Epidemiology Sensitivity and Specificity – Flashcards
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Diagnostic test
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Used to determine the presence of absence of a disease when a subject shows signs or symptoms of a disease
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Screening test
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Identifies asymptomatic individuals who may have the dz
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What do you do after a positive screening test?
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A diagnostic test to establish a definitive dx
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What are two types of errors your tests can have?
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Biological Non-biological
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Analytic Sensitivity (Se)
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Lowest concentration of substance that the rest is capable of detecting
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Analytic Specificity (Sp)
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If a test reacts to more than one substance, it is not specific to that one substance
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What do Diagnostic Se and Sp depend on?
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Analytic Se and Sp (But they're different concepts!)
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Gold standard
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Test that diagnoses all diseased animals and misdiagnoses none
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Measures of imperfection
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Diagnostic sensitivity and specificity
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Diagnostic Sensitivity
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Proportion of diseased animals that test positive (Few false negatives)
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Diagnostic Specificity
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Proportion of non-diseased animals that test negative (Few false negatives)
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When do you want high sensitivity?
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Before moving animals from endemic to dz free areas
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When do you want high specificity?
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To confirm disease Before initiating aggressive therapeutics
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What value is useful in animals of unknown disease status
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Predictive values
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Predictive value definition
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The probability that an animal has or does not have the disease, given the test result
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Positive predictive value
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PV(+)=a/(a+b) Probability an animal is positive if it tested positive
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Negative predictive value
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PV(-)=d/(c+d) Probability an animal is negative if it tested negative
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PV is influenced by
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Se, Sp, prevalence (aka prior probability)
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As PV(+) increases, PV(-)
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Decreases
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What kind of test increases PV(+)?
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More specific
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What is PV(+) in a population where all animals are infected?
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100%
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As prevalence increases, PV(+) ______ and PV(-) _______
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increases decreases
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Using a more specific test _______ PV(+)
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Increases (fewer false positives)
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What's the best way to maximize PV(+)?
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Using a test with high Sp
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What's the best way to maximize PV(-)?
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Using a test with high Se
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When consequences of missing a diseased animal are highly undesirable, what do you want?
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A test with high Se and high PV(-) (false negatives are bad)
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When consequences of misdiagnosing a disease in a healthy animal is highly undesirable, what do you want?
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A test with high Sp and high PV(+) (false positives are bad)
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When using tests, what should you always verify about the population used for evaluation of the test?
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That it's similar to the population likely to be seen in your practice
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True prevalence
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True state of nature
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Apparent prevalence
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The result we get due to imperfections in the diagnostic tests used =TP+FP
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If we know Se and Sp of the diagnostic test, what can we estimate?
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The true prevalence from the apparent prevalence
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True prevalence math
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TP+FN=(AP+Sp-1)/(Se+Sp-1)
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If your true prevalence is not between 0 and 1, what does it mean?
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That your Se and Sp are not accurate for your population
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What is included in a reference range?
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Range of values in 95% of healthy reference population
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What is excluded in a reference range?
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The highest and lowest 2.5%
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What does the cut-off point of the reference range affect?
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The test's diagnostic Se and Sp
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If you raise the cutoff above which the test is declared positive, what happens?
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Se decreases Sp increases
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When you reduce the cutoff above which a test is declared negative, what happens?
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Se increases Sp decreases
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So what do you want to do when you select a cutoff for a test?
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Optimize Se and Sp
