Pulmonary embolism diagnosis

Associate Editors-in-Chief: Ujjwal Rastogi, MBBS [mailto:urastogi@perfuse.org]

Overview
Pulmonary Embolism is difﬁcult to diagnose which is often missed because of non-speciﬁc clinical presentation. As immediate treatment is highly effective, early diagnosis is fundamental and important.

Diagnosis
The diagnosis of PE is based primarily on the clinical evaluation combined with diagnostic modalities such as spiral CT, V/Q scan, use of the D-dimer and lower extremity ultrasound.

Pretest Probability
In spite of all of nonspecific clinical and lab findings, it has been shown that clinicians are actually fairly good at assigning meaningful clinical probabilities for PE.
 * In PIOPED, 67% of the patients labeled as having a high clinical probability (>80% likelihood) had PE, as compared with only 9% of those give a low clinical probability (<20% likelihood).
 * Unfortunately, the majority of patients (64%) were assigned an intermediate clinical probability (20 – 80% likelihood), reinforcing the fact that a clinical diagnosis can be difficult.

High Pretest Probability
Many authors, reserve the term high pretest probability for those patients with a clinical presentation consistent with PE, in whom an alternative diagnosis is not apparent (e.g. pneumonia) and who have known risk factors for venous thromboembolism (VTE).

Low Pretest Probability
Low pretest probability patients include those patients with an alternative diagnosis to explain the clinical findings or those without risk factors.

Intermediate Pretest Probability
Intermediate probability patients include those patients not fitting either high or low pretest probability definitions.

Predicting the Risk of Pulmonary Embolism
The decision to do medical imaging is usually based on clinical grounds, i.e. the medical history, symptoms and findings on physical examination.

The most commonly used method to predict clinical probability is the Wells score, a clinical prediction rule, the use of which is complicated by multiple versions being available.
 * Development of the Wells score
 * In 1995, based on literature search and clinical criteria, Wells et al developed a prediction rule to predict the likelihood of PE.

There are additional prediction rules for PE, such as the Geneva rule. More importantly, the use of any rule is associated with reduction in recurrent thromboembolism.
 * The prediction rule was revised in 1998. This prediction rule was further revised and simplified during a validation by Wells et al in 2000.
 * In the 2000 publication, Wells proposed two different scoring systems using cutoffs of 2 or 4 with the same prediction rule. In 2001, Wells published results using the more conservative cutoff of 2, to create three categories.
 * An additional version, the "modified extended version", using the more recent cutoff of 2 but including findings from Wells's initial studies were proposed.
 * Most recently, a further study reverted to Wells's earlier use of a cutoff of 4 points to create only two categories.

Wells score
The Wells score:
 * Clinically suspected DVT  (leg swelling, pain with palpation)  - 3.0 points
 * Alternative diagnosis is less likely than PE - 3.0 points
 * Tachycardia - 1.5 points
 * Immobilization/surgery in previous four weeks - 1.5 points
 * History of DVT or PE - 1.5 points
 * Hemoptysis - 1.0 points
 * Malignancy (treatment for within 6 months, palliative) - 1.0 points

Traditional interpretation (Wells criteria)
 * Interpretation of the Wells score
 * Score >6.0 - High (probability 59% based on pooled data )
 * Score 2.0 to 6.0 - Moderate (probability 29% based on pooled data )
 * Score <2.0 - Low (probability 15% based on pooled data )

Alternate interpretation (Modified Wells criteria)
 * Score > 4 - PE likely. Consider diagnostic imaging.
 * Score 4 or less - PE unlikely. Consider D-dimer to rule out PE.