Most clinical laboratories consider values greater than 2 standard deviations from the mean as abnormal. Increasing the cut off levels would decrease the number of false positive but would increase the number of false negative. The test would be more specific but less sensitive. If the management of disease involves a high mortality rate cut off level for the diagnostic test should be adjusted to provide greater specificity. Range of values of sensitivity and specificity that a test can take on over a range of possible cutoff values.
Predictive value gives the probability of the disease state once the patient s test result is known.
Highly sensitive and specific test may have poor predictive value if the prevalence of disease in the population is low. The predictive value cannot be generalized to a new population, because the prevalence of disease may differ between two populations. Your pretest probability of infection is 0.70. You decide that you will treat the patient with antibiotics as long as the probability of endocarditis is greater than 0.05. I would do an erythrocyte sedimentation rate .