Can I use a D-dimer blood test to diagnose DVT?
D-dimer is a breakdown product of a thrombus and is an indirect marker of an active clot. A very sensitive assay will detect a thrombus, but there are many other conditions that can cause a false positive result. A negative D-dimer test can rule out DVT. However, this test should not be used to confirm DVT since its specificity is low. The D-dimer test should only be used to rule OUT thrombosis in patients with a low clinical suspicion of DVT, calculated using the modified Well’s criteria. The original Well’s criteria1 classified the pretest (clinical) probability into low, moderate, and high categories. The modified Well’s system classifies patients as likely or unlikely to have DVT. These modified criteria were used in a randomized trial that assessed the diagnostic value of the D-dimer test.2 Patients who were clinically unlikely to have DVT did not benefit from an ultrasound test if their D-dimer result was negative. Their rate of VTE (0.4%) during 3 months of follow-up was compara
