In reply: We thank Mountain for his comments. As he points out, it is only possible to infer cause and effect from observational data. The fact that referrals for D-dimer levels and computed tomography pulmonary angiography increased during the same period does not prove that one “drove” the other, although we believe this is a reasonable inference. Importantly, D-dimer testing was introduced as a “rule-out” test in low-risk patients, in an effort to reduce unnecessary referrals for venous thromboembolism imaging; but we found no evidence in our observational data that this had occurred.
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