Diagnostic Tests

Question 1

when is a Diagnostic test used

Answer 1

A diagnostic test may be used:
by a clinician, together with a clinical examination, to diagnose or exclude a particular disorder in a patient;

as a screening test to ascertain which individuals in an apparently healthy population are likely to have (or sometimes, not have) the disease of interest.

Individuals flagged in this way will then usually be subjected to more rigorous investigations in order to have their diagnosis confirmed.

Question 2

what is the equation for sensitivity

Answer 2

a/(a+c)

remember 2x2 table

Question 3

what is sensitivity

Answer 3

Sensitivity = Given the patient has the disease, what is the proportion of times the test is positive

are the number of false negatives (e.g. test is negative when they have disease).

So sensitivity is also 1 minus proportion of false negatives.

Sensitivity = the probability that someone with disease has a positive test result.

Question 4

what is the equation for specificity

Answer 4

d/(b+d)

Question 5

what is specificity

Answer 5

Given a subject is free of disease, specificity is the proportion of times the test will be negative

false positives, i.e. test is positive when person does not have disease.

So Specificity is 1 minus proportion of false positives

Specificity = the probability that a person without disease has a negative test result

Question 6

Do sensitivity and specificity give us the probability of the test giving the correct diagnosis, whether it is positive or negative

Answer 6

Sensitivity & specificity do not give us this

Question 7

what is the PPV

Answer 7

Positive predictive value (PPV) is the probability that someone has the disease when the test is positive, OR the proportion of individuals with disease when the test is positive.

Depends on prevalence

Question 8

Equation for PPV

Answer 8

a/(a+b)

Question 9

what is NPV

Answer 9

Negative predictive value (NPV) is the probability that someone is without disease when the test is negative, OR the proportion of individuals without disease when the test is negative

depends ON prevalence

Question 10

equation for NPV

Answer 10

D/(C+D)

Question 11

is PPV the proportion of patients with positive test results who are correctly diagnosed

Answer 11

yes

Question 12

is NPV the proportion of patients with negative test results who are correctly diagnosed

Answer 12

yes

Question 13

what test gives a direct assessment of the usefulness of the test in practice.

Answer 13

PPV and NPV

Question 14

Do sensitivity and specificity depend on prevalence of abnormality

Answer 14

NO the PPV and NPV do

Question 15

what is the disadvantage of sensitivity & specificity

Answer 15

The disadvantage of sensitivity & specificity is that they do not assess the accuracy of the test in a clinically useful way

Question 16

what tests depend on prevalence of abnormality

Answer 16

PPV and NPV

Question 17

Are both sensitivity and specificity conditional on having or not having the disease

Answer 17

Both sensitivity and specificity are conditional on having or not having the disease. They are properties of the test, not the disease.

Question 18

what are properties of the test, not the disease

Answer 18

sensitivity and specificity

Question 19

equation for accuracy

Answer 19

(a+d)/N

Question 20

Equation for prevalence

Answer 20

(a+c)/N

Question 21

FUNCTION OF A SCREENING

Answer 21

Screening – on (apparently) healthy people. Need high sensitivity to rule out those without disease

Question 22

Function of diagnostic

Answer 22

Diagnostic – on people with a high suspicion of disease . Need high specificity to include those with disease

Question 23

How is the Receiver Operating Characteristic (ROC) curve obtained

Answer 23

The Receiver Operating Characteristic (ROC) curve is obtained by plotting sensitivity (true positive rate) vs 1 – specificity (false positive rate) for every distinct cut-off value.

Question 24

what are some assumptions about the ROC curve

Answer 24

Plot sensitivity vs 1-specificity for different cut-offs.

An ROC curve that is no better than chance will lie along 45o line.

The best cut-point is the one nearest the upper left-hand corner.

When comparing two curves the one with the largest area under the ROC is considered more accurate.

Question 25

what is a Likelihood ratio

Answer 25

For any test result we can compare the probability of getting that result, if the patient truly had the condition of interest, with the corresponding probability if they were healthy.
The ratio of these probabilities is called the likelihood ratio (LR).

Question 26

equation for LR+

Answer 26

Sensitivity/ (1-specificity)

Question 27

equation for LR-

Answer 27

(1-sensitivity)/ specificity

Question 28

what is Bayes Theorem

Answer 28

A very simple but important result in statistical theory.

This property enables you to calculate (conditional) probabilities when it is not possible to observe the proportions directly.

Question 29

Bayes theorem equation

Answer 29

(sensitivity x prevalence) / probability of a positive result

Question 30

what is the use of a large LR+

Answer 30

A large LR(+), e.g. > 10, suggests test may be useful in ruling in a diagnosis

Question 31

what does a small LR- suggest

Answer 31

A small LR(-), close to zero (e.g. <0.01) suggests the test may be useful in ruling out a diagnosis

Question 32

is the PNZ is close to 1-prevalence is the test useful

Answer 32

no

Question 33

are sensitivity and specificity independent of the disease prevalence

Answer 33

yes