Screenings in Medicine

Question 1

In a 2x2 table for screening tests, what are the labels for the rows and columns?

Answer 1

Rows: screening test pos/neg
Columns: disease presence “truth”

Question 2

What is a “true positive”? What box in a 2x2 table do these people live in?

Answer 2

Someone has the disease and the screening test comes back positive. Test is accurate so it is a “true” measure. Live in box “A”

Question 3

What is a “true negative”? What box of the 2x2 table do these people live in?

Answer 3

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”

Question 4

What is a “false negative”? What box of the 2x2 table do these people live in?

Answer 4

When the screening test comes back negative when the person actually does have the disease. The test is not accurate. Live in box “C”

Question 5

What is a “false positive”? What box of the 2x2 table do these people live in?

Answer 5

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”

Question 6

What is the sensitivity of a screening test?

Answer 6

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.

Question 7

If you have a highly sensitive screening test, what is the rate of false negatives?

Answer 7

Very low

Question 8

How would you calculate the sensitivity of a screening test?

Answer 8

Sensitivity= TP/(all diseased)x100%

Question 9

When measuring the sensitivity of a screening test, would you be analyzing rows or columns?

Answer 9

Columns

Question 10

What is the specificity of screening tests?

Answer 10

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

Question 11

A highly specific screening test has what kind of rate of false positives?

Answer 11

Very low

Question 12

How would you measure the specificity of a screening test?

Answer 12

Specificity=TN/(all not diseased)x100%

Question 13

What is the positive predictive value of a screening test?

Answer 13

How accurately a positive test predicts the presence of disease.
Measures the percentage of TP’s in patients with a positive test (correct predilection)

Question 14

When measuring specificity of a screening test, are you considering the rows or columns of the 2x2 table?

Answer 14

Columns

Question 15

How would you calculate the positive predictive value of a screening test?

Answer 15

PPV=TP/(all positive tests)x100%

Question 16

When considering the positive predictive value of a screening test, are you looking at the rows or columns of a 2x2 box?

Answer 16

Rows

Question 17

What is the negative predictive value of a screening test?

Answer 17

How accurately a negative test predicts the absence of disease.
Percentage of TN’s in patients with a negative test (correct prediction)

Question 18

How would you calculate the negative predictive value or a screening test?

Answer 18

NPV=TN/(all negative tests)x100%

Question 19

When considering the negative predictive value of a screening test, are you looking at the rows or columns of a 2x2 box?

Answer 19

Rows

Question 20

What is the diagnostic accuracy (DA) or diagnostic precision (DP) of a screening test?

Answer 20

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

Question 21

How would you calculate the diagnostic accuracy (DA) or diagnostic precision (DP) for a screening test?

Answer 21

DA/DP=(TP+TN)/(all patients)x100%

Question 22

When considering the diagnostic accuracy (precision) of a screening test, are you looking at the rows or columns of a 2x2 box?

Answer 22

Neither, it measures A+B/ total

Question 23

What is a likelihood ratio of a screening test measuring?

Answer 23

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

Question 24

What is the likelihood ratio positive (LR+) in a screening test?

Answer 24

Probability of a positive test in the presence of disease / probability of a positive test in the absence of disease

Question 25

What is the likelihood ratio negative (LR-) in a screening test?

Answer 25

Probability of a negative test in the presence of disease / probability of a negative test in the absence of disease

Question 26

What are the 2 ways you can calculate the likelihood ratio positive of a screening test?

Answer 26

1. Sensitivity / (1-specificity)

2. [(A/(A+C))/(B/(B+D))]

Question 27

What are the two ways you can calculate the likelihood ratio negative of a screening test?

Answer 27

1. (1-Sensitivity) / Specificity

2. [(C/(A+C))/(D/(B+D))]

Question 28

What should the result of a calculation for LR+ and LR- be to demonstrate that the tests are most beneficial?

Answer 28

LR+ should be >10

LR- should be <0.1

Question 29

What is validity and what are the two types of validity?

Answer 29

• Ability to accurately discern between those that do and those that do not have the disease
• Internal and external validity

Question 30

What is internal validity?

Answer 30

Extent to which results accurately reflect what was being assessed (true situation of study population)

Question 31

What is external validity?

Answer 31

Extent to which results are applicable to other populations (not included in the original study; aka generalizability)

Question 32

What is reliability?

Answer 32

Ability of a test to give the same result on repeated uses

***reproducibility/consistency

Question 33

Since a valid test is always reliable, is a reliable test always valid?

Answer 33

NO, a bad test can be reproducible without being accurate

Question 34

What is used when a screening test has multiple cutoff values? Why?

Answer 34

ROCs (receiver operator curves)
More efficient way to show a relationship between sensitivity and specificity for tests with numerical (continuous) outcomes

Question 35

what phase in clinical trial?

testing of drug on 20-100 healthy human volunteers for preliminary dosing, toxicity, pharmacokinetics, and safety. Goal is not to determine efficacy.

Answer 35

phase 1

Preclinical is in vitro and in vivo animal studies

Phase 2 is preliminary testing of drug in 100-300 pts for phase 1 qualities+efficacy

Phase 3 is same as phase 2 but with 300-3,000 pts

Phase 4 is postmarketing surveillance