Diagnostics (Exam 1)

Question 1

What is a diagnostic test?

Answer 1

Any device or process designed to detect or quantify a sign, substance, tissue change, or body response in an animal

Question 2

What are advantages of virus isolation (VI)?

Answer 2

Allows identification of unknown agents
Virus available for further study
Sensitive method
Detects “live” virus

Question 3

What are disadvantages of virus isolation (VI)?

Answer 3

SLow
Expensive
Requires animals, eggs, tissue culture
Can’t detect inactivated virus, requires fresh tissues, blood, etc., and careful handling, shipping

Question 4

What is a direct FA test?
What is is its sensitivity?

Answer 4

Fluorescent Antibody
Not highly sensitive
Virus specific antibody bound to FITC tag, glows green when positive

Question 5

What is a hemagglutination inhibition (HI) test used for detecting?

Answer 5

2 viruses can agglutinate RBCS
Parvovirus
Influenza

Question 6

What is an indirect FA test? What is a disadvantage?

Answer 6

Has intermediate antibody that allows attachment of another antibody
Can degrade virus

Question 7

What is a PCR? What is its sensitivity?

Answer 7

Detects amplified nucleic acids (DNA, reverse txn for RNA viruses)
Very sensitive
DOESNT measure antigen

Question 8

What is an Agar Gel Immunodiffusion (AGID) test?

Answer 8

Uses known amount of antibodies and antigen
Must incubate for 24 hours

Question 9

What is an Indirect ELISA (serology) test? What is its sensitivity?

Answer 9

Much quicker
More sensitive (more false positives)
Detects if antibody is present

Question 10

What is a titer?

Answer 10

2 fold serial dilutions of serum
Higher titer = lower risk = more Abs in circulation
Lower titer = lower risk
Highest dilution you still see antibodies

Question 11

When is serum neutralization used?

Answer 11

Used for titers for most viruses

Question 12

What are issues in animal disease diagnosis?

Answer 12

Infection DOES NOT equal disease
What is the cut off point in disease diagnosis?
Who decides on the cut off point that will separate the healthy and diseases?
Presence of false positives and false negatives

Question 13

Define: Screening Tests

Answer 13

Tests used for clinically healthy animals

Question 14

Define: Confirmatory/Diagnostic Tests

Answer 14

Tests for clinically diseased animals

Question 15

Characteristics of Screening Tests

Answer 15

Generally cheap (parvo snap)
Detect numerous possible cases of the disease but less specific in identifying real cases
Fast and reasonable accurate
Not meant to supply complete information about the pathogen
Need follow up with confirmatory/diagnostic test

Question 16

Characteristics of Confirmatory/Diagnostic Tests

Answer 16

Tests for clinically diseased animals
To confirm diagnosis or classify disease status among animals suspected to have the disease
Ex. positive fluorescent antibody test for Rabies

Question 17

What are common goals in using diagnostics?

Answer 17

Establish a diagnosis in symptomatic patients
Provide prognostic information
Guide for selection of treatment or control stratergies
Monitor effects of treatment

Question 18

Define: Precision

Answer 18

Replication of test results when the same samples are re-tested
Reproducibility, reliability, repeatability

Question 19

Define: Validity

Answer 19

Ability to distinguish between who has a disease and who does not
Accuracy

Question 20

What can cause variation in test results?

Answer 20

Variation within animal subjects
Variation in the reading of test results by the same reader (measuring tool)
Variation between those reading the test results

Question 21

What is the gold standard of a diagnostic test?

Answer 21

A diagnostic test or benchmark that is the best available under reasonable conditions
The CRITERION STANDARD

Question 22

What are the four dimensions in which validity is expressed?

Answer 22

Sensitivity
Specificity
Positive predictive value
Negative predictive value

Question 23

Define: Sensitivity (Words + Mathematically)

Answer 23

The probability of the test to generate positive results among animals that actually possess the disease
Sensitivity = True Positive/Total Diseased

Question 24

Define: Specificity (Words + Mathematically)

Answer 24

The probability of a test to generate negative results among animals that are genuinely free of the disease
Specificity = True Negative /Total Non-Diseased

Question 25

Define: True Positive

Answer 25

Actual Infection

Question 26

Define: True Negative

Answer 26

Absence of infection

Question 27

Define: False Positive

Answer 27

Positive result of non-diseased individual

Question 28

Define: False Negative

Answer 28

Negative result of diseased individual

Question 29

What happens when you increase the sensitivity of a test?

Answer 29

Increased sensitivity = increased false positives

Question 30

When should you select a test with high sensitivity?

Answer 30

When missing a diseased animal would be costly or dangerous (foot in mouth)
When screening for a disease or pathogen in imported animals
Demonstration of absence of a disease in a disease-free zone

Question 31

What happens when you increase the specificity of a test?

Answer 31

Increased specificity = increased false negatives

Question 32

When should you select a test with high specificity?

Answer 32

When confirming a diagnosis
When increased false positives would cause physical, emotional, and financial damages (e.g. cancer)

Question 33

Define: Positive Predictive Value (Words + Mathematically)

Answer 33

It answers the question: “if the test result is positive, what is the probability that the animal actually has the disease?”
PPV = True Positive/Total Test Positive

Question 34

Define: Negative Predictive Value (Words + Mathematically)

Answer 34

It answers the question: “if the test result is negative, what is the probability that the animal does not have disease?”
NPV = True Negative/Total Test Negative

Question 35

What happens to predictive value with a higher prevalence?

Answer 35

With a higher prevalence, higher predictive value