This song by TheMashedPotatoMan is a hilarious and memorable reminder of one very important aspect of the D-dimer assay, which is this:


Which means that:

  1. If it’s positive, that doesn’t necessarily mean the patient has a bad, pathologic thrombus somewhere (it might be that the patient has physiologic clotting somewhere)
  2. If it’s negative, that is darn good proof that the patient does NOT have a thrombus somewhere (because even a teensy thrombus would make the D-dimer assay positive)
  3. Given 1. and 2., you should NOT order a D-dimer on a patient that you think has a low chance of really having a thrombus (because it will likely come back falsely positive, and that will trigger bigger, more expensive, totally unnecessary testing).

Listen to the song – it explains these points nicely. And makes me want to eat spaghetti – sounds like something that should be playing in a little family-owned Italian restaurant.


Here are his lyrics:

I saw a woman in ED and she said she’s breathless
Her peak flow is low, her chest is full of a wheeze
She’s clearly asthmatic
So why did they ever consider she might have PEs

Why, Why, Why D-dimer
Why, Why, Why D-dimer
Can’t you see its low specificity
Means you get a false positive and a pointless CT

I saw a man with cancer and he said he’s breathless
He took to his bed then suddenly collapsed to the floor
It’s clearly an embolus
With a false negative d-dimer I had to ignore

Why, Why, Why D-dimer
Why, Why, Why D-dimer
I implore of you see someone breathless just pause
Don’t send a d-dimer with an obvious alternative cause
And remember to work out your pretest probability score.

Very good points. If the patient has a low pretest probability (meaning that it’s pretty unlikely the patient has a PE, like the woman with asthma in the song), don’t order a D-dimer. It is such a sensitive test that it will probably come back falsely positive (because she’s clotting somewhere, but she’s just not making a big thrombus in her lung). Then you’ll be obliged to order a CT. Waste of time and money and patient comfort/worry!

However, if you’re seriously wondering about PE (like you very well should be in a patient with cancer, like the man in the song, since adenocarcinoma increases your risk for thrombosis), and you order a D-dimer and it comes back negative, well, then you have to give up that line of thinking, because a negative D-dimer assay is very good evidence that the patient does NOT have thrombus.

Bottom line: Use the D-dimer to rule OUT a clot, and only use it on patients you think have a HIGH likelihood of having a clot.