A recent patient I saw in the emergency department was a fifties year-old woman with a family history of aortic dissections presenting with “chest pain” per the triage note. On my history and exam, she more endorsed vague neck and epigastric discomfort (which had now resolved), and had no other classic findings for a dissection (e.g. hemodynamic instability, asymmetric pulses or blood pressures, abnormal neurologic findings, etc.). She also had a a normal chest x-ray and a negative initial workup for ACS, including a normal ECG and undetectable troponin. In terms of other life-threatening diagnoses, she did not PERC out, and had a Wells score that suggested the D-Dimer would be an appropriate test to rule out pulmonary embolism.
When I discussed with her the potential utility of getting a CT scan of her chest to evaluate for an aortic dissection — she asked me about how much radiation exposure this involved, and shared her (valid and very appropriate) concerns about getting too much radiation. She had many CT scans for various reasons over the years she felt, and did not want any additional unnecessary radiation.
I talked to her more about this and tried to start some shared decision making by sharing a favorite infographic of mine about radiation amounts in diagnostic imaging, and (to myself) pondered a clinical question: If the D-dimer test was low, did that along with the low-ish pretest probability, safely decrease the likelihood of dissection enough to forego a CT scan? There is an emerging literature on the use of dimer testing to rule out aortic dissections, but how good is it? Do you use the same cut-off as in pulmonary embolism? Should that cutoff be age adjusted? And what are the test characteristics in this context? I had no idea, so that’s what today’s post-didactics reading was about.
I read through “A Systematic Review and Meta-analysis of D-dimer as a Rule-out Test for Suspected Acute Aortic Dissection” by Asha et al., which reviews the work of 30 studies and combines the data for 4 studies using a standard cutoff of 0.50 μg/mL to estimate sensitivity, specificity, and positive and negative likelihood ratios of a D-dimer. As the abstract conclusion reads:
“Overall, sensitivity and negative likelihood ratio were 98.0% (95% confidence interval [CI] 96.3% to 99.1%) and 0.05 (95% CI 0.03 to 0.09), respectively. These measurements had little statistical heterogeneity. Specificity (41.9%; 95% CI 39.0% to 44.9%) and positive likelihood ratio (2.11; 95% CI 1.46 to 3.05) showed significant statistical heterogeneity. When applied to a low-risk population as defined by the American Heart Association (prevalence 6%), the posttest probability for acute aortic dissection was 0.3%.”
So there you have it. Obviously, there’s more to it, and the actual paper is worth reading — it discusses some of the drawbacks of the included studies, specifically unanswered questions about bias and the generalizability to ED populations given the high prevalence of disease in the included cohorts. Limitations aside, basic conclusion that was in low risk patients, a negative D-dimer confers an even lower risk of acute aortic dissection, and it may be reasonable (don’t you love that phrase?) to consider using this result to inform your decision-making regarding the utility of imaging. Of course, one must also consider the rate of false positives, and the potential harms of resultant downstream testing as has been discussed regarding testing for PE.
I think that one of the more important (and potentially easily-overlooked, as when it comes to all clinical decision tools or supports, or anything that serves as a Bayesian modifier) points I took away from this review is that while this is a potentially useful test in this context, pretest probability matters. As the abstracts of some of the included studies say: “When applied to a low-risk population…”, “…in patients with low likelihood of the disease”, “…the presence of ADD risk score 0 or ≤ 1 combined with a…” and so on. You should only really hang your hat on a negative dimer assay when you think the probability is low in the first place. Another question to consider though, is how low is the pre-test probability to suggest you *shouldn’t* order a dimer to r/o dissection? And how many people with potential dissections that might be caught and thereby managed earlier PERC’d out of receiving a test that might reveal this diagnosis (though might also subject them to an unnecessary scan for PE)?
As the full text of the article states:
It would be pertinent to comment on the many case reports of patients with confirmed acute aortic dissection but a negative D-dimer result. It should first be recognized that these cases did not have a risk-stratification applied and also that no test, no matter how good, including the reference standards for the disease, has 100% accuracy. These cases mostly represent a subgroup of patients with a thrombosed false lumen or an intramural hematoma who seem particularly likely to have a lower or negative D-dimer result. The studies in this meta-analysis included such patients, which means that the high sensitivity and excellent negative likelihood ratio were achieved with the inclusion of these problematic cases.
It is always worth remembering that rare diseases are rare, and that in a patient with a low pretest-probability of having a disease, any test can be construed as to have a high sensitivity when applied to the wrong population. For instance, I can figure out who is low risk for aortic dissection in most chest pain patients with the “Bryan” rule — I just ask if their name is Bryan, spelled with a y. If negative, they are extremely unlikely to have an aortic dissection. Of course, if they do, my test will likely miss them but the point remains. In the patient described above, even though the D-dimer was negative, this patient was not low risk by the fairly-conservative AHA acute aortic dissection risk score (pictured below), and therefore the sensitivities and specificities cited in the articles presented in this meta-analysis don’t apply to their case.
In cases where acute aortic dissection is suspected as a likely potential diagnosis, a D-dimer is probably not an appropriate test to replace definitive diagnostic imaging of the aorta– specifically, as stated by previous guidelines from the AHA: computed tomography (CT), magnetic resonance imaging (MRI), or transesophageal echocardiography. Let this inform your discussions of shared decision making in the emergency department, and document accordingly, and hopefully you’ll be able to adopt a strategy to help everyone sleep better at night.
Nazerian P1, Morello F2, Vanni S1, Bono A3, Castelli M1, Forno D3, Gigli C1, Soardo F3, Carbone F3, Lupia E3, Grifoni S1. Combined use of aortic dissection detection risk score and D-dimer in the diagnostic workup of suspected acute aortic dissection.
Int J Cardiol. 2014 Jul 15;175(1):78-82. PMID: 24838058
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