Monday, August 15, 2005

Diagnosis of pulmonary embolism: Are we there yet?

A systematic review in the April 27 issue of JAMA was blogged about by Inner Visions on May 8, and has more recently been given the status of POEM as abstracted in the current issue of Cleveland Clinic Journal of Medicine. The take home message was that a normal CT done with the appropriate technique equals pulmonary angiography (PAG) for the exclusion of pulmonary embolism (PE). This sounds simple enough, but questions remain about how this evidence changes our diagnostic approach. What is the initial test of choice? Does CT replace nuclear ventilation and perfusion (V/Q) scanning?

This paper doesn’t answer these questions. Such questions require a broader perspective which takes into account the results of PIOPED I and PIOPED II. The JAMA review evaluated the performance of CT by the standard of freedom from events over three month follow up and compared those results with similar data previously reported for PAG. PIOPED I evaluated V/Q scanning against the reference standard of PAG. PIOPED II evaluated CT against a multi-dimensional diagnostic strategy using non invasive tests and occasional use of PAG. The multi-dimensional strategy was validated as a reference standard by clinical follow up for freedom from events. PIOPED II has not been published, but its results were presented at the Radiological Society of North America 90th Scientific Assembly and Annual Meeting late last year, abstracted here at Medscape (free registration required).

While consideration of all this evidence together with cost and patient tolerability provides no simple answers, I believe the following statements are supported.

1) Invasive PAG is seldom indicated.
2) A normal V/Q scan has the best negative predictive value (NPV) of all tests (note I said normal, not “low probability”). (In PIOPED I no patients with normal V/Q had PE!).
3) When CT results are discordant with clinical probability the PPV and NPV are poor. This was a significant finding in PIOPED II, similar to the findings for discordant V/Q results in PIOPED I. This means that, just as with V/Q scanning, results of CT must be integrated with other clinical data.
4) A CT scan costs more than a V/Q scan. [1].
5) There is no clear modality of choice for initial testing. Careful clinical assessment determines the choice of the initial test(s) which might include D-dimer, leg compression ultrasonography, V/Q scanning or CT.

This POEM adds to our knowledge of testing for PE but it’s just one piece of the puzzle. Be careful of simplistic interpretations.

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