Question

A) Sequential screening is performed to reduce the number of false-positives whereby persons who test positive on the first screening test are re-tested a second time. A county-level screening program was implemented whereby 1000 persons were tested for a viral disease in sequence using the same test. The prevalence of the viral disease at the county-level is 15%. The test has a sensitivity of 90% and specificity of 95%.

Calculate the PPV (positive predictive value) of persons who undergo both tests.

B)

Sequential screening is performed to reduce the number of false-positives whereby persons who test positive on the first screening test are re-tested a second time. A county-level screening program was implemented whereby 1000 persons were tested for a viral disease  in sequence using the same test. The prevalence of the viral disease at the county-level is 15%. The test has a sensitivity of 90% and specificity of 95%.

Calculate the NPV (negative predictive value) of persons who undergo both tests.

NOTE: Cells values represent persons so should be round to the whole number. Round predictive values to one digit after the decimal and round up for values of 5 or more.

Answer 1

positive predictive value are proportion of positive test that are true positive and represent the presence of disease

PPV can be calculated from sensitivity , specificity and prevalence

  

=

= 0.7606

PPV is 76.06% or 76.1%

negative predictive value are proportion of negative test that are true negatives and represent the absence of disease

=

= 0.9818

NPV is 98.18% or 98.2%