Question

  

A new antibody test detects serum antibodies against virus X (sensitivity 99%, specificity 90%). When applied in a group of hospitalized patients diagnosed as having virus X infections, the test is found to have a positive predictive value of 85%. When used to screen a group of healthy blood donors for virus X infections, the test is found to have a positive predictive value of 30%. Which of the following best explains this difference between the positive predictive values?

A.The prevalence of virus X infection is higher among the hospital patients than among blood donors

B.Lead time bias occurs among the blood donors

C.Measurement error was introduced into the procedure.

D.Cases of virus X infection are more severe in the hospital

A new blood test has been developed to screen for disease Z. Researchers establish 50 units as a cut point above which a test is considered positive and thereby indicative of disease. The test manufacturers determine that the test's sensitivity is unacceptably low. However, the manufacturers are not concerned with the specificity and do not want the cost of the test to rise. How can they improve the sensitivity of the test?

A.Test each person's blood twice.

B.Raise the cut point above 50 units.

C.They cannot improve this test and should begin work developing a new test.

D.Lower the cut point below 50 units.

A new screening test for Lyme disease is developed for use in the general population. The sensitivity and specificity of the new test are 60% and 70%, respectively. Three hundred people are screened at a clinic during the first year the new test is implemented. Assume the true prevalence of Lyme disease among clinic attendees is 10%.

Calculate the following value

The predictive value of a positive test is:

A.94.0%

B.18.2%

C.6.0%

D.33.0%

E.22.2%