What is a D-dimer test?
A D-Dimer test is a blood test that may be done when a doctor suspects a blood clot. The problem is that it doesn’t give a clear, yes or no, answer as to whether or not you have a clot. This makes it quite complicated to understand, and once you start adding the correct jargon like ‘specificity’ and sensitivity’ you end up with the kind of jumble that makes for excellent exam questions for medical students!
Understanding the results of a D-dimer test
I prefer to use a rugby analogy; I’ve chosen this not because of my rugby knowledge (those of you who know me are probably falling around laughing by now) but because we live in New Zealand, where rugby is a national obsession and because I’m trying to repair the damage Paul did by suggesting rugby may be a bad choice of sport if you’re taking anticoagulants!
Anyway, suppose you wanted to find out who was in a school’s first fifteen rugby team. You might go to a school assembly (all pupils are there, nobody sick) and ask “Who plays rugby for the school?” Those who say they don’t are definitely not in the first 15, but for those who said yes you don’t know if they are in the first fifteen, the second fifteen, the blue team, the yellow team or whatever (NZ schools have lots of sports teams!). You have reduced the number of possible players, but not precisely identified the first 15. You have used this question to exclude people who don’t play rugby for the school.
The d-dimer test is used in a similar way as an exclusion test. You do the test on somebody with a suspected blood clot, if the result is negative you know that that person doesn’t have a clot. But if a person has a positive test they may have a blood clot or there may be some other explanation for the positive result. The d-dimer test can be positive in infections, inflammation, after trauma and many other medical conditions.
It’s possible that some pupils decide to trick you and say they don’t play rugby for the school when actually they’re in the first fifteen: in the same way, rarely, a negative D-dimer test can occur even though someone has a clot. This is called a false negative.
Why would a doctor do a D-Dimer test?
Doctors listen to a patients’ stories, examine them and possibly complete a scoring system such as the WELLS score. This gives them a good idea of the likelihood of a blood clot.
If it seems very likely that someone has a blood clot, then doctors should proceeded immediately to tests that can give a definite diagnosis, like CTPA of the lungs looking for PE or an ultrasound scan of the leg looking for a DVT. A D-dimer test doesn’t need to be done as a negative result would be ignored anyway in case it was a false negative.
A D-dimer test is of most use when a doctor is pretty sure the person doesn’t have a clot but just wants an extra safeguard. It’s a much cheaper and easier procedure than a CTPA or leg ultrasound. If the result is negative, the patient is spared further tests. If it’s positive the doctor can investigate further.
Any real-life stories?
Paul’s pulmonary embolism was diagnosed because a cautious GP decided not to trust Paul’s word as a hematologist that the pain he was getting was not likely to be a PE. Paul had his D-dimer blood test done and returned to work in the hospital. He was sitting at his microscope when the phone call came, telling him the test result was very high and he should get himself downstairs to the emergency department for a CTPA!
More information please
If you’d like more intense information in the form of a quiz, try this post from ‘Life in the Fast Lane” a blog aimed at emergency department doctors.
Did you have a D-Dimer test as part of your investigations? Did it help with the diagnosis?