Pioped criteria for determining the probability of pulmonary embolism on a V/Q Scan

High Probability (80-100% likelihood for PE):

1. Greater than or equal to 2 large mismatched segmental perfusion defects or the arithmetic equivalent in moderate or large and moderate defects. A high probability lung scan confirms a very high likelihood for pulmonary embolism and justifies treatment with anticoagulation (unless contraindicated).
Caveat - It has been suggested that 2.5 mismatched large segmental defects (or the arithmetic equivalent) is a better threshold for calling a scan high probability, as it associated with a 100% probability of PE in the PIOPED population.

Intermediate Probability (20-80% likelihood for PE):

1. One moderate to 2 large mismatched perfusion defects or the arithmetic equivalent in moderate or large and moderate defects.

2. Single matched ventilation-perfusion defect with a clear chest radiograph.
Caveat - Single ventilation-perfusion matches are borderline for "low probability" and thus should be categorized as "intermediate" in most circumstances by most readers, although individual readers may correctly interpret individual scintigrams with this pattern as "low probability".

3. Difficult to categorize as low or high, or not described as low or high.

Low Probability (0-19% likelihood for PE):

1. Perfusion defects matched by ventilation abnormality provided that there are: (a) clear chest radiograph and (b) some areas of normal perfusion in the lungs. Extensive matched V/Q abnormalities are appropriate for low probability, provided that the CXR is clear.

2. Any perfusion defect with a substantially larger chest radiographic abnormality.

3. Any number of small perfusion defects with a normal chest radiograph.

4. Nonsegmental perfusion defects (e.g., cardiomegaly, enlarged aorta, enlarged hila, elevated diaphragm).

5. Multiple matched V/Q abnormalities, even when relatively extensive, are low probability for PE [11]. The prevalence of PE in patients with extensive matched V/Q defects and no CXR abnormality was 14% (low probability).


1. No perfusion defects or perfusion exactly outlines the shape of the lungs seen on the chest radiograph (note that hila and aortic impressions may be seen and the chest radiograph and/or ventilation study may be abnormal).