Exam 1 - Introduction to Infectious Disease Diagnostics

Question 1

with diagnostic tests, when should you screen your patients?

Answer 1

prior to breeding, exposure, or acquisition
prior to domestic or international travel
for regionally common diseases
for highly dangerous diseases
prior to making a preventative care decision

Question 2

when using diagnostic tests for diagnosing a patient, when should you use them?

Answer 2

after a known/suspected exposure
after a period of compatible clinical signs
prior to making a therapeutic care decision

Question 3

screening tests are highly _________ while confirmatory tests are highly _________ (sensitive/specific)

Answer 3

sensitive

specific

Question 4

why do you want your screening tests to be highly sensitive?

Answer 4

you’re using it to rule out a disease - you will pursue more diagnostics to confirm whether or not your results were correct

Question 5

T/F: screening tests may tolerate more false positives

Answer 5

true

Question 6

what are the benefits of using screening tests for large numbers/populations?

Answer 6

they are simple & fast
affordable
& generate simple results

Question 7

confirmatory tests are used to ______ disease

Answer 7

diagnose

Question 8

T/F: confirmatory tests may tolerate more false negatives

Answer 8

true

Question 9

what patients should you use confirmatory tests for?

Answer 9

smaller numbers of patients with the highest index of suspicion

Question 10

what are the cons of using confirmatory tests?

Answer 10

more expensive, inconvenient, & nuanced interpretation

Question 11

T/F: a diagnostic test result is a diagnosis

Answer 11

FALSE - you must interpret the test result in regards to your patient

Question 12

T/F: tests yield results, not answers

Answer 12

TRUE

Question 13

_______/_______ are features of the tests

Answer 13

sensitivity/specificity

Question 14

_______/_______ _______ _______ are features of how you use a test

Answer 14

positive & negative predictive value

Question 15

T/F: you have some control over your positive & negative predictive values

Answer 15

true - you are determining how you will use the test

Question 16

in regards to interpreting a test result with positive & negative predictive values, what are you considering?

Answer 16

knowledge of the patient
knowledge of the disease in question
knowledge of how the test works

Question 17

what is sensitivity?

Answer 17

probability that an animal that has the disease will test positive

(probability that an animal has a positive test result given it has the disease)

evaluating your test!!!

Question 18

what is specificity?

Answer 18

probability that an animal that does not have the disease will test negative

(probability that an animal has a negative test result given it does not have the disease)

likelihood that a healthy, disease free animal will get a negative test result

Question 19

what is positive predictive value?

Answer 19

probability that an animal that tests positive has the disease

(probability of an animal having the disease given the test is positive)

probability that a sick animal has a positive test result

Question 20

what is negative predictive value?

Answer 20

probability that an animal that tests negative does not have the disease

(probability of an animal not having the disease given a negative result)

likelihood that a healthy, disease free animal will get an accurate negative test result

Question 21

predictive values depend on the probability of what?

Answer 21

the animal having the disease before you test it - if your probability of disease is high (pre-test probability), then your PPV is inherently high & NPV is low

Question 22

how is sensitivity calculated?

Answer 22

TP/(TP + FN) = sensitivity

A/(A+C)

Question 23

how is specificity calculated?

Answer 23

TN/(FP+TN) = specificity

D/(B+D)

Question 24

T/F: false positives do not affect sensitivity

Answer 24

true

Question 25

how is PPV calculated?

Answer 25

TP/(TP + FP) = PPV A/(A + B)

Question 26

how is NPV calculated?

Answer 26

TN/(FN + TN) D/(C + D)

Question 27

what is prevalence?

Answer 27

frequency of an occurrence (diagnosis) in the population at the moment (# of animals just like your patient that have the disease right now)

Question 28

__________ is a reflection of the population your patient is a member of

Answer 28

prevalence

Question 29

what is incidence?

Answer 29

frequency of an occurrence (diagnosis) that will happen in the population over some set time (projection of future values in a year)

Question 30

what is population?

Answer 30

the group in which your patient is a member of

Question 31

why is prevalence important to consider diagnostically?

Answer 31

you need to consider it when choosing a diagnostic test! want to use the test to the best of its ability - high prevalence groups, use in certain settings

Question 32

what may happen if you apply a diagnostic test in a low prevalence group?

