Screenings in Medicine Flashcards
In a 2x2 table for screening tests, what are the labels for the rows and columns?
Rows: screening test pos/neg
Columns: disease presence “truth”
What is a “true positive”? What box in a 2x2 table do these people live in?
Someone has the disease and the screening test comes back positive. Test is accurate so it is a “true” measure. Live in box “A”
What is a “true negative”? What box of the 2x2 table do these people live in?
Someone who doesn’t have the disease and the screening test comes back negative. The test is accurate so it is a “true” measure. Live in box “D”
What is a “false negative”? What box of the 2x2 table do these people live in?
When the screening test comes back negative when the person actually does have the disease. The test is not accurate. Live in box “C”
What is a “false positive”? What box of the 2x2 table do these people live in?
When the screening test comes back positive when the patient does not actually have the disease. The test is not accurate. Live in box “B”
What is the sensitivity of a screening test?
How well a test can detect the presence of disease when in fact disease IS present.
Proportion of time that a test is positive in a patient that does have disease.
If you have a highly sensitive screening test, what is the rate of false negatives?
Very low
How would you calculate the sensitivity of a screening test?
Sensitivity= TP/(all diseased)x100%
When measuring the sensitivity of a screening test, would you be analyzing rows or columns?
Columns
What is the specificity of screening tests?
How well a test can detect absence of disease when in fact the disease is absent.
Proportion of time that a test is negative in a patient that does NOT have the disease
A highly specific screening test has what kind of rate of false positives?
Very low
How would you measure the specificity of a screening test?
Specificity=TN/(all not diseased)x100%
What is the positive predictive value of a screening test?
How accurately a positive test predicts the presence of disease.
Measures the percentage of TP’s in patients with a positive test (correct predilection)
When measuring specificity of a screening test, are you considering the rows or columns of the 2x2 table?
Columns
How would you calculate the positive predictive value of a screening test?
PPV=TP/(all positive tests)x100%
When considering the positive predictive value of a screening test, are you looking at the rows or columns of a 2x2 box?
Rows
What is the negative predictive value of a screening test?
How accurately a negative test predicts the absence of disease.
Percentage of TN’s in patients with a negative test (correct prediction)
How would you calculate the negative predictive value or a screening test?
NPV=TN/(all negative tests)x100%
When considering the negative predictive value of a screening test, are you looking at the rows or columns of a 2x2 box?
Rows
What is the diagnostic accuracy (DA) or diagnostic precision (DP) of a screening test?
Proportion of the time that a patient is correctly identified as either having a disease or not having a disease with a positive or negative test, respectively
How would you calculate the diagnostic accuracy (DA) or diagnostic precision (DP) for a screening test?
DA/DP=(TP+TN)/(all patients)x100%
When considering the diagnostic accuracy (precision) of a screening test, are you looking at the rows or columns of a 2x2 box?
Neither, it measures A+B/ total
What is a likelihood ratio of a screening test measuring?
Ratio of probability of a given test results (pos or neg) for a person WITH the disease / probability of the same test result (pos or neg) for a person WITHOUT the disease
What is the likelihood ratio positive (LR+) in a screening test?
Probability of a positive test in the presence of disease / probability of a positive test in the absence of disease
What is the likelihood ratio negative (LR-) in a screening test?
Probability of a negative test in the presence of disease / probability of a negative test in the absence of disease
What are the 2 ways you can calculate the likelihood ratio positive of a screening test?
- Sensitivity / (1-specificity)
2. [(A/(A+C))/(B/(B+D))]
What are the two ways you can calculate the likelihood ratio negative of a screening test?
- (1-Sensitivity) / Specificity
2. [(C/(A+C))/(D/(B+D))]
What should the result of a calculation for LR+ and LR- be to demonstrate that the tests are most beneficial?
LR+ should be >10
LR- should be <0.1
What is validity and what are the two types of validity?
- Ability to accurately discern between those that do and those that do not have the disease
- Internal and external validity
What is internal validity?
Extent to which results accurately reflect what was being assessed (true situation of study population)
What is external validity?
Extent to which results are applicable to other populations (not included in the original study; aka generalizability)
What is reliability?
Ability of a test to give the same result on repeated uses
***reproducibility/consistency
Since a valid test is always reliable, is a reliable test always valid?
NO, a bad test can be reproducible without being accurate
What is used when a screening test has multiple cutoff values? Why?
ROCs (receiver operator curves)
More efficient way to show a relationship between sensitivity and specificity for tests with numerical (continuous) outcomes
