Diagnostic Tests

Question 1

Definition of diagnostic test

Answer 1

Performed to aid diagnosis/disease detection

Question 2

Definition of sensitivity

Answer 2

Correct identification of diseased with test

Question 3

Definition of specificity

Answer 3

Correct identification of healthy

Question 4

Definition of positive predictive value

Answer 4

Probability that the positive result is found in the diseased

Question 5

Definition of negative predictive value

Answer 5

Probability of the correct identification of the healthy

Question 6

Definition of the likelihood ratio

Answer 6

Use sensitivity and specificity of test to determine whether a test result usefully changes the probability that a condition exists

Question 7

Definition of bias

Answer 7

If the average is not close to the true value in a distribution graph

Question 8

Definition of imprecision

Answer 8

If there is a large distribution of values

Question 9

Definition of line identity

Answer 9

Line where x=y at 45 degrees from origin

If values of the 2 measurements are almost equal => high levels of agreement

Question 10

Definition of the line of regression

Answer 10

Line that describes how x changes y

Question 11

Definition os association

Answer 11

Relationship between the exposure and outcomes

Question 12

What are diagnostic tests
When are they used
How can they be used

Answer 12

Performed to aid diagnosis/detection of disease

Used in

• radiology
• laboratories
• physiology

Can be a definitive diagnosis
Part of a diagnosis
Part of a screening test

Question 13

Describe the ideal diagnostic test

Why is this never going to happen

Answer 13

Correct identification of diseased with +ve test result
Correct identification of healthy with -ve test result
Cheap and painless

False positives and false negatives will occur
-will be 4 possible outcomes

Question 14

What are the 4 ways of assessing the effectiveness of a diagnostic test

Answer 14

Sensitivity
-correct identification of diseased

Specificity
-correct identification of healthy

Positive predictive value
-+ve results due to diseased individual

Negative predictive value
—ve result due to healthy individual

Question 15

How would you calculate the confidence interval for proportions or percentages

Answer 15

P+- 1.96 √p(1-p)/N

N can be the denominator of the previously used equation

Question 16

How would you calculate the sensitivity

Answer 16

No of diseased +ve to test/Total no of diseased

Question 17

How would you calculate the specificity

Answer 17

No of healthy -ve to test/total no of healthy

Question 18

How would you calculate the positive predictive value

Answer 18

No of diseased +ve/no of +ve results

Question 19

How would you calculate the negative predictive value

Answer 19

No of healthy -ve/no of -ve results

Question 20

How would you calculate the accuracy of the test

Answer 20

TP + FN / (TP+TN+FP+FN)

Question 21

How would you calculate the positive likelihood ratio

Answer 21

Sensitivity/1-specificity

Question 22

How would you calculate negative likelihood ratio

Answer 22

1-sensitivity/specificity

Question 23

How can you involve disease prevalence values in the positive likelihood ratio

Answer 23

PPV/1-PPV = P/1-P x LR+

Question 24

How would you calculate the amount of agreement between qualitative test results
What is k?

Answer 24

K = observed % agreement - expected % agreement/100 - expected % agreement

K=probability that judges will agree if there is no agreement by chance

Question 25

How would you calculate the observed % agreement

How would you calculate the expected % agreement

Answer 25

Observed % agreement = no of agreements/all obs

Expected % agreements = (no of normal obs in 1 x no of normal obs in 2) + (no of abnormal obs in 1 and no of abnormal obs in 2) / total obs ^2

Question 26

How can you interpret kappa values

Answer 26

<0, no agreement

0. 0-0.20, slight agreement
1. 21-0.40, fair agreement
2. 41-0.60, moderate agreement
3. 61-0.80, good agreement
4. 81-1, excellent agreement

Question 27

How would you process continuous measurements

Answer 27

Continuous measurements may be dichotomised => qualitative +ve or -ve result

Question 28

How would you choose the cut off point for continuous measurements in diagnostic tests

Answer 28

Choose sensitivity or specificity required

If you move cut point to increase sensitivity, specificity will be sacrificed

Question 29

Why do we have overlap between the ranges that give positive and negative results in continuous measurements

Answer 29

Patient variation (stress)
Measured variation (GP, nurse)
Measurement conditions (temp)
Instrument condition variation (sphygmomanometer)

All are sources of bias and imprecision

Question 30

What is the difference between bias and imprecision

Answer 30

Bias, if average is not close to the true value

Imprecision, if there is a large distribution of values

Question 31

How is agreement between sets of continuous measurements measured

Answer 31

Line of identity drawn

Question 32

How would you test for agreement

Answer 32

Bias, mean diff with 95% CI
Precision, SD of 95% range

If both are small => good agreement

Question 33

What is the difference between agreement and association

Answer 33

Association,

• relationship between exposure and outcomes
• tested for with a correlation coefficient
• can involve the line regression

Agreement

• assess the departure of 2 measurements from line of identity
• tested for with SD and CIs