Venous thromboembolism: diagnosis and management of pulmonary embolism

To the Editor: The clinical update on venous thromboembolism by Lee and colleagues advises that “Ventilation perfusion (V/Q) isotope scanning reliably establishes the diagnosis of PE [pulmonary embolism] if the V/Q features suggest a high probability of PE . . .”.1 Although this is probably true for patients with intermediate or high pretest probability, a discordant result (low pretest probability and high probability V/Q) should be regarded with suspicion.

From the original PIOPED data, high probability V/Q was predictive of angiographically confirmed PE in 80% of patients,2 which drops to 56% by Bayesian analysis if the pretest probability is low. False positive results may be due to previous PE or unrelated parenchymal lung disease. There is significant potential morbidity associated with a false positive result for PE, both from the acute anticoagulation and for future presentations with PE-type symptoms, where PE will be accorded a higher probability because of the previous documented diagnosis.

As Lee and colleagues also state, D-dimer testing must be combined with an estimate of pretest probability to be useful. They advocate excluding PE on the basis of low pretest probability and negative D-dimer result. However, a negative D-dimer result (rapid enzyme-linked immunosorbent assay [ELISA] type) may be used to exclude PE in intermediate as well as low probability patients.3 This is dependent on the type of assay available as well as the local PE prevalence, and local guidelines should therefore be developed.