Physicians at an ambulatory clinic are validating a rapid point-of-care urine antigen test for *Legionella pneumophila*. During a localized outbreak, 800 adult patients with symptoms of community-acquired pneumonia undergo testing with both the rapid antigen test and a reference standard polymerase chain reaction (PCR) assay. The PCR assay detects *Legionella* in 150 patients. The rapid antigen test is positive in 105 of these PCR-positive patients. Additionally, the rapid antigen test is positive in 35 patients who are negative by PCR. What is the positive predictive value of the rapid antigen test in this patient population? (Express your answer as a percentage).
Answer: 75 %
Answer
75
The positive predictive value (PPV) is the probability that a patient with a positive test result actually has the disease. It is calculated by dividing the number of true positives (TP) by the total number of positive test results (TP + false positives [FP]). In this scenario, the TP is 105 (patients with PCR-confirmed infection who tested positive). The FP is 35 (patients without PCR-confirmed infection who tested positive). Therefore, the PPV is calculated as 105 / (105 + 35) = 105 / 140 = 0.75, which corresponds to 75%.
Step-by-Step Solution
Key Concept
Positive Predictive Value (PPV)
Estimated Time:1m 0s