Quick D-dimer test eliminates need for CTA for PE

Jun 13, 2004

Computed tomography angiography has rightly gained popularity in recent years as a means for diagnosing pulmonary embolism. But what if there were a test that could rule out PE just like CTA, yet without the radiation and contrast exposure?

In a retrospective study of 419 patients who underwent both a D-dimer assay and a CTA for suspected PE, researchers found the exams were in 100% agreement on a large segment of cases. Therefore, they determined, just a D-dimer test would have sufficed for up to 60% of the patients in their study.

The study, by Dr. Peter Abcarian and colleagues at the Kaiser Foundation Hospital in Honolulu, is published in the latest issue of the American Journal of Roentgenology.

The advent of multidetector CT for pulmonary angiography has provided a diagnostic exam for PE that is more reliable and easier to obtain than traditional methods, the authors noted.

"The wide acceptance of pulmonary CT angiography by referring clinicians has led to a significant increase in the number of pulmonary CT angiograms obtained at our hospital, most (≈90%) of which are negative for pulmonary embolism," they wrote (AJR, June 2004, Vol. 182: pp. 1377-1381).

But there is a downside to MDCT's usefulness, they stated. "Because CT examinations are costly and involve radiation and contrast exposure, an inexpensive rapid adjunctive test with high negative predictive value would be useful to diminish the number of negative pulmonary CT angiograms."

D-dimer is a fibrin degradation fragment produced in the presence of blood clots. Earlier studies have found the absence of serum D-dimer to have strong negative predictive value for acute venous thromboembolism.

The Kaiser study, however, is the first to look at what the researchers describe as "a rapid, highly sensitive, quantitative assay" and its potential for obviating the need for subsequent CTA in suspected PE.

The quantitative serum D-dimer measurements were performed using a latex agglutination turbidimetric immunoassay method (STA-Liatest D-DI, Diagnostica Stago, Parsippany, NJ) with a fully automated coagulation analyzer (STA-Compact, Diagnostica Stago).

"In addition to being highly sensitive, these automated assays have a fast turnaround time of one hour or less, are available 24 hours a day, show good duplication of results around the normal and abnormal cutoff values, and are relatively immune to biologic interference from variables such as rheumatoid factor, lipemia, and hyperbilirubinemia," the authors wrote.

In the study, pulmonary CT angiography was performed on an MDCT scanner (LightSpeed