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Evaluation of diagnostic tests (view source)
Revision as of 12:06, 19 January 2011
, 12:06, 19 January 2011→Evaluating diagnostic tests
* '''Sensitivity:''' the probability that an infected animal is correctly classified as positive by the diagnostic test
* '''Sensitivity:''' the probability that an infected animal is correctly classified as positive by the diagnostic test
* '''Specificity:''' the probability that an uninfected animal is correctly classified as negative by the diagnostic test
* '''Specificity:''' the probability that an uninfected animal is correctly classified as negative by the diagnostic test
* '''Positive predictive value:''' the probability of obtaining a positive result in a diseased compared with non-diseased animal
* '''Positive predictive value:''' the probability that an animal that produces a positive test result is truly diseased
* '''Negative predictive value:''' the probability of obtaining a negative result in a non-diseased compared with diseased animal
* '''Negative predictive value:''' the probability that an animal that produces a positive test result is truly diseased
* '''Likelihood ratio positive test (LR+):''' likelihood of obtaining a positive test in a disease compared with non-diseased animal
* '''Likelihood ratio negative test (LR-):''' likelihood of obtaining a negative test in a diseased compared with non-diseased animal
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As the prevalence of the disease in the population increases animals are more likely to have the disease. Therefore the probability that an animal which tests positive truly has the disease increases i.e. PV+ increases and the probability that an animal which tests negative has the disease decreases i.e. PV- decreases. If the disease is rare and there is a low prevalence in the population animal's are less likely to have the disease, therefore the likelihood that an animal which tests positive is truly positive is low i.e. PV+ is low and the likelihood that an animal which tests negative does not have the disease is high i.e. PV- is high.
As the prevalence of the disease in the population increases animals are more likely to have the disease. Therefore the probability that an animal which tests positive truly has the disease increases i.e. PV+ increases and the probability that an animal which tests negative has the disease decreases i.e. PV- decreases. If the disease is rare and there is a low prevalence in the population animal's are less likely to have the disease, therefore the likelihood that an animal which tests positive is truly positive is low i.e. PV+ is low and the likelihood that an animal which tests negative does not have the disease is high i.e. PV- is high.
==Selecting a cut-off for diagnostic tests==
==Selecting a cut-off for diagnostic tests==