Diagnostic Tests

Question 1

diagnosis

Answer 1

classify animals as having a disease (or other health related state) or not

Question 2

uses of diagnostic tests

Answer 2

clinical medicine: treatment and prognosis

surveillance: identify changes in disease status

international trade: keep infected animals out

research: identify and monitor research subjects

Question 3

Dichotomous tests

Answer 3

only 2 possible answers

positive or negative

e.g radiographs, MRI, PM

Question 4

continuous tests

Answer 4

continuum of possible answers, still need to classify as positive or negative

e.g serum chemistry, antibody titers, CBC

Question 5

hematology diagnostic tests

Answer 5

measures amount of different cell types in a venous blood sample

Question 6

biochemical diagnostic tests

Answer 6

measures enzymes, metabolites, proteins, etc-usually in venous blood sample

Question 7

immunological diagnostic tests

Answer 7

use antigen to measure antibodies or vice versa

Question 8

example of immunologic tests

Answer 8

SNAP FIV/FeLV

Question 9

pathogen detection tests

Answer 9

detects the pathogen itself (or specific parts of it)

ex: microscopy, culture, virus isolation, PCR

Question 10

T/F some test for epidemiological investigations “fingerprint” pathogens or discriminate infected from vaccinated animals (DIVA)

Answer 10

true

Question 11

T/F usually cant measure the disease itself

Answer 11

true

Question 12

what is measured?

Answer 12

‘something’ (chemical, antigen, etc) that is present in a certain quantity when an animal has a certain disease/underlying pathogen

test value

Question 13

how do diagnostic tests work?

Answer 13

sample taken –> test is run and test value is produced –> decision i (+/-) is made by test or diagnostician (test result)

Question 14

cut off value

Answer 14

determine experimentally as the value that minimizes false positive/ false negative results

Question 15

T/F usually there is a clear separation in the test values between diseased and non diseased

Answer 15

false

Question 16

T/F some non-diseased animals may have higher test values than some of the disease and vice versa

Answer 16

true

Question 17

T/F can have false positives/negatives

Answer 17

true

Question 18

True positive

Answer 18

diseased animal that tests positive

Question 19

true negative

Answer 19

non-diseased animal that tests negative

Question 20

false negative

Answer 20

diseased animal that test negative

Question 21

false positive

Answer 21

non-diseased animal that tests positive

Question 22

how are gold standard tests used

Answer 22

true disease status of individuals is often determined using a gold standard test, against which the performance of a new test is evaluated

Question 23

disadvantages of gold standard test

Answer 23

labor intensive, impractical, highly invasive, slow and/or expensive

Question 24

every test is evaluated by how accurately it classifies:

Answer 24

diseased individuals as test positive

non-diseased individuals as test negative

Question 25

how to evaluate a test

Answer 25

test group of disease animals test a group of non-diseased animals establish a cut-off value

Question 26

what are the 2 parameters to evaluate diagnostic test

Answer 26

sensitivity (Se) Specificity (Sp)

Question 27

diseased animals are used to determine

Answer 27

sensitivity (of the new test)

Question 28

non diseased animals are used to determine

Answer 28

specificity (of the new test)

Question 29

T/F Se and Sp are proportions

Answer 29

true

Question 30

determining sensitivity

Answer 30

test a group of diseased animals proportion of diseased animals that the test correctly classifies as positive

Question 31

what are some factors that can cause sensitivity to be low?

Answer 31

few/small amounts of whatever the test measures samples are degraded before testing

Question 32

T/F a sensitivity of 100% has no false negatives

Answer 32

true classifies all diseased animals as positive

Question 33

In a low Se test (75%)

Answer 33

25% of diseased not detected and are false negatives

Question 34

T/F Se doesn't tell us anything about how well the test performs on non-diseased animals

Answer 34

true

Question 35

determining Sp

Answer 35

test a group of non-diseased animals proportion of non-diseased animals that the test correctly classifies as negative

Question 36

what are some factors that can cause specificity to be low?

Answer 36

cross-reactivity (mistakenly identified as another) samples are contaminated

Question 37

T/F high SP (100%) has no false positives

Answer 37

true correctly classifies all non-diseased animals as negative

Question 38

in a low Sp test (75%)

Answer 38

25% of non- diseased animals classified as positive (false positive)

Question 39

T/F Sp doesnt tell you anything about how well the test performs on diseased animals

Answer 39

true

Question 40

what is a balanced test

Answer 40

Equal Se and Sp equally misclassifies diseased and non-diseased

Question 41

SnNOut

Answer 41

sensitive test when Negative rules disease Out **correctly classifies ALL diseased animals as POSITIVE**

Question 42

SpPIn

Answer 42

Specific test when Positive rules disease in **remember-looks at non-diseased animals so if positive can't rule it out**

Question 43

T/F test with 100% sensitivity will have no false negatives but may have false positives if Sp is low

Answer 43

true any negative results are from non-diseased animals

Question 44

T/F test with 100% specificity will have no false positive results but may have false negatives with low Se

Answer 44

true any positive results are from diseased animals (rules it in)

Question 45

maximize sensitivity when:

Answer 45

need to detect all diseased or infected animals

Question 46

maximize specificity when:

Answer 46

cost of false positive is high and dont care if there are lot of false negatives ex: need to cull positive animals

Question 47

how to use Sp and Se in series

Answer 47

1st test: high Se - detects most/ all of diseased animals in population - negatives are true negatives 2nd test: high Sp - test all that tested positive on the first test - all test positives from second test are true positives (eliminates false positives from the first test)

Question 48

T/F Se and Sp tell you the probability that an animals which tests positive is truly diseased

Answer 48

false tells you the probability of a diseased animal testing positive or non-diseased testing negative

Question 49

PPV

Answer 49

positive predictive value true positive/test positives

Question 50

NPV

Answer 50

negative predictive value true negative/test negatives

Question 51

how to calculate Se

Answer 51

true positives/ (true positives = false neagtives)

Question 52

how to calculate Sp

Answer 52

true negatives/ (true negatives + false positives)