6: What does a positive test mean? Flashcards
Objectives of testing
- 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
What are challenges with testing?
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
Can tests give false positives and false negatives
Yes
Reasons for false positives
Cross reactivity
Technical error
Previos infection/vaccination
Non-specific binding
Subjective interpretation
Reasons for false negatives
Low level of target
Sample quality and timing
Technical error
Antigenic variation
Inhibitory substance
Consequence of ignoring that tests are imperfect @ individual level
Accurate diagnosis
Treatment efficacy
Cost efficiency
Animal welfare
Consequences of ignoring that tests are imperfect @ Population level
Disease surveillance & control
Public health
Food safety
Economic impact
Evaluating risk factors
Sensitivity
The proportion of animals with a disease that will have a positive test result = proportion of correctly identified diseased animals
Sensitivity equation
Sensitivity = true positives / (true positives + false negatives)
How to interpret sensitivity of 80%
80% of animals with a disease will be correctly classified as disease positive
What does sensitivity value tell us about false negatives?
The false negative fraction - 1- sensitivity = false negative fraction
Example: sensitivity of 80% - 1-0.8 = 20% false negatives
Specificity
The proportion of animals that are disease-free that are correctly classified as disease-free = proportion of correctly classified non-diseased animals
Specificity equation
Specificity = true negatives / (true negatives + false positives)
How to interpret specificity of 95%
95% of animals that are disease-free will be correctly classified as disease-free
What does specificity tell us about the false positive fraction?
1 - Sp = False positive fraction
Example - specificity of 95% = 1-0.95 = 0.05
False positive fraction = 5%
Does Sensitivity tell us about false negative or false positive fraction?
seNsitivity = false Negative fraction
Does specificity tell us about the false positive or false negative fraction?
sPecificity = false Positive fraction
Gold standard test (or Jesus himself)
A gold standard is a test or procedure that is absolutely accurate OR best available test
- confirmed clinical cases
Gold standard test (or Jesus himself)
A gold standard is a test or procedure that is absolutely accurate OR best available test
- confirmed clinical cases
All previously discussed sensitivity & specificity were EPIDEMIOLOGICAL (<3) Se/Sp. What is analytic Se?
Lowest detectable concentration of a compound/analyte
All previously discussed sensitivity & specificity were EPIDEMIOLOGICAL (<3) Se/Sp. What is analytical Sp?
Capacity to react with only one compound/analyte
Positive predictive value (PPV)
The proportion of test positive animals that are truly disease positive
How to interpret a PPV of 80%
Given that the animal has tested positive, it has an 80% chance of actually having the disease
Negative Predictive Value (NPV)
The proportion of test negative animals that are truly free of that disease
How to interpret a NPV of 80%?
Given that the animal has tested negative, it has an 80% chance of truly being disease free; q
Pre-test & post-test probabilities refer to
The probability of being infected before or after a test is done
Pre-test probability
Prevalence in population
Post-test probability
PPV of the test in that population
True prevalence
The ACTUAL level of disease within the population - rarely actually known
Apparent prevalence
The prevalence of disease as detected by tested
Screening tests are those applied to
Apparently healthy members of a population to detect seroprevalence of certain diseases, or the presence of disease agents, or subclinical disease
Diagnostic tests
Used to confirm or classify disease status, provide a guide to selection of treatment, or provide an aid to prognosis
If you have questions please ask Nicole she loves this stuff so much <3
<3 happy to help epi is my true love
