Question

A screening test for a newly discovered disease is being evaluated. In order to determine the effectiveness of the new test, it was administered to 900 workers. 150 of the individuals diagnosed with the disease tested positive. A negative test finding occurred in 60 people who had the disease. A total of 50 persons not diseased tested positive for it. Assume the prior probability is not known.

What was the sensitivity of the test?

What was the specificity of the test?

What was the predictive value of a positive test?

What was the predictive value of a negative test?

What is the overall accuracy of the test?

What is the LR+ of the test?

What is the LR– of the test?

Which of the following is an appropriate interpretation for LR–?

Answer 1

Sensitivity= true positive/ true positive + false negative

= 150/150+ 60= 0.72

Specificity= true negative/ true negative+ false positive

= 750/750+ 50= 0.93

Predictive value for positive test= true positive/ true positive+ false positive

= 150/150+ 50= 0 75

Predictive value for negative test= true negative/true negative+ false negative

= 750/750+60= 0.92

Positive lr= sensitivity/ 1-specificity

= 0.72/1-0.93= 10.28

Negative lr = 1-sensitivity/ specificity

= 1- 0.72/0.93= 0.3

Interpretation

Lr negative

Decrease 0.3 probablity of having disease given the test is negative

Lr positive

Increase 10.28 probablity of having disease given test is positive.