Screening In Medicine Flashcards by Emily Cross

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?)

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4 possible outcomes to screening

true/false positive

true/false negative

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true positive

the test correctly reports a positive result

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true negative

the test correctly reports a negative result

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false positive

the test incorrectly reports a positive result when the patient does not have disease

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false negative

the test incorrectly reports a negative result when the patient does have disease

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describing the accuracy of a screening test

sensitivity

2. specificity

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sensitivity

how well a test can detect presence of disease when it is present

positivity of test in the diseased

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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 %

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specificity

how well a test can detect absence of disease when it is absent

negativity of test in healthy

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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

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calculate specificity

= TN / (TN+FP) x %
or
= D / (B+D) x %

TN/all non-diseased

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what calculations answer how accurate a test is?

sensitivity

specificity

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sensitivity and specificity are all about predicting ……?

accuracy of the test

not about predicting the patient’s disease state

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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

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PPV

how accurately a pos. test predicts the presence of disease

PPV = TP/(TP+FP)
= TP/all pos. tests

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NPV

how accurately a neg. test predicts the absence of disease

NPV = TN/(TN+FN)
or
= TN/all neg. tests

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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.

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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

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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

diagnostic accuracy | also called 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

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