We’re back with our second entry in a series of clinical topics of importance submitted by EPPA’s Director of Quality; Dr. Peter Currie. In these posts, we’ll unpack a particular clinical topic in detail. Today’s post continues the storyline from our first entry describing tests used to rule-out major cardiac events. We hope this series will illuminate topics our scribes work with every day and help other readers better understand the complexities of medical discourse. Enjoy.
As I wrote last time, when we are evaluating a patient with chest pain, there is a “trifecta” of deadly conditions that we are primarily concerned with: acute coronary syndrome (compromised blood flow to the heart), pulmonary embolism (blood clot in the lung), and aortic dissection (tear in the wall of the aorta). The diagnosis of pulmonary embolism (PE) has evolved over the years. The diagnostic test of choice at this time is the CAT scan but this test has issues. Cost is one, but radiation, and the associated increased risk of cancer, is another big concern. Additional concerns are dye allergy (this CAT scan requires an IV dye injection to which some patients are allergic) and contrast nephropathy (CAT scan dye can potentially harm kidneys, particularly the compromised kidneys of older patients and diabetics).
Lastly, getting a CAT scan is a time-consuming process which has implications for throughput and wait-times. So, how can a patient be evaluated for PE without the risks, costs, and time involved in getting a CAT scan every time we are concerned but without compromising safety?
Some brilliant researchers discovered that a blood test called D-dimer might be able to help. D-dimer is a fibrin degradation product, basically a protein created with blood clots are being broken down. Research has demonstrated that the d-dimer is elevated in patients who have PE and further research showed that this test alone can be used in select patient populations, in lieu of a CAT scan, to rule-out PE. There are some caveats however.
D-dimer cannot be used to exclude PE in high-risk patient populations so we use either clinical gestalt or a validated/researched scoring rule (the most popular is Wells Criteria) to categorize our patients as low, intermediate, or high risk. Once categorized, we apply the appropriate test.
However, d-dimer has one big flaw: it can be elevated for a host of reasons beyond PE. These reasons include pregnancy, older age, trauma, infection, surgery, malignancy, and many others. For this reason, d-dimer is said to have a low specificity which refers to the extent to which the test is specific to the condition (PE in this case). As you can imagine, many of the patients whom we are evaluating for PE have the above conditions. Because of this, we often jokingly talk about winning the d-dimer lottery when we’ve navigated through the above conditions and get a negative d-dimer allowing us to rule-out PE and move along in the diagnostic process.
Peter T. Currie, MD, FACEP, FAAEM
Medical Director of Quality, EPPA