What effect does increasing the peak systolic velocity criterion have on predictive values in carotid testing for severe stenosis?

Increasing the peak systolic velocity criterion in carotid testing serves to enhance the specificity of the test for diagnosing severe stenosis. As the threshold for what is considered a positive result for severe stenosis is raised, the positive predictive value (PPV) tends to increase. This happens because a higher peak systolic velocity indicates a more significant narrowing of the artery, making it more likely that a positive test truly corresponds to severe stenosis.

Moreover, as the criterion becomes more stringent, it can lead to a decrease in the negative predictive value (NPV). This is due to the higher likelihood that cases with lower velocities may be falsely classified as negative, causing some true severe stenosis cases to be missed. Thus, while PPV is improved due to a higher proportion of true positives among the positive test results, a decrease in the NPV reflects a reduction in the ability to accurately identify cases that do not have severe stenosis among those with lower velocities.

This dynamic illustrates the trade-off often encountered in diagnostic testing, where increasing specificity can compromise sensitivity, thereby influencing predictive values in opposite directions.