Researchers are evaluating a diagnostic test for Legionella infection that relies on detection of bacterial antigens in the serum. The test is compared to bacterial cultures, which are the gold standard for diagnosis. A sample of 200 patients is selected. Among 100 patients with culture-positive infection, the antigen test was positive in 90 patients. Among 100 patients with negative cultures, the test was positive in 30 patients. What was the positive predictive value of the new serum antigen test in this sample?
A)30%
B)70%
C)75%
D)88%
E)90%

Question

Researchers are evaluating a diagnostic test for Legionella infection that relies on detection of bacterial antigens in the serum.  The test is compared to bacterial cultures, which are the gold standard for diagnosis.  A sample of 200 patients is selected.  Among 100 patients with culture-positive infection, the antigen test was positive in 90 patients.  Among 100 patients with negative cultures, the test was positive in 30 patients.  What was the positive predictive value of the new serum antigen test in this sample?
A)30%
B)70%
C)75%
D)88%
E)90%

Quizplus · Accepted Answer

Explanation: 11ec2b69_c104_2f0a_8f63_657f6755f42a_MD0005_00 The positive predictive value (PPV) is one of a number of parameters used to assess the accuracy and utility of diagnostic tests. It is defined as the percentage of individuals that actually have the disease out of all positive test results (ie, the probability that a patient actually has the disease given a positive test result). PPV is calculated as PPV = TP / (TP+FP). In this example, it can be easily solved for by completing the 2×2 contingency table, which gives a PPV of 75%. parameters 11ec2b69_c104_2f0b_8f63_4144664967f4_MD0005_00 (Choice A) The false positive rate is the likelihood that the test will be positive in a patient without the disease. In this case, the false positive rate (FP / [FP+TN]) of the serum antigen test is 30%. (Choice B) Specificity is the likelihood that a patient without the disease will test negative. In this case, the specificity of the serum antigen test is 70%. (Choice D) The negative predictive value (NPV) is the likelihood of not having the disease given a negative test result. In this case, the NPV of the serum antigen test is 88%. (Choice E) Sensitivity refers to the likelihood that a patient with the disease will test positive. In this scenario, the sensitivity of the serum antigen test is 90%. Educational objective: The positive predictive value conveys the probability that a patient actually has the disease given a positive test result.

Researchers Are Evaluating a Diagnostic Test for Legionella Infection That