This song by TheMashedPotatoMan is a hilarious and memorable reminder of one very important aspect of the D-dimer assay, which is this:
It’s a SUPER-SENSITIVE test.
Which means that:
- 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)
- 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)
- 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 OUTÂ a clot, and only use it on patients you think have a HIGHÂ likelihood of having a clot.
Hi!
Thank you for the great posts.
Regarding the D-Dimer, i was told by a senior resident that there were cases he had seen in the ED of people with negative D-Dimer and deep vein thrombosis/chronic pulmonary embolism. Is that possible?
(If you ever had the time, maybe you could also address the chronic pulmonary embolism issue.)
If possible, could you also explain what is it that you mean by `physiologic clotting`(maybe offer some examples).
Thank you,
Alexandra
Hi Alexandra – it would be very unlikely to have a negative D-dimer assay if you have a thrombus. The test is just SO sensitive that it really should be positive if there is a thrombus. Physiologic clotting refers to the normal wear and tear on blood vessels (you constantly are repairing little tears in capillaries – but this is not considered pathologic). Some people say even the process of venipuncture (with immediate initiation of clot formation) can make the D-dimer assay positive. This might be a stretch – but that’s how sensitive the test is!
Thanks a lot!
Alexandra
I read in Harrison 18ed fig 300-3 algorithm. It says we do a d-dimer for low likelihood of PE. For high likelyhood we directly go for imaging.
I understand and agree with your logic but its contradicting Harrison makes me unhappy. Please comment.
No that makes absolute sense! If the likelihood of PE is low, then you do a D-dimer to rule it out (high sensitivity!). For high likelihood PE (that means that you’re pretty sure it is a PE), then doing a D-dimer to prove that it’s a PE is not useful (you only want to do a D-dimer to rule out PE). To prove that it IS a PE (which is what you’d want to do if you are pretty sure that it is a PE), you need a test which will not have a lot of false positives (like imaging!).
Thanks for explaining mam.
In the song he actually says the D-dimer for the cancer patient was a false negative.