Q. What is the difference between FDPs and D-dimers?
A. Fibrin degradation products (FDPs) and D-dimers are both little fibrin-containing molecules that you can measure in the blood. There’s an important difference between the two, though, that I’ll mention in a minute.
Making a clot
When you make a blood clot, the first thing you do is make a little mushy plug of platelets to fill in the hole in the vessel. Then, you make fibrin (from a bigger molecule called fibrinogen) which sticks to the platelet plug like superglue and seals it up.
If you want to break down the clot, a substance called plasmin can attack the fibrin in the plug, chopping it up into little fragments called fibrin degradation products, which you can measure in the blood. This breakdown process actually starts happening almost right away, as soon a clot is being formed (in this context, it’s called remodeling). Which makes sense, if you think about it – you want to keep the clot limited to the necessary size/place (otherwise, it would fill up the whole vessel, which could be a disaster).
Testing for thrombi
If you want to find out if someone has a thrombus (a big clot, like a pulmonary embolus, that is causing problems for the patient), you can look for fibrin degradation products in the blood, the idea being that if you see a lot of them, it probably means the patient has a thrombus somewhere that is being remodeled. There are a couple problems with the test though. One is that the test is SUPER SUPER sensitive. Meaning that even small, normal clots give off enough FDPs to make the FDP test register as positive. More on that in a minute.
The second problem is that FDPs can be created from circulating fibrinogen too (not just from busting up a clot). So if you see FDPs, you don’t really know if they came from a clot or not.
D-dimers vs. FDPs
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.
Best use of the D-dimer assay
That’s all well and good – but we’re still left with the issue of the super-sensitivity of the test. If you see D-dimers in the blood, it might mean that there is a thrombus somewhere – or it might mean that the patient simply has a normal clot or two. Another way to put this is that the test is SUPER sensitive, but not very specific. How to get around this problem?
The best way to deal with this is to use the test to rule out a thrombus but not to rule in a thrombus. If the D-dimer (or FDP) assay is positive, it doesn’t really tell you a heck of a lot – so it’s not great for ruling in a thrombus. However, if the test is negative, you can be dang sure that the patient does not have a thrombus. So it’s great for ruling out a thrombus.
The test is often for this purpose in the emergency room, in patients that come in with symptoms of pulmonary embolus (PE). If you do a D-dimer assay and it comes back negative, then you know that there isn’t a PE, and you better start looking for something else.
really interesting for me.
Brilliant! Very clear and logical. Thank you!
every passing second while am here in this site i learn more and more . thank you
Wonderful explAnation
Great explanation
AWESOME!!!!!!!!!!!!
You are best teacher..
Allah help u every moment of life..
excellent………studying these two words for last 5 years but doesnt know the concept. thanks a lot.
God bless you dear Dr. Kraft!
u really make things easy to understand…
Massive topic with a simple explanation. Great!
Thank you, it was very clear 😀
thank you that’s very helpful
A 2 year medical student here and I absolutely enjoyed reading your post on the differences between FDP’s and d-dimers.
Thank You!
Thank you for posting this wonderfully and simply put explanation. i learned alot. After reading this once, i don’t feel the need to cram! Thank you!
wonderful and a joy to read so logical thank you 🙂
very useful
This is a really interesting article.
It is explained very nicely. Thank you.
thanx very clear and concise
Thank you so much. It is super high-yield for me.
Good information which I like. Thanks a lot
Very good explanations. Thank you.
I have gotten the concept quite clearly. Simply put yet capturing all the ambiguities that cause difficulty. Thank you Prof.
really helpful! finally understand the difference between D-dimer and FDP test.
Excellent… Very very nice point’s
Tnx
excellent!you teach it as a simple language.more helpful to us.Thank u
excellent
Excellent explanation.
Can I marry your brain??
GREAT EXAPLAINNNN
Thank u, simple and clear
super way of explaining things. Thank you
Thank you and God bless
Thank you! This was a GREAT explanation.
ZIIING! I get it now! MLS student here …. My professor’s lecture notes simplify the textbook descriptions, but having the concepts explained in a different way like this REALLY helped me. I doubt anyone’s going to see this but thank you, whoever
I saw it, Anna!! Thank you – that really makes my day 🙂 YAY!
Very excellent explanation
God Bless You….
Simply precise & concise I appreciate the way the difference b/w the two turned up so interestingly that kept us all involved indeed. Allah the mighty bless you for your selfless efforts always.
Really very nice .. clearly explained.. Thank you
what a wonderful way to simplify the things and make others understand this difference. Thanks a lot
very well explained