FDP vs D DIMER : THE GAMES OF SENSITIVITY AND SPECIFICITY

FDP vs D DIMER

First we need to understand clot formation with this perspective

CLOT FORMATION

First platelet plug is formed than fibrin (from a bigger molecule called fibrinogen) sticks to the platelet plug and seals it up. 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.

NOW we need to Know how FDP and D DIMERs are produced ?

Plasmin can attack the fibrin in the plug, chopping it up into little fragments called fibrin degradation products, which can BE measured in the blood. This breakdown process actually starts happening almost right away, as soon a clot is being formed.

What is the difference between FDPs and D-dimers?

Fibrin degradation products (FDPs) and D-dimers are both little fibrin-containing molecules formed by fibrin degredation. D-dimers are different from FDP as  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 degredation occurs, some of the resulting fragments will contain these cross-links and are called D-dimers.

SO D DIMERS ARE ESSENTIALLY A TYPE OF FDP

SO FDP IS AN UMBRELLA UNDER WHICH D DIMER COMES

SO  TESTING FOR FDP

[1] Test is SUPER SUPER sensitive. Meaning that even small, normal clots give off enough FDPs to make the FDP test register as positive.

[2] The second problem is that FDPs can be created from circulating fibrinogen too. So if you see FDPs, you don’t really know if they came from a clot or not and hense NON SPECIFIC

D-dimers are more specific for actual clots than FDPs are – because you only get D-dimers from the breakdown of real clots AND 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. However it still has high sensitivity andlow specificity so it cant be confirmatory to rule in a thrombus in presence of D DIMER

Best use of the D-dimer assay = So it’s great for ruling out a thrombus.

SO FOR DIC : MOST SENSITIVE = FDP , MOST SPECIFIC = D DIMER