End tidal carbon dioxide tension (P(ET,CO(2))) is a surrogate for dead space 
ventilation which may be useful in the evaluation of pulmonary embolism (PE). We 
aimed to define the optimal P(ET,CO(2)) level to exclude PE in patients 
evaluated for possible thromboembolism. 298 patients were enrolled over 6 months 
at a single academic centre. P(ET,CO(2)) was measured within 24 h of 
contrast-enhanced helical computed tomography, lower extremity duplex or 
ventilation/perfusion scan. Performance characteristics were measured by 
comparing test results with clinical diagnosis of PE. PE was diagnosed in 39 
(13%) patients. Mean P( ET,CO(2)) in healthy volunteers did not differ from P( 
ET,CO(2)) in patients without PE (36.3+/-2.8 versus 35.5+/-6.8 mmHg). P(ET,CO(2 
)) in patients with PE was 30.5+/-5.5 mmHg (p<0.001 versus patients without PE). 
A P(ET,CO(2)) of >or=36 mmHg had optimal sensitivity and specificity (87.2 and 
53.0%, respectively) with a negative predictive value of 96.6% (95% CI 
92.3-98.5). This increased to 97.6% (95% CI 93.2-99.) when combined with Wells 
score <4. A P(ET,CO(2)) of >or=36 mmHg may reliably exclude PE. Accuracy is 
augmented by combination with Wells score. P( ET,CO(2)) should be prospectively 
compared to D-dimer in accuracy and simplicity to exclude PE.