In the absence of any weighted clinical signs of a pulmonary embolism (PE), what D-dimer measurement would warrant an expensive CT workup?
—RICH MOORE, PA-C, Bulls Gap, Tenn.
Attempts at more conservative diagnostic therapy prior to doing a CT are evaluating the relationship between elevated D-dimer levels and PE. However, many conditions can elevate D-dimer, and it is not specific. Selected D-dimer assays are highly sensitive and used as a probability of exclusion. A negative finding may not provide a sufficient diagnostic decision. A positive finding alone may not be specific for PE. There is no standard D-dimer level. Different labs use different units of measurement, which leads to subjective interpretation. There are many types of tests available, but they are not equivalent. For example, consider the latex D-dimer test and the enzyme linked immunosorbent assay (ELISA). The ELISA may take up to two weeks to determine a result. A single lab value alone should not determine the need for a CT scan. Risk factors, clinical presentation, chest x-ray, arterial blood gas measurement, and an ECG should be considered.
Because there is no standard D-dimer level, clinicians evaluating patients for suspected PE must rely on normal and cutoff levels used by their own institutions. In one study, retrospective analysis revealed that no patient with a D-dimer less than normal, defined as <275 ng/mL, was diagnosed with PE (J R Soc Med. 2005;98:54-58). That being said, the current recommendation is that a normal D-dimer alone does not adequately exclude a PE. For more information, see Am Fam Physician. 2004;69:2829-2836. The Institute for Clinical Systems Improvement offers a Technology Assessment Report on D-dimer testing (go to www.icsi.org and search for D-dimer testing. Accessed July 15, 2009).—Debra Kleinschmidt, PhD, PA (130-7)