Arterial blood gas values cannot rule out PE.
"Instead, they should continue to rely on D-dimer and clinical prediction models, such as the Wells criteria, or go on to diagnostic imaging," said Dr. Tara Keays of the University of Ottawa Hospital.
Researchers have tried to come up with bedside investigation tools to rule out this common and lethal disease without the need for more invasive and expensive diagnostic imaging before starting anticoagulation therapy. The most successful results involve blood testing for D-dimer, a fibrin degradation product that is produced only after a clot has formed and is in the process of being broken down, in combination with a clinical algorithm.
Recent studies combining D-dimer with arterial blood gas (ABG) values as a diagnostic method showed impressive negative predictive values, but efforts to validate the results were unsuccessful.
In a retrospective study presented by Dr. Keays, her team studied the role of ABG and D-dimer values using data from a double-blind, randomized controlled trial comparing bedside diagnostic tests with ventilation/perfusion (V/Q) scanning for excluding pulmonary embolism (PE).
Exclusion criteria included mechanical ventilation and anticoagulation. Of 824 patients screened, 458 were eligible and 399 consented, and 278 had ABG drawn initially. Of the total cohort of 399 patients, 14.3% were diagnosed with PE.
"Looking at the continuous ABG values, there was no significant difference in the mean PaC[O.sub.2], Pa[O.sub.2], or Aa gradient between the two groups. However, when we looked at the proportion of abnormal ABGs and D-dimer values, there was a significant difference," Dr. Keays explained.
"Our clinical prediction rule simply states that PE is possible if D-dimer is positive or if the D-dimer is negative and there's an abnormal PaC[O.sub.2] and an abnormal Aa gradient," she said. But a subsequent attempt to validate these findings in a retrospective analysis of 246 patients was not as clear-cut. "Normal ABG data in combination with negative D-dimer [do] not allow safe exclusion of PE without going on to diagnostic imaging," Dr. Keays concluded.
BY BRUCE K. DIXON
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|Title Annotation:||Pulmonary Medicine|
|Author:||Dixon, Bruce K.|
|Publication:||Internal Medicine News|
|Date:||Feb 15, 2006|
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