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.

To the Editor: I read with interest the article by Lee et al regarding the investigation and treatment of pulmonary embolism (PE).1 The investigation of patients presenting with PE as a diagnostic possibility is of great interest to emergency physicians, and such presentations are a daily occurrence in emergency departments around the country.

Unfortunately, only a small amount of text is devoted to describing the relative merits of ventilation perfusion (V/Q) scanning and computed tomography pulmonary angiography (CTPA), and no guidance is provided as to which is the test of choice when both are available. The British Thoracic Society has recommended CTPA as the lung imaging modality of first choice for patients presenting with non-massive PE.2 There is a large and increasing body of evidence that CTPA provides superior specificity to V/Q scanning in the detection of PE. CTPA also provides the opportunity of establishing diagnoses other than PE and, in addition, a negative multi-slice CTPA is of sufficient sensitivity to enable the withholding of anticoagulation.3 It is also my experience that CTPA is easier to obtain out of hours, compared with V/Q scanning.

The authors state that V/Q scanning “reliably establishes the diagnosis of PE if the V/Q scan features suggest a high probability of PE . . .”.1 Unfortunately, this statement is incorrect. It is essential that V/Q scan results be interpreted in the light of the patient’s clinical probability for PE. In the PIOPED study, only 56% of patients with high probability V/Q scan reports had pulmonary embolism if the pretest probability was low.4

No mention is made of the special situation of pregnant women presenting with pleuritic pain, or which lung imaging test is considered “safest” for both mother and baby. Although the risks of PE are generally agreed to be increased in pregnancy, it is my experience that pregnant women are extremely reluctant to undergo any form of diagnostic investigation that exposes the fetus to radiation.

The Wells criteria have been validated for the assessment of PE in emergency department patients only, and provide a means for clinicians with little experience to make an accurate assessment of an individual patient’s clinical probability of PE.5 Once initiated, clinical assessment of the patient with possible PE is straightforward. The key question facing emergency physicians is this: is there a group of patients that have such low probability for PE that no investigation at all is required?

In reply: Pulmonary embolism (PE) remains a complex diagnosis despite the availability of validated prediction models and D-dimer testing to direct the need for diagnostic imaging.

We agree with Bailey that the ability to exclude the diagnosis of PE on clinical grounds in patients with a low pretest probability is highly desirable. Unfortunately, clinical features lack sensitivity and specificity for the diagnosis of PE, and clinical prediction models, laboratory investigations, and diagnostic imaging are likely to remain an integral part of the clinical work-up.

As suggested by Bragg, it may be possible to simplify the diagnostic approach by using a highly sensitive D-dimer assay, and simplified pretest probability models have been proposed. However, this may come at a cost of reduced specificity,1 which leads to unnecessary diagnostic imaging studies and thus limits the clinical utility of these approaches. Further improvements in the diagnostic approach to PE are clearly needed.

There are emerging data demonstrating the accuracy of computed tomography pulmonary angiography (CTPA) for the diagnosis of PE. However, CTPA has limitations (a large contrast load, high radiation dose, and lack of sensitivity of first generation scanners for small thrombi2), some of which are evident in the recently published validation study referred to by Bailey:3 25% of screened patients with suspected PE were not eligible for this study because of renal impairment, a contraindication to CT, or other reasons.

The diagnostic algorithm that we provided in our review suggests that either ventilation perfusion (V/Q) scanning or CTPA can be used for patients with suspected PE who require diagnostic imaging,2 with the choice determined by patient factors and availability.

The diagnosis of PE during pregnancy is challenging because of concerns about radiation exposure and uncertainty about whether CTPA or V/Q delivers more radiation to the fetus.4 Furthermore, clinical decision rules have not been validated in pregnancy. However, recommendations from experts and professional bodies suggest that V/Q scanning can be used in combination with compression ultrasound to establish or exclude the diagnosis of PE during pregnancy in most cases with minimal fetal radiation exposure.5,6

The comments by Bailey and Bragg concerning the interpretation of high probability V/Q scan results highlight the pitfalls of performing diagnostic imaging without considering the patient’s pretest probability of PE. Although a high probability V/Q scan is diagnostic in patients with a moderate or high pretest probability of PE (prevalence of disease ≥ 90%), the prevalence of disease is only about 50% in those with a low pretest probability.7,8 Therefore, V/Q scanning should not be performed in patients with a low pretest probability unless the D-dimer test is positive. In this situation the algorithm for moderate or high pretest probability should be followed,2 and a high probability scan reliably establishes the diagnosis.