PIOPED II: High-Probability V/Q with High Pretest Probability

B. High probability V/Q scan — combined with high pre-test probability, the positive predictive value for PE approaches 96%

When a HIGH probability V/Q scan is combined with HIGH clinical pre-test probability (e.g., Wells score high risk), the positive predictive value (PPV) for PE approaches 96%. This combination is considered sufficient to diagnose PE and begin anticoagulation without CTPA in most guidelines. Conversely, a NORMAL V/Q scan combined with LOW pre-test probability effectively rules out PE (NPV ~99%). The V/Q scan's clinical utility depends on integrating scan result with pre-test probability: intermediate scans in intermediate-probability patients represent the most diagnostically challenging scenario and often require CTPA.