In light of the discussion a few weeks ago about the stability of
sensitivity and specificity in different populations, and a very nice
review of diagnostic test parameters by DLS in acp journal club this
week, I have questions about the use of likelihood ratios and nomograms
of the sort cited from Fagan. If sensitivity and specificity of a given
test varies in different populations (where spectrum of disease and
confounding conditions will vary), couldn't it be misleading to take
likelihood ratios derived from a broad population (e.g. ferritin levels
from a consecutive series of patients evaluated for anemia in your
clinic) and apply them to a specific patient for whom one has estimated
pretest probability of anemia based on clinical judgment (e.g.
menstruating young female complaining of fatigue)? Don't we need to
know sensitivity and specificity of a given ferritin level in a
population of menstruating women with fatigue, at which point we can
calculate post-test probability without relying on estimates of pretest
probability and likelihood ratios? It seems that likelihood ratios rely
on the assumption that sensitivity and specificity are constant across
distint spectrums of patients and disease severity, which last weeks
discussion implied was demonstrably not the case. Any thoughts?
(Please note new street address and phone #)
David Atkins, MD, MPH
Center for Practice and Technology Assessment
Agency for Health Care Policy and Research
6010 Executive Blvd, Suite 300
Rockville, MD
(301) 594-4016
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