Determining the Significance of a Test Result

How do we determine the significance of a test result in a given patient? In deciding the value that makes a lab test “positive”, we almost always make a compromise between a value that generates too many false positives (FP) versus a value with too many false negatives (FN). What we choose depends on the purpose of the test. In most situations we want a balance. However, sometimes we want to capture as close as possible to 100% of people with the disease. Therefore, we accept some false positives. For example, in newborn screening for phenylketonuria (PKU), the false positive rate is higher than we would like. This high false positive rate creates a need for further testing which generates a lot of anxiety in families waiting for the result of confirmatory testing. However, since we do not want to miss anyone with the disease we tolerate these anxieties.