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Today I entered a rabbit hole and learned a ton about d-dimer. It was humbling to realize how little I knew about this pesky blood test. Here are some things I've learned (1/7)
D-dimer is one of several fragments made when plasmin cleaves fibrin. Simply, when a clot is broken down, d-dimer is made [among other stuff]. So what is d-dimer made of? It is made of one part factor 13 and two parts fibrin d-domains. So fibrin+factor 13+fibrin = d-dimer. (2/7)
D-dimer assays have been around since the 1970s (!). Back then, they were used to check for evidence of DIC [disseminated intravascular coagulation]. Over time, the test became more refined! (3/7)
Nowadays d-dimer is used to RULE OUT VTE and does so with sensitivity of about 95%. The specificity is less impressive [about 50%]. Sometimes it is also used to assess whether someone with unprovoked VTE should received longer therapy. (4/7)
D-Dimer is NOT specific for VTE. A lot of things can increase it: age (esp age >70), pregnancy, ethnicity (African origins), and pretty much anything that causes inflammation (e.g., infection, cancer, rheumatologic diseases, IBD). (5/7)
D-dimer testing is a dog’s breakfast. Odds are your lab does things differently then the lab across the street. In total, there are over 30 different assays. The assays differ in all sorts of things I barely understand [method of testing, instrumentation, calibration]. (6/7)
Take home points: d-dimer is sensitive for VTE but not specific. Many things can cause increased D-dimer. “Cut-off” values you read in a paper are likely not applicable to where you work. So ask your lab what testing approach is used there. When in doubt, call heme. 
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