D-Dimer Logic

D-dimer is a decision aid, not a clot locator. A raised result shows that cross-linked fibrin has been formed and degraded somewhere in the body; it does not establish a clinically important venous thromboembolism (VTE) or identify its site. A low result becomes useful only when the patient's prior probability is low enough and the assay is sensitive enough for a negative result to be trusted. Start with the patient rather than the laboratory request. Use a validated clinical assessment such as Wells or revised Geneva before ordering the test. A patient with high or likely probability proceeds to imaging, whereas an appropriately selected patient with low or intermediate probability may be ruled out by a negative high-sensitivity quantitative result. A patient who is already low risk and satisfies every PERC criterion needs no D-dimer at all; testing can create a false-positive cascade to imaging without adding safety. This sequence prevents two opposite errors: treating an isolated raised result as proof of thrombosis, and using a negative result to reassure a patient whose clinical probability is too high for D-dimer...