Lecture 21-23 - Screenings in Medicine

Question 1

What two questions are important to ask of medical screening tests?

(L21 S4)

Answer 1

• how ACCURATE is the test

- how CONFIDENT can you be about the prediction of disease status from the results

Question 2

What are the four outcomes from a screening test and what differentiates them?

(L22 S6-9)

Answer 2

True positive (A):
-CORRECT reporting of a POSITIVE result and patient actually HAS the disease

True negative (D):
-CORRECT reporting of NEGATIVE result and patient actually DOES NOT have disease

False positive (B):
-INCORRECT reporting of POSITIVE result and patient DOES NOT have disease

False negative (C):
-INCORRECT reporting of NEGATIVE result and patient DOES have disease

Question 3

What two factors can be used to assess how accurate a screening test is in determining disease status?

(L21 S10)

Answer 3

• sensitivity

- specificity

Question 4

How are sensitivity and specificity evaluated after they have been calculated?

(L21 S10)

Answer 4

-comparison to known disease status using a “gold standard”

Question 5

What does sensitivity represent?
What does a high sensitivity indicate about a test?

(L21 S11)

Answer 5

• how well a test can detect disease pressence when the disease is actually present
• sensitivity = TP / (TP + FN) or A / (A + C)

-high sensitivity indicates a low false negative rate

Question 6

What does specificity represent?
What does a high specificity indicate about a test?

(L21 S12)

Answer 6

• how well a test can detect disease absence when the disease is actually absent
• specificity = TN / (TN + FP) or D / (B + D)

-high specificity indicates a low false positive rate

Question 7

What two factors can be used to assess confidence in prediction of disease presence or absence from a screening test?

(L21 S15)

Answer 7

• positive predictive value

- negative predictive value

Question 8

What does positive predictive value (PPV) represent?

L21 S16

Answer 8

• how accurately a positive test predicts the presence of disease
• PPV = TP / (TP + FP)
• PPV = A / (A + B)

Question 9

What does negative predictive value (NPV) represent?

L21 S17

Answer 9

• how accurately a negative test predicts the absence of disease
• NPV = TN / (TN + FN)
• NPV = D / (C + D)

Question 10

Prevalence of a disease in a community is a factor in __________ and __________ but is not a factor in __________ and __________.

(L22 S18-21)

Answer 10

PPV; NPV; sensitivity, specificity

Question 11

The lower the specificity, the lower the __________.

L22 S18-21

Answer 11

PPV

Question 12

What does diagnostic accuracy/precision represent?

L22 S22

Answer 12

• proportion of screenings where a patient is correctly identified
• DA/DP = (TP + TN) / (all patients)
• DA/DP = (A + D) / (A + B + C + D)

Question 13

What does likelihood ratio represent?

L22 S24

Answer 13

• ratio of the probability of a person with disease receiving a test result and the probability of somebody without disease getting that same result
• can be calculated for both a positive result (LR+) or a negative result (LR-)

Question 14

What is likelihood ratio positive (LR+) and how is it calculated?

(L22 S25)

Answer 14

-ratio of the probabilities of a positive test in those with disease and the probability of a positive test of those without disease

• sensitivity / (1 - specificity)
• (A / (A + C)) / (B / (B + D))

Question 15

What is likelihood ratio negative (LR-) and how is it calculated?

(L22 S26)

Answer 15

-ratio of the probabilities of a negative test in those with disease and the probability of a negative test of those without disease

• (1- sensitivity) / specificity
• (C / (A + C)) / (D / (B + D))

Question 16

To be considered useful, a test should have a LR+ of __________ and a LR- of __________.

(L22 S27)

Answer 16

> 10; <0.1

Question 17

What is validity?

L23 S28

Answer 17

• ability to accurately identify those with disease and those without disease
• precision of results

Question 18

Differentiate between internal validity and external validity.

(L23 S28)

Answer 18

Internal validity:
-how accurately a studies results reflect the true conditions of a study population

External validity:

• how accurately a studies results reflect true conditions of a population other than the one that was studied
• generalizability

Question 19

What is reliability?

L23 S29

Answer 19

• the ability of a test to give the same results with repeated used
• degree of reproducibility or consistency

Question 20

What relationship do validity and reliability have?

L23 S29

Answer 20

• valid tests are always reliable

- reliable tests are not always valid

Question 21

What is a cutoff value?

What effect does changing a cut off value have on the outcomes of a test?

Answer 21

-the value of a test used to differentiate between presence of disease or absence of disease