Respiratory disease detection using clinical category information
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populations at different frequencies. For example, it may be possible that respiratory disease occurrence might be different in one specific group of animals compared to another. If the
prevalence of a disease is different in different populations then applying the same diagnostic test to those different populations may produce different predictive values
because predictive value estimates are influenced by the prevalence.
For respiratory disease, gross necropsy has a positive predictive value PPV of 0.9 and a negative predictive value NPV of 0.85.
This means that if a gross necropsy is performed during a voyage and the gross necropsy diagnosis is respiratory disease, then that animal has a 90 probability of truly having
respiratory disease as the cause of death positive predictive value.
Conversely if a gross necropsy is performed and the gross necropsy diagnosis is not respiratory disease, then that animal has an 85 probability of having died from a cause
other than respiratory disease negative predictive value.
Having information on diagnostic test performance and gold standard test outcomes also provides a more detailed understanding of prevalence estimates. The prevalence of
respiratory disease as a cause of death is the probability that a mortality case died of respiratory disease.
The diagnostic test result gross necropsy classified animals as test positive or test negative and using these numbers we can generate a prevalence estimate of respiratory disease:
100215 = 47.
The term apparent prevalence is used to refer to the prevalence estimate derived from the diagnostic test gross necropsy because it is really a prevalence of a positive test result and
not the prevalence of true disease since the diagnostic test is not perfect.
If a gold standard test result is available then these results provide an estimate of the true prevalence, based on the final cause of death results.