John T. Philbrick, MD; Steven Heim, MD, MSPH; Joel M. Schectman, MD, MPH
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Philbrick JT, Heim S, Schectman JM. d-Dimer and Venous Thromboembolism. Ann Intern Med. 2004;141:482. doi: 10.7326/0003-4819-141-6-200409210-00020
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Published: Ann Intern Med. 2004;141(6):482.
TO THE EDITOR:
Although Stein and colleagues' conclusion, “a negative quantitative rapid ELISA result is as diagnostically useful as a normal lung scan or negative duplex ultrasonography finding,” is technically correct (1), the issues surrounding this use of d-dimer are more complex. For a test used to rule out disease, high sensitivity is not the only important characteristic. Specificity also plays a key role and limits the use of d-dimer for venous thromboembolism (VTE) in at least 2 ways.
First, the likelihood ratio for a negative test result, calculated as (1 − sensitivity)/specificity, is inversely proportional to specificity. Few tests have invariable sensitivity and specificity. Instead, these indexes vary with the clinical characteristics of patient samples. This is particularly true for the specificity of d-dimer. Many conditions besides thromboembolism cause “elevated” d-dimer levels. When large numbers of patients with these conditions, such as cancer, trauma, surgery, and advanced age, are included in a study, specificity will be very low, the likelihood ratio for a negative test result will not be as small as reported by Stein and colleagues (1), and d-dimer levels of those with VTE will sometimes be indistinguishable from levels in those without (2). Thus, unless the clinician orders d-dimer testing only in patients without conditions known to cause elevated levels, results will not be diagnostically useful for VTE.
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