[Music]
hello and welcome to epidemiology are
you a medical student or a doctor or
some other kind of clinician or maybe a
health scientist of some kind either way
you probably know that by the time a
patient comes to see you and is showing
symptoms they've already got a disease
state that's gone beyond a certain
progression and it would have been nice
probably to have been able to detect
them before there is symptomatic but how
do you identify a potential patient
before they're showing symptoms that's
why we have screening tests you probably
know about screening test screening
tests for things like taking people's
blood pressures in the mall or even
digital rectal exams or pap smears in in
the office these are kinds of tests we
do to identify people for later
investigation so in this lecture we are
going to learn some things about how to
assess the qualities of screening tests
epidemiologically you're going to learn
how to set up a contingency table that's
how we put our numbers together to
compute the appropriate indicators for
the quality of a screening test you're
going to be able to compute and
interpret certain functions of a
screening test in particular the
sensitivity and specificity your all a
problem already heard those terms
probably for reading the descriptions of
tests like a pregnancy test on side of
the box it has numbers relating to its
sensitivity and specificity you'll be
able to compute and interpret and the
positive predictive value and the
negative predictive value of a test and
also you'll be able to compute and
interpret this thing we called the
likelihood ratio so screening tests are
meant to identify individuals who are
probably in the early stages of disease
and that allows us to have an earlier
intervention that we assume gives them a
better chance of being cured or to
improve their prognosis in some sense
things that affect the ability of a
screen test to be useful include its
potential to over diagnose an issue if
you or diagnosed people you're finding
people who don't actually have the
disease but you think they do and that
causes some stress that causes financial
burns on a healthcare system and so
forth
sometimes a screening test can miss
diagnose a situation and that's
confusing for a lot of different reasons
sometimes a test itself could
painful or damaging we don't like that
we try to afford those situations and
sometimes a screening test doesn't offer
any ability to help a patient by which I
mean some diseases there is no advantage
to finding out if someone has it earlier
that doesn't change the prognosis any
maybe you could argue that
psychologically it's useful for a
patient to know if they have a disease
earlier rather than later but
functionally and practically and from
the perspective of public health
officials it's not that advantageous and
a systematic level to do so an important
point is that screening tests are not
diagnostic tools they are meant to
identify individuals who are likely
deserving of a deeper investigation
what kind of deeper investigation a more
invasive probably expensive test so that
means that to compute the Diagnostics of
a screening test we need some objective
measurement of what constitutes truth or
real disease status a screening test
gives us a suspicion of disease status
we need another test a more invasive
expensive inaccurate test to confirm
that suspicion that's for the purposes
of computing our Diagnostics on everyday
basis we don't do this keep in mind if
someone test positive on a screening
test then they go for a deeper
investigation with a specialist probably
if it has negative they go along their
happy way never knowing anything more
about that disease state until an
opportunity arises again for them to be
tested this means that everything we
know about screening tests are in study
communities research programs only in
research programs or in studies do we
know what happens to those who test
negative and every day those who test
positive are followed up on so let's
think about how we follow up on certain
positive tests so a pap smear is usually
followed up with a biopsy if you test
positive and a pap smear there'll be a
deeper investigation to determine if
that positive test was real if you test
positive on a home pregnancy test you
think you're pregnant you probably get
an ultrasound to confirm that suspicion
if you test negative you probably won't
if you had a digital rectal exam you'd
probably get a biopsy as well if you
test positive on the I'm in the d-r-e
[Music]