Sens/spec, +/-LRs, PPV/NPVs
lehmann at JHMI.EDU
Wed Sep 12 01:00:28 UTC 2018
Please don't forget to discuss thresholds. Prevalence, along with LR+ or LR-, gives positive/negative PV (=posterior)), don't mean anything without an honest discussion of thresholds (whether it's the threshold for "pay attention" or for "test" or for "treat" (without testing). For instance, I asked residents for 20 years, what's their lowest absolute probability of bacterial meningitis for getting an LP on a one year home with a fever (or the highest probability to send the infant home), and the mode was 1/1,000, with a range of 1/1,000,000 (this resident was a lawyer before she was a doctor) and 1/10 (we educated this resident that she was wrong, using the other residents' responses as data).
I know there has been research done on the failure of thresholds for explaining variability in performance, but it's got to be useful in communication. (Needs to be tested. Still. Or have I missed the paper?)
On Sep 11, 2018, at 6:52 PM, Tom Yuen <0000001243181998-dmarc-request at LIST.IMPROVEDIAGNOSIS.ORG<mailto:0000001243181998-dmarc-request at LIST.IMPROVEDIAGNOSIS.ORG>> wrote:
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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