epidemiology

Question 1

Why do a diagnostic or screening test?

Answer 1

1. Diagnose a sick animal or clinical case.
2. Determine the status of an animal exposed to a diseased animal.
3. Show individual animals are free from infection.
4. Estimate prevalence of infection in a population.
5. Show that a population is free from infection (may be country, zone, kennel, herd).
6. Determine immune status of individual animals or populations (post-vaccination)

Question 2

Four possible outcomes of a test:

Answer 2

Question 3

sensitivity

Answer 3

How likely you are to find an animal that truly has the disease
Sensitivity is less than 100% if there are false negatives
ex: 95% sensitivity means that the test will find 95 of every 100 diseased animals

Question 4

specificity

Answer 4

How likely am I to be specific about the disease and not lump in something else?
Specificity is less than 100% if there are false positives
ex: 95% specificity means that 5 of every 100 healthy animals will be misdiagnosed as diseased/infected

Question 5

Positive Predictive Value (PPV)

Answer 5

What proportion of positive tests are correct?
ex. if PPV = 95%, then 95% of test positive animals actually have the disease (the other 5% are false positives and are healthy without the disease)
depends on prevalence

Question 6

Negative Predictive Value (NPV)

Answer 6

What proportion of negative tests are correct?
ex. if NPV = 95%, then 95% of animals testing negative are really free of the disease (the other 5% are false negatives and have disease but were not detected)
depends on prevalence

Question 7

accuracy

Answer 7

Do not believe it
changes as prevalence changes