forgive the length but i mislead gp-uk and nobody picked me up on it :-o
adrian wrote:
> One of my patients was distinctly miffed that the FOB he paid for as
> part of his screen at the Nuffield didn't pick up his colon cancer.
>
> It was only one test, and AFAIK the proponents of FOB as screening use
> three tests.
>
> THere does need to be attention to the power of these
> investigations,
apropos an earlier comment i made in reply to mary:
> >if the pre-test probability is low the positive predictive
> value of the
> >test plummets and the test becomes a liability
> Owen Dempsey
> Would a negative test encourage you to prescribe if the pre-test
> probability was high?
> Mary
i said the specificity (ability of the test to detect the true
negatives) would rise
however i scurried back to my books
and i was wrong; suprising but true.......knew it didn't sound right
its interesting (to me); but the sensitivity and specificity of a test
(positivity in disease, and negativity in health) are unaffected by
prevalence: however the negative predictive value, (the post-test
probability of being healthy if you have a negative test), actually
falls if the prevalence rises, so the answer to mary's question is that
it depends on how high the prevalence is; eg
for a test with 95% sens and spec and for a prevalence of 50% the
negative predictive value is 95%; quite helpful you may think........
however:
if the prevalence is 95% the negative predictive value is only 50%; and
illuminatingly the complement to this the posterior probability of
disease following the negative test result is also 50%
clinically the test wouldn't seem to be adding much to your decision
making and could easily be misleading; and falsely reassuring
took me a while to come to grips with this yesterday; enjoyed it;
patients obviously will have to rely on (trust?) us when it comes to
advising on these matters; the BUPA screening methods are highly
unethical IMHO
lecture over (sacketts big book pp90-91)
regards all
owen dempsey
GP
West Yorks
UK
'trained to move at a slow and loping pace'
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|