Lecture 9 - Which Investigation do I order? Diagnostic Test Accuracy

Question 1

Sensitivity

Answer 1

Probability of a positive test in a diseased person (PID)

Question 2

Specificity

Answer 2

Probability of a negative test in a non-diseased person

Question 3

How is test accuracy determined?

Answer 3

• Appropriate spectrum of patients is used
• Each patient in the study undergoes BOTH the diagnostic test in question AND the reference (gold) standard
• Comparison between the two test is blind and independent

Question 4

What is the formula for sensitivity?

Answer 4

TP / (TP + FN)

or

a/(a+c)

Question 5

What is the formula for specificity?

Answer 5

TN / (TN + FP)

or

d/(b+d)

Question 6

What is Positive Predictive Valve (PPV)?

Answer 6

Probability of disease given a +ve result

The proportion of individuals who test positively (a + b) AND truly have the disease (a)

Question 7

What is Negative Predictive Value (NPV)?

Answer 7

Probability of NO disease given a -ve result

The proportion of individuals who test negatively (c + d) AND truly do not have the disease (d)

Question 8

What do the predictive values of a test depend on?

Answer 8

• Accuracy of the test

- Pre-test probability (or prevalence of the disease)

Question 9

How do you know if a test is helpful?

Answer 9

The Youden Index

Question 10

What is the formula for the Youden Index and what are its parameters for a useful test?

Answer 10

Youden Index = sensitivity + specificity -1

for a test to be useful, Youden index should be >0

Question 11

SpPin

Answer 11

With a highly specific test, a positive result rules in the diagnosis

Question 12

SnNout

Answer 12

With a highly sensitive test, a negative result rules out the diagnosis

Question 13

What is the formula for PPV?

Answer 13

PPV = a / (a +b)

or

PPV = TP / (TP + FP)

Question 14

What is the formula for NPV?

Answer 14

NPV = d / (c + d)

or

NPV = TN / (FN + TN)

Question 15

What do likelihood ratios describe?

Answer 15

How much the odds change after applying results of a test

Question 16

What is the formula for LR+?

Answer 16

Sensitivity / (1 - Specificity)

Question 17

What is the formula for LR-?

Answer 17

(1-Sensitivity) / Specificity

Question 18

What is the formula for LR?

Answer 18

[Chance of test result in disease] / [Chance of test result in no disease]

Question 19

What does LR > 1 mean?

Answer 19

Test results more likely to result in disease

Question 20

What does LR < 1 mean?

Answer 20

Test results more likely to result in no disease

Question 21

What does LR = 1 mean?

Answer 21

Test results equally likely in diseased and non diseased

Question 22

What are the advantages of LR over PPV and NPV?

Answer 22

• less likely to change w/ the prevalence of the disorder
• they can be calculated for several levels of the symptom/sign or test
• they can be multiplied together to combine the results of multiple diagnostic test
• can be used to calculate post-test probability for a target disorder