.ORG>
Sent: Tuesday, September 11, 2018 4:52:51 PM
To: IMPROVEDX at LIST.IMPROVEDIAGNOSIS.ORG
Subject: Re: [IMPROVEDX] Sens/spec, +/-LRs, PPV/NPVs

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Many thanks to everyone's reply, especially from Dr Brush and John Ely for his thoughtful response.

I think the bottom line that my residents needed to answer was basically if there was any difference and/or advantage between sensitivity/specificity and +LR/-LR.  PPV/NPV, as John (Ely) pointed out is dependent on disease prevalence.

But it seems to me, based on everyone's reply is that there doesn't seem to be much PRACTICAL difference between sens/spec and LR.  Just different ways of expressing essentially the same thing. (if I have this horribly wrong please correct me as I am giving a followup lecture Thursday and will be addressing the questions they have last week!)

Essentially a test with a high +LR and a high specificity- if positive in a patient with a moderate pretest prob of a disease, dramatically increases your posttest prob of them truly having the disease.  I think my residents were struggling (as was I) with whether it mattered which statistic (LR vs. spec/sens) we looked at- and it doesn't seem to matter.

Thank you again everyone, while I am a first-time poster I have been following this listserv for years and am humbled by the collective wisdom assembled here.

Tom Yuen, MD

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