Test Characteristics

Question 1

False positive

Answer 1

Test is positive (suggests disease) but the patient has no disease;

Question 2

False negative

Answer 2

The test is negative (suggests no disease) but the patient has the disease;

Question 3

Sensitivity

Answer 3

The proportion of those with a disease which it correctly identifies;
A/A+C; true positive/true positive+false negatives

Question 4

Specificity

Answer 4

Proportion of those without the disease which it correctly identifies;
D/B+D; true negative/ true negative + false positive;

Question 5

Positive predictive value (PPV)

Answer 5

Probability that a subject has the disease given that they have a positive result;
PPV = true positives/ true positives + false positives

Question 6

Negative predictive value (NPV)

Answer 6

The probability that the subject does not have the disease given that they have a negative result;
True negative/true negative + false negative

Question 7

The PPV is determined by?

Answer 7

Prevalence, specificity and sensitivity;

Question 8

Diagnostic test

Answer 8

To confirm or exclude the presence of a disease

Question 9

Screening test

Answer 9

To assess the risk of disease in an asymptomatic person to determine the need for further confirmatory diagnostic testing

Question 10

Prognostic test

Answer 10

To assess the risk of future disease and the need for preventative measures;

Question 11

Screening tests
Purpose - 
Target population - 
Test method -
Positive result threshold - 
Positive result - 
Cost -

Answer 11

To detect potential disease indicators;
Large number of asymptomatic but potentially at risk individuals;
Simple, acceptable to patients and staff;
Towards high sensitivity so not to miss potential disease;
Indicates suspicion of disease;
Cheap - benefits justify costs

Question 12

Diagnostic tests 
Purpose - 
Target population - 
Test method -
Positive result threshold - 
Positive result - 
Cost -

Answer 12

Establish presence/absence of disease;
Symptomatic individuals to establish diagnosis or asymptomatic with a positive screening test;
Maybe invasive, expensive - justifiable;
Higher specificity - more weight given to accuracy, precision than to patient acceptability;
Definite diagnosis;
Higher cost - justified

Question 13

The commonly used anti TTG antibody has …

Answer 13

sensitivity and specificity both in high 90s % (few tests are this good);

Question 14

Likelihood ratio of a test

Answer 14

Likelihood that a positive test would be expected in a patient with the disease compared to the likelihood in a patient without;

Likelihood of patient with disease/ likelihood of patient without disease;

Question 15

Post test odds

Answer 15

Pre test odds * Likeliood ratio

Question 16

Post test probability

Answer 16

Post test odds/post test odds + 1