On the Shoulders of Giants
Patients in an ED setting with a low pretest probability based on the Wells score, and a negative D-dimer were safely ruled out for pulmonary embolism (PE) without further diagnostic imaging.
Why does this matter?
Imaging for PE is not benign. Greater radiation exposure means greater risk of malignancy, though small, in the future. Strategies that allow the disease to be ruled out with clinical criteria or simple lab tests obviate the need for V/Q or CT of the chest. The original Wells score was validated in 1998 in a higher PE prevalence, non-ED population. This study focused on ED patients and found the probability of disease at low, moderate, and high pretest probability tiers was lower than the original Wells study. It is important to know that this study used a rapid, whole blood D-dimer test, called SimpliRED. The ELISA-based or second generation agglutination D-dimer tests have been shown to have superior diagnostic accuracy* and are now preferred and are likely what is done in your ED. MDCalc is the go to source to learn about Wells and other PE scoring instruments. Not only is MDCalc a medical calculator, it's a teaching tool.
Wells, wells, wells...
This was a prospective study of 930 Canadian ED patients who presented with suspicion for PE. They used the Wells score to risk stratify patients into low, intermediate, and high pretest probability groups. All had a SimpliRED D-Dimer performed. Those with low pretest probability and negative D-dimer were considered ruled out for PE; 1 had subsequent PE in 3-month follow up. Moderate and high pretest probability patients had a D-dimer performed and all also underwent V/Q scanning. The most important findings were that the prevalence of PE was 1.3%, 16.2%, and 40.6% in low, moderate, and high pretest probability patients based on Wells score. The SimpliRED D-dimer had a negative predictive value of 99.5%, 93.9%, and 88.5% in the low, moderate, and high pretest probability cohorts. With current generation D-dimer assays, patients with both low to moderate pretest probability and negative D-dimer are considered ruled out for PE. Another way to stratify is to dichotomize the results into PE unlikely/PE likely. "PE unlikely" plus a negative D-dimer is ruled out; "PE likely" goes straight to CT pulmonary angiogram with no D-dimer. See MDCalc for more.
Excluding pulmonary embolism at the bedside without diagnostic imaging: management of patients with suspected pulmonary embolism presenting to the emergency department by using a simple clinical model and d-dimer. Ann Intern Med. 2001 Jul 17;135(2):98-107.
There is an entire Cochrane review on using D-dimer to exclude PE. It is long, but the plain language summary is very quick to read and helpful.
*Accuracies of commonly used D-dimer assays from the JAMA article linked above - sensitivity, specificity, negative likelihood ratio.
- Organon Teknika Latex Immunoassay: 96%, 45%, 0.09
- Vidas Rapid ELISA: 90%, 45.1%, 0.22
- SimpliRED: 84.8%, 68.4%, 0.22