Answer 32

may end up with false positives

Question 33

_______ patients on whom to run ____ test

Answer 33

preselect any

Question 34

when considering prevalence & choosing a diagnostic test, what 3 things should you consider?

Answer 34

decide in advance what that patient's pre-test probability is decide in advance what you will do with a positive result decide in advance what you will do with a negative result

Question 35

why are false positives important to consider when it comes to PPV?

Answer 35

a small number of false positives can greatly affect your PPV

Question 36

if you get a positive result on a test with a lower PPV, how would you give your positive result more value?

Answer 36

retest using a different test retest using the same test - different lot number retest patient in a few weeks

Question 37

what would your estimate prevalence be on a sick puppy with bloody diarrhea having parvo? (low/medium/high)

Answer 37

high

Question 38

what 3 components of diagnostic test characteristics should you consider?

Answer 38

how the test works limits of the method cross-reactions or other pitfalls

Question 39

rational use of any diagnostic test requires an understanding of what?

Answer 39

patient risk (pre-test probability/prevalence) patient history, physical/clinical presentation, & lifestyle

Question 40

what does NAATs stand for? what common test does this group include?

Answer 40

nucleic acid amplification tests PCR tests

Question 41

what are examples of immunoassay diagnostic tests?

Answer 41

antigen/antibody tests, serology, IFA/IHC, ELISA, agglutination, & immunodiffusion

Question 42

what are examples of microscopic identification tests?

Answer 42

fecal floats, skin scrape, urine sediment, blood smear

Question 43

what are the benefits to using microscopic identification as a diagnostic approach?

Answer 43

provides direct evidence, often rapid/patient side, & sample represents a small amount of a specific site

Question 44

what are the downfalls of using microscopic identification as a diagnostic approach?

Answer 44

requires expertise, can't see viruses, may not be enough bacteria present to make an identification, & can't determine species of bacteria

Question 45

what is this?

Answer 45

blastomycosis

Question 46

what is this?

Answer 46

babesia in a blood smear

Question 47

what may you use a culture for?

Answer 47

extracellular bacteria (aerobic/anaerobic), fungi, urine, discharge/swabbed materials, aspirated materials, & indwelling devices

Question 48

what are the benefits to using culture as your diagnostic approach?

Answer 48

direct evidence of viable agent, agent species can be identified, & agent species can be tested for susceptibility

Question 49

what are the downfalls to using culture as your diagnostic approach?

Answer 49

some organisms are hard to grow/won't grow without special mediums (obligate intracellular), early antibiotic treatment can wipe out the pathogen you're looking for, need sufficient numbers of pathogens, & competition within the sample (some species will grow readily & take over)

Question 50

what are you looking for if using isolation in cell culture diagnostically?

Answer 50

intracellular bacteria, viruses, protozoa, & pathogens to use in vaccines

Question 51

what are the benefits to using isolation in cell culture?

Answer 51

vaccine development! & confirms organisms are viable

Question 52

what are the downfalls to using isolation in cell culture?

Answer 52

it's slow, technically challenging, requires prior knowledge of probably pathogen (what are you trying to isolate), & high risk of false negatives

Question 53

who is quarantined? for how long?

Answer 53

quarantine the exposed - as long as the incubation period

Question 54

who is isolated? for how long?

Answer 54

infected people - until the shedding period ends

Question 55

T/F: there is more signal from the immune system than the pathogen requiring careful interpretation of immunoassays

Answer 55

true

Question 56

T/F: if using an antigen test to detect antibodies to a pathogen in a patient, the immunoassay is able to differentiate between infection from exposure

Answer 56

false - there is also a lag time after infection for antibodies

Question 57

when using an immunoassay diagnostic test, what are you looking for?

Answer 57

evidence of a pathogen by presence of either antigen or antibody

Question 58

what is NAATs?

Answer 58

nucleic acid amplification tests

Question 59

when would you use NAATs/PCR?

Answer 59

detecting nucleic acid - use for bacteria, fungi, DNA/RNA, organisms that are slow to culture, viruses, & organisms that are dangerous to culture

Question 60

what are the benefits of NAATs?

Answer 60

nucleic acid sequences are specific to their pathogen tests can detect dead pathogens tests can be optimized to many tissues/fluids & DNA is stable to use in these tests

Question 61

what are the downfalls of NAATs?

