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39 Cards in this Set
- Front
- Back
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What is the difference between serum chemistry results and infectious disease test results?
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serum chemistry results are on a continuous scale, while infectious disease test results are either positive or negative based on cutoff points
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What is considered the normal range for a biochemical profile?
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mean +/- two standard deviations
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What values are considered normal for a biochem profile if the data isn't normally distributed?
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2.5-97.5 percentile
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What is a titer?
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highest dilution that produces a test reaction
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What scale of dilution is usually used for titers?
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2 fold
(can see 10 fold, or 2 then 10 fold) |
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define seropositive
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animal that has a detectable titer at or above the cutoff
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define seronegative
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animal that does not have a detectable titer at the cutoff
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What is a cutoff value?
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threshold for what is considered a positive or negative test
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What do circulating antibodies indicate?
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infection
exposure recovered from a previous infection maternal antibodies in neonates |
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What do circulating antigens indicate?
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current infection only
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define seroprevalence
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proportion of seropositive animals in a population
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What does seroprevalence depend on?
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rate of seroconversion
half-life of the antibodies length of time after seroconversion *doesn't necessarily mean there is a high rate of infection |
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What change in titer is considered significant?
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four fold change, may indicate infection (but not always)
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define analytical sensitivity
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lowest level of substance that can be detected by an assay
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define analytical specificity
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how well the test detects only one substance (lack of cross reactivity)
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define prevalence
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probability that a randomly selected animal has the disease
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define apparent prevalence
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probability that a randomly selected animal has a positive test result
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define diagnostic sensitivity
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probability that an animal has a positive test result given it has the disease
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define diagnostic specificity
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probability that an animal has a negative test result, given it doesn't have the disease
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define positive predictive value
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probability that the animal tests positive when it has the disease
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define negative predictive value
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probability that an animal that tests negative will not have the disease
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Why are predictive values more useful clinically than sensitivity and specificity?
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they use the test result to make an inference about the true disease status which is more useful in the clinic
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What do predictive values depend on?
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prevalence
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What is a gold standard?
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perfectly predicts which animals do and do not have disease
*in real life, the test with the highest sensitivity and specificity |
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What is the relationship between true prevalence and apparent prevalence?
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as true prevalence increases, so does apparent prevalence
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How do cutoff values affect sensitivity and specificity?
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if the cutoff value is increased, specificity increases and sensitivity decreases (and vice versa)
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What effect does prevalence have on PPV and NPV?
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increased prevalence increases the PPV and decreases the NPV (and vice versa)
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What assumptions are generally made about sensitivity and specificity?
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constant, do not change based on the population in which they are used (this may not be true!)
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What is a receiver-operating characteristic curve?
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graphical representation of all possible outcomes of Se and Sp for a test (based on the possible cutoff values)
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How are ROC curves interpreted?
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generally the point closest to the northwest corner of the graph is the Se and Sp with the greatest accuracy for a given test
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When should you NOT select the most accurate point on an ROC curve as the cutoff for a test?
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when the sensitivity or specificity needs to be very high (ie: the risk of having a positive animal test negative is unacceptable or vice versa)
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How can ROC curves be used to compare different tests?
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plot them all on the same graph, see which one has the greatest area under the curve (closest to the NW corner). That one is the most accurate.
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What is a kappa statistic?
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measure of agreement
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how is kappa interpreted?
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less than 0 = poor agreement
1 = perfect agreement |
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When is kappa used?
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to compare two or more observers or tests
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What assumptions are made when kappa is used?
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observers/tests are independent conditional on disease status
observers/tests are blinded to the other's results |
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What is conditional independence?
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second test sensitivity is not dependent on the results of the first
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What does it mean to have serial test interpretation?
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one test applied first, then the positive animals are tested with another test
*animal must test positive on both to be considered positive *increases specificity, decreases sensitivity |
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What is parallel testing?
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multiple tests are applied at the same time, animal is considered positive if either test is positive
*increases sensitivity, decreases specificity |
