Diagnostic & Screening Tests in Respiratory Diseases

Question 1

What is the purpose of a diagnostic test?

Answer 1

To confirm the presence of disease

Question 2

Who do you give diagnostic tests to?

Answer 2

Patients who already have a clinical suspicion of disease

Question 3

What is the purpose of a screening test?

Answer 3

To identify patients who may have a disease or an early form of disease or a risk factor for a disease to allow for early intervention

Question 4

Who do you give screening tests to?

Answer 4

Patients who have had no clinical assessment before hand

Question 5

What is a true negative?

Answer 5

People who don’t have the disease and the test shows they don’t have the disease

Question 6

What is a false negative?

Answer 6

People who have the disease and the test shows they don’t have the disease

Question 7

What is a false positive?

Answer 7

People who don’t have the disease and the test shows the do have the disease

Question 8

What is a true positive?

Answer 8

People who have the disease and the test shows they have the disease

Question 9

What is sensitivity?

Answer 9

The percentage of people with the disease that test positive

Question 10

How is sensitivity calculated?

Answer 10

TP/(TP+FN)

Question 11

What is specificity?

Answer 11

The percentage of people without the disease that test negative

Question 12

How is specificity calculated?

Answer 12

TN/(TN+FP)

Question 13

What is the positive predictive value?

Answer 13

The percentage of positive tests that are actually positive

Question 14

How is the positive predictive value calculated?

Answer 14

TP/(TP+FP)

Question 15

What is the negative predictive value?

Answer 15

The percentage of negative tests that are actually negative

Question 16

How is the negative predictive value calculated?

Answer 16

TN/(TN+FN)

Question 17

Are sensitivity, specificity, PPV and NPV constant for a test?

Answer 17

Sensitvity and specificity are but PPV and NPV are not

Question 18

What are PPV and NPV dependent on?

Answer 18

The prevalence of disease

Question 19

What will the PPV be if the prevalence is low?

Answer 19

Low

Question 20

What will the NPV be if the prevalence is low?

Answer 20

High

Question 21

If a test has a high threshold for the definition of the disease what will the sensitivity and specificity be?

Answer 21

Low sensitivity but high specificity

Question 22

What are the axes in a ROC curve?

Answer 22

1-specificity vs sensitivity

Question 23

What does a linear graph on a ROC curve show?

Answer 23

That the test doesn’t discriminate between people with the disease who test positive and people without the disease who test positive

Question 24

What does the area under the curve measure?

Answer 24

The discriminating ability of the test

Question 25

What biases are there in screening tests?

Answer 25

Selection bias, lead time bias and length time bias

Question 26

What is the selection bias in a screening test?

Answer 26

Healthy people are more likely to be screened because they are more motivated to undertake healthy behaviour

Question 27

What is the lead time bias in a screening test?

Answer 27

A screening test might show a longer survival time than a diagnostic test - but this is only because the test is done earlier not that the test makes them live longer - the time of death could potentially be the same - early detection not prolonged survival

Question 28

What is the length time bias in a screening test?

Answer 28

Diseases that have a slow enough progression to allow for a screening test have better outcomes than faster diseases - detecting of a non aggressive disease