Pulmomary embolism in Bradford, UK: role of end-tidal CO2 as a screening tool
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* Ioannis Gounaris

Editor – I read with interest Riaz and Jacob's article on using end-tidal CO2 as a screening tool for pulmonary embolism (*Clin Med* April 2014 pp 128–33).

I would like to point out that the estimates of the performance of D-dimers and Wells’ score presented in the article, including the area under curve (AUC) figures, are severely biased as a positive D-dimer or high Wells’ score were used to select patients for inclusion in the cohort in the first place. For example, the reported AUC of 0.52 for the Wells’ score should be interpreted as the ability of different values *above* the threshold to discriminate between patients with pulmonary embolism (PE) compared to those without or, to put it otherwise, whether a threshold different to the current one would be more appropriate. Similarly, the reported performance of end-tidal carbon dioxide (ETCO2) applies only to patients preselected for a computed tomography pulmonary angiogram (CTPA) on the basis of a positive D-dimer test or elevated Wells’ score, but cannot be assumed to apply to the general population of patients presenting to the hospital with suggestive respiratory symptoms.

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