Screening In Medicine Flashcards
2 questions that patients should ask their doctor when a medical screening test is recommended
- how accurate is the test?
2. how confident are you in the results ? (that its a true positive?)
4 possible outcomes to screening
true/false positive
true/false negative
TP
true positive
the test correctly reports a positive result
TN
true negative
the test correctly reports a negative result
FP
false positive
the test incorrectly reports a positive result when the patient does not have disease
FN
false negative
the test incorrectly reports a negative result when the patient does have disease
describing the accuracy of a screening test
- sensitivity
2. specificity
sensitivity
how well a test can detect presence of disease when it is present
positivity of test in the diseased
relate sensitivity and proportion
proportion of time that a test is positive in a patient that does have disease
= TP / (TP+FN) x %
or
= A/A+C x %
specificity
how well a test can detect absence of disease when it is absent
negativity of test in healthy
relate specificity and proportion
proportion of time that a test is negative when patient does not have disease
a highly specific test has a low FP rate
calculate specificity
= TN / (TN+FP) x %
or
= D / (B+D) x %
TN/all non-diseased
what calculations answer how accurate a test is?
sensitivity
specificity
sensitivity and specificity are all about predicting ……?
accuracy of the test
not about predicting the patient’s disease state
factors to determine your confidence in the results of a test
- positive predictive values
- neg. predictive value
to describe the accuracy of prediction of disease
PPV
how accurately a pos. test predicts the presence of disease
PPV = TP/(TP+FP)
= TP/all pos. tests
NPV
how accurately a neg. test predicts the absence of disease
NPV = TN/(TN+FN)
or
= TN/all neg. tests
if community prevalence of a disease increases
of false neg./pos. can increase
PPV greatly increases
NPV slowly decreases
but sensitivity/specificity never changes because it’s about the test not the pop.
as you approach a prevalence of 100%
PPV will jump towards a 100% because any test can predict the disease since almost everyone has the disease
true negative/false negative
sensitivity
TP/(TP+FN)
true negative/false positive
specificity
TN/(TN+FP)
as sensitivity decreases, the _____ will decrease, why?
NPV
because FN is in both equations
as sensitivity decreases, the _____ will decrease, why?
NPV
because FN is in both equations
in what situation would the sensitivity/specificity and the PPV/NPV be the same for two different cities?
sensitivity/specificity is always the same because it’s about the test
PPV/NPV can only be the same if the prevalence of the cities is the same
DA
diagnostic accuracy
also called DP
DP
diagnostic precision
also called DA
define DA
proportion of total screenings that a patient is correctly identified as a TP or TN
DA equation
=(TP+TN) / (TP+FP+FN+TN)
or
= (TP+TN) / all patients
LR
likelihood ratio
define LR
ratio of the probability of a test result for a diseased person / divided by the probability of the same test result for a non-diseased person
calculate LR
equation depends on what you’re calculating for
positive or negative results
LR is a ratio of ______ .
2 probabilities
where the diseased persons always live in numerator
non-diseased always live in denominator
LR+
positive test in diseased / positive test in non-diseased
or
sensitivity/(1-specificity)
[(A/(A+C)) / (B/(B+D))]
LR+ is not
simple division
it is the division/ratio of 2 percentages
LR-
negative test in diseased / negative test in non-diseased
(1-sensitivity) / specificity
compare equations of LR+ vs. LR-
LR+ numerators A and B
sensitivity / (1-specificity)
LR- numerators C and D
(1-sensitivity) / specificity
what should our LR+/LR- results be in a good test?
+ should be large
- should be small
LR+/- results to demonstrate the test is beneficial
LR+ >10
LR-<0.1
a _____ test is always reliable, but a ______ test is not always valid.
valid
reliable
interpret a LR+ of 1.0
the likelihood ratio, positive of 1.0. tells us that the test is just as likely to positive in diseased as it is to be positive in non-diseased
a equal likelihood of a positive test in diseased and positive test in non-diseased
LR- describes negative in diseased and negative in non-diseased
define validity
ability to accurately discern between those who are diseased vs. non-diseased
validity is analogous to ?
precision
define internal validity
extent to which results accurately reflect the study population
define external validity
extent to which results are applicable to other populations
another term for external validity
generalizability
define reliability
ability of a test to give the same result on repeated uses
reliability is analogous to ?
reproducibility or consistency
what is the impact of modifying the cut-off value on the sensitivity and specificity of a test?
increasing cutoff increases false negatives (increases specificity)
decreasing cutoff increases false positives (increases sensitivity)
when specificity is on the left
