Diagnostic Tests

Question 1

Specificity

Answer 1

percent of healthy animals that test negative

a highly specific test will rarely misclassify animals that are not diseased

Question 2

Sensitivity

Answer 2

percent of diseased animals that test positive

a sensitive test will rarely misclassify animals with disease

Question 2

Why are sensitivity and specificity preferred to positive/negative predictive values?

Answer 2

sensitivity and specificity do not change as the prevalence of disease changes

PPV and NPV do

Question 3

What conditions change sensitivity and specificity?

Answer 3

1. type of cases change (mild v. severe)
2. test is applied to a population with different characteristics that change the likelihood of false positives and negatives

Question 4

Positive predictive value

Answer 4

the percent of animals that test positive and really have the disease

Question 5

Negative Predictive Value

Answer 5

percent of animals that test negative that really don’y have the disease

Question 6

Testing in series (3)

Answer 6

1. retesting only positive samples with a different test
2. both tests must be positive to diagnose the patient
3. trying to remove false positives

increase sepcificity, decrease sensitivity

Question 7

Testing in parallel (3)

Answer 7

1. test all samples with 2 different tests
2. patient is positive if either test is positive
3. goal is to find all positive cases

increase sensitivity, decrease specificity

Question 8

Determining disease in continuous outcome tests (3)

Answer 8

1. use numerical results
2. derive a cut-off value for positive/negative cases
3. determine likelihood of disease given a test value

Question 9

Diagnostic Stewardship

Answer 9

choosing the right test for the right patient to prompt the correct action

Question 10

Pathogen Detection (pros + cons)

Answer 10

pros
* definitive answer
* infection and vaccine differ
* quantitative information

cons
* pathogen must be present in sufficient amounts at time of sample
* sensitive to sampling handling and postmortem change
* may be hard to ID

Question 11

Antibody-based tests (pros + cons)

Answer 11

pros
* sampling is quick and easy
* can sample living or dead
* many testing options

cons
* depends on humoral immune response
* wait until titers can be measured
* individuals respond differently to exposure
* cross-reacting
* titers don’t equal disease

individuals stay positive for a long period, vaccines can cause seropositives

Question 11

Antigen-based tests (pros + cons)

Answer 11

pros
* definitive answe
* vaccine and natural infection differ

cons
* hard to sample
* antigen may be detectable for limited time
* cross-reacting organisms
* antigens levels may be too low or denatured

individuals only positive when agent is present

Question 12

Nucleic Acid-based tests

sequencing of a pathogen-specific DNA/cDNA/rRNA

Answer 12

pros
* sensitive
* fast
* provides agents specific genetic code
* DNA more stable than other samples

cons
* nucleic acid must be extractable
* overly sensitive
* fresh samples needed for RNA
* samples may contain PCR inhibiting substances

individuals only positive when agent is present