Napkin Drawing #2: D-dimer, FDP, and Rabbit Holes

By Mark Ard

I vividly remember this topic from Kaplan Q bank, UWorld, and my Step 1 test. It always boiled down to the same question, what’s the difference between elevated d-dimer and elevated fibrin degradation products (FDP). (If Step 1 questions were written like that, life would be so much easier).

Of course, a good Step 1 question will assume you know the basics and wants a deeper understanding. Did this patient recently have a radical prostatectomy and release a large amount of urokinase? Do you think his nosebleed is from picking it too aggressively, or is it the first sign of DIC, followed by bleeding out of every orifice and IV site? Does the fact that his liver enzymes are sky high explain the high FDP and low d-dimer (the answer is yes, the liver makes anti-plasmin).

Before we get to page 359 of my 2013 First Aid book where I have this drawn so eloquently, I want to warn you against going down rabbit holes. I reviewed this question after a practice block and two hours later, I had a napkin-drawing worthy of a spot in the the Louvre right next to the Mona Lisa. I think it earned me a point, maybe two, since I also spent a while understanding the clotting cascade. Was it worth it? Is masochism ever worth it? (Sometimes?) Here are a few tips for going down rabbit holes:

• Don’t do it while in the middle of bulk studying. If you’re answering and reviewing 46 questions during a three-hour study block, make a note to come back to the topic at the end of the day or week.
• Start with Wikipedia. Sometimes the Qbank explanation doesn’t really focus on the question you have. Familiarize yourself with the topic (even if it’s not technically correct). Stick to static pages that look like someone spent more than the average rant time to explain.
• Ask fellow students. Student Doctor Network, Reddit, and other forums with relatively knowledgeable people can engage in discussion if things still don’t make sense.
• Go to the source. There’s a reason they suggested you buy Harrison’s Principles of Internal Medicine. There’s also a reason many schools have online access. Focus on the images and captions. I think this is a must unless you are very trusting of someone named GuyWhoDoesStuff on SDN (he has like 3,000 posts).
• Tread lightly on Pubmed. This is the definition of a rabbit hole. “d-dimer” yields roughly 7,000 results. I would try Google Scholar and stick to first page articles or reviews.
• Give yourself a time limit. Since the birth of my daughter, that limit has gone from one hour to 10 minutes. We all have priorities. Decide what type of nerd you want to be (and when you draw this on a white board while getting pimped on medicine rounds…you will be called many things)

As for the drawing, I suggest getting familiar with the results, the role, activators and deactivators of plasmin, and the tie in of the clotting cascade. The rule of thumb is that an elevated D-dimer means a clot has formed (clotting cascade, XIIIa has made cross links) and an isolated Fibrin Degredation Product means something activated plasmin (and it might not be a clot). The other rule of thumb is that these tests are sensitive and not specific, so either elevated means…well…not too much in practice. Rest assured though, the complicatedness of the topic makes it Step 1 drool-worthy.

How do you deal with rabbit holes? Share your recommendations and experiences below.