Answer 61

RNA is easily degradable presence of a pathogen doesn't equal causation high tech/labor intensive extremely sensitive - may be overly sensitive

Question 62

what kind of virus is canine distemper virus?

Answer 62

enveloped, ssRNA, morbilivirus - RNA is less stable

Question 63

what animals serve as the primary reservoir for distemper? what about other outside sources?

Answer 63

dogs coyotes, dingos, raccoon, fox, badger, etc

Question 64

when does shedding begin in an animal infected with distemper?

Answer 64

day 7 post infection

Question 65

how is distemper virus shed?

Answer 65

primarily in respiratory secretions, but also urine & other tissues

Question 66

when are clinical signs seen in animals infected with distemper?

Answer 66

clinical signs seldom seen until day 9 or later

Question 67

what is the pathogenesis of distemper virus?

Answer 67

virus enters respiratory system - replicates in epithelium & enters tissue macrophages which are then carried to lymphoid tissue multiplies extensively in lymphoid tissues (lymph nodes, tonsil, thymus, GALT) hematogenous spread back to the epithelium (includes CNS) resulting in viremia & cell-associated viremia footpad tropism - hard pad disease

Question 68

what is the presentation of acute encephalitis in an animal infected with distemper?

Answer 68

typically young/immunocompromised dogs viral replication in CNS without inflammation CSF tap may be normal will not see antibodies at this time because immune system isn't working

Question 69

what is the presentation of sub-acute to chronic encephalitis in an animal infected with distemper?

Answer 69

dogs with a moderate immune response CNS invasion occurs as lymphoid tissue is recovering CSF tap should detect inflammation

Question 70

what is the presentation of old dog encephalitis in an animal infected with distemper?

Answer 70

rare, chronic, progressive form replication-incompetent CDV in neurons

Question 71

what are the classical respiratory clinical signs seen in an animal infected with distemper virus?

Answer 71

serous to muco-purulent oculonasal discharge viral pneumonia usually complicated by secondary bacterial infection

Question 72

what are the classical gastrointestinal clinical signs seen in an animal infected with distemper virus?

Answer 72

anorexia & vomiting diarrhea may become severe - intussusceptions

Question 73

what are the classical neurological clinical signs seen in an animal infected with distemper virus?

Answer 73

seizures, tremors, myoclonus - often manifest after systemic signs have resolved

Question 74

in what order do clinical signs show up in an animal infected with distemper virus?

Answer 74

respiratory, GI, then neurological

Question 75

what is the typical history of a dog with suspect distemper?

Answer 75

undervaccinated animal respiratory then gi & then maybe neuro signs

Question 76

what are the typical physical exam findings of a dog with suspect distemper?

Answer 76

sick, febrile dog with oculonasal discharge

Question 77

what lab work abnormalities would you expect to get with an animal infected with distemper virus?

Answer 77

lymphopenia but otherwise unremarkable

Question 78

what may you see with microscopic identification of distemper virus?

Answer 78

inclusion bodies

Question 79

what would be your next tier diagnostics pursued for distemper virus?

Answer 79

maybe immunoassays - paired titers IFA of skin biopsy also used

Question 80

what is a paired titer?

Answer 80

samples taken 3 weeks apart from CSF & blood - looking for rise/fall of antibodies

Question 81

If an animal presented clinically healthy & later went on to exhibit more severe clinical signs of distemper, what would you expect their titer results to be if you took the first sample upon initial presentation?

Answer 81

4 fold increase

Question 82

if you use an immunoassay looking for the presence of antibodies on an animal that has suspect distemper, & has never been vaccinated, what would be supportive of distemper?

Answer 82

presence of antibodies - positive result

Question 83

why is a PCR test utilized as a next tier diagnostic for canine distemper virus?

Answer 83

easy to use - use urine, blood, or CSF can also do nasal, pharyngeal, or conjunctival swab

Question 84

T/F: a distemper vaccine can potentially cause a transient positive RT-PCR

Answer 84

true - would be somewhat of a false positive result

Question 85

what should you consider before using an immunoassay test on a possible distemper patient?

Answer 85

timing vaccine status locations to test (often used test)

Question 86

why are RT-PCR tests often used for diagnosing possible distemper patients?

Answer 86

widespread pathogen - many fluid & tissue sources can be used for testing