Enter the D-dimer. D-dimers are little chunks of broken up fibrin, like FDPs, but with an important difference: they contain an extra little linkage. When fibrin seals up a clot, there’s actually an extra, final step in which factor XIII creates little cross-links between the fibrin molecules. When the fibrin in a clot is busted up, some of the resulting fragments will contain these little cross-links. These little fragments contain one “E” fibrin subunit and two “D” subunits (check out the diagram above), and are called D-dimers.
D-dimers are more specific for actual clots than FDPs are – because you only get D-dimers from the breakdown of real clots (not from the breakdown of fibrinogen). Tests that specifically look for D-dimers were developed in the 1990s, and most labs use these D-dimer assays now instead of assays that measure FDPs.