Evaluating diagnostic tests Flashcards

1
Q

How is prevalence calculated from a 2x2 table?

A

a+c/N
a = true positive, c = false negative
N = total population

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How is sensitivity calculated from a 2x2 table?

A

True positive rate
a/(a+c)
1- proportion of false negatives
P(T+|D+) (test positive given disease positive)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How is specificity calculated from a 2x2 table?

A
True negative rate
d/(b+d)
d = true negative
b = false positive
1-proportion of false positives
P(T-|D-)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How is the positive predictive value (PPV) calculated from a 2x2 table?

A

a/a+b

Probability that someone has the disease when the test is positive

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How is negative predictive value (NPV)calculated from a 2x2 table?

A

d/c+d

Proportion of individuals without the disease when the test is negative

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the effect of prevalence of disease on PPV and NPV?

A

PPV - increased with prevalence

NPV - decreased with prevalence

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is more important in a screening test?

A

High sensitivity to rule out those without disease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is more important in a diagnostic test?

A

Needs high specificity to include those with the disease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the likelihood ratio for a positive test?

A

Probability of a positive result with a disease/probability of a positive result without disease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How is LR+ calculated and what does this mean?

A

Sensitivity/1-specificity

LR+>1 means that the test is more likely to give a positive result if patient has the disease than if they did not

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How is LR- calculated and what does this mean?

A

1-Sensitivity/Specificity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How are pre-test odds calculated?

A

prevalence/(1-prevalence)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How are post-test odds calculated and what does this mean?

A

pre-test odds x LR+

Quantifies what a positive test reults provides about whether an individual truly has the disease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How can a cut-off be decided for diagnosis based on continuous measurement?

A

Using an ROC curve (receiver operating characteristic)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How is an ROC curve plotted?

A

X axis = 1-specificty (false positive probability)
Y axis = sensitivity |(true positive probability)
For each diagnostic cut-off

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

How is the ROC curve used to decide the diagnostic cut-off point?

A

The value nearest the upper left-hand corner

17
Q

When comparing ROC curves, what suggests a better diagnostic test?

A

Larger area under the curve (c-statistic)

Perfect area = 1