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Epidemiology > 6: What does a positive test mean? > Flashcards

6: What does a positive test mean? Flashcards

1
Q

Objectives of testing

A
  • confirm presence of disease
  • assess severity or progression of disease
  • estimate likely response to treatment (prognosis)
  • evaluate response to treatment
  • satisfy regulatory requirements
  • determine status of an animal or group of animals
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2
Q

What are challenges with testing?

A

Tests aren’t perfect
Results are subject to interpretation
Different tests measure different things
Motivation for testing influences interpretation
We test individuals or groups
Testing itself can have effects on health outcomes
Tests cost money

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3
Q

Can tests give false positives and false negatives

A

Yes

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4
Q

Reasons for false positives

A

Cross reactivity
Technical error
Previos infection/vaccination
Non-specific binding
Subjective interpretation

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5
Q

Reasons for false negatives

A

Low level of target
Sample quality and timing
Technical error
Antigenic variation
Inhibitory substance

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6
Q

Consequence of ignoring that tests are imperfect @ individual level

A

Accurate diagnosis
Treatment efficacy
Cost efficiency
Animal welfare

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7
Q

Consequences of ignoring that tests are imperfect @ Population level

A

Disease surveillance & control
Public health
Food safety
Economic impact
Evaluating risk factors

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8
Q

Sensitivity

A

The proportion of animals with a disease that will have a positive test result = proportion of correctly identified diseased animals

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9
Q

Sensitivity equation

A

Sensitivity = true positives / (true positives + false negatives)

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10
Q

How to interpret sensitivity of 80%

A

80% of animals with a disease will be correctly classified as disease positive

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11
Q

What does sensitivity value tell us about false negatives?

A

The false negative fraction - 1- sensitivity = false negative fraction
Example: sensitivity of 80% - 1-0.8 = 20% false negatives

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12
Q

Specificity

A

The proportion of animals that are disease-free that are correctly classified as disease-free = proportion of correctly classified non-diseased animals

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13
Q

Specificity equation

A

Specificity = true negatives / (true negatives + false positives)

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14
Q

How to interpret specificity of 95%

A

95% of animals that are disease-free will be correctly classified as disease-free

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15
Q

What does specificity tell us about the false positive fraction?

A

1 - Sp = False positive fraction

Example - specificity of 95% = 1-0.95 = 0.05

False positive fraction = 5%

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16
Q

Does Sensitivity tell us about false negative or false positive fraction?

A

seNsitivity = false Negative fraction

17
Q

Does specificity tell us about the false positive or false negative fraction?

A

sPecificity = false Positive fraction

18
Q

Gold standard test (or Jesus himself)

A

A gold standard is a test or procedure that is absolutely accurate OR best available test
- confirmed clinical cases

19
Q

Gold standard test (or Jesus himself)

A

A gold standard is a test or procedure that is absolutely accurate OR best available test
- confirmed clinical cases

20
Q

All previously discussed sensitivity & specificity were EPIDEMIOLOGICAL (<3) Se/Sp. What is analytic Se?

A

Lowest detectable concentration of a compound/analyte

21
Q

All previously discussed sensitivity & specificity were EPIDEMIOLOGICAL (<3) Se/Sp. What is analytical Sp?

A

Capacity to react with only one compound/analyte

22
Q

Positive predictive value (PPV)

A

The proportion of test positive animals that are truly disease positive

23
Q

How to interpret a PPV of 80%

A

Given that the animal has tested positive, it has an 80% chance of actually having the disease

24
Q

Negative Predictive Value (NPV)

A

The proportion of test negative animals that are truly free of that disease

25
Q

How to interpret a NPV of 80%?

A

Given that the animal has tested negative, it has an 80% chance of truly being disease free; q

26
Q

Pre-test & post-test probabilities refer to

A

The probability of being infected before or after a test is done

27
Q

Pre-test probability

A

Prevalence in population

28
Q

Post-test probability

A

PPV of the test in that population

29
Q

True prevalence

A

The ACTUAL level of disease within the population - rarely actually known

30
Q

Apparent prevalence

A

The prevalence of disease as detected by tested

31
Q

Screening tests are those applied to

A

Apparently healthy members of a population to detect seroprevalence of certain diseases, or the presence of disease agents, or subclinical disease

32
Q

Diagnostic tests

A

Used to confirm or classify disease status, provide a guide to selection of treatment, or provide an aid to prognosis

33
Q

If you have questions please ask Nicole she loves this stuff so much <3

A

<3 happy to help epi is my true love

Epidemiology (9 decks)

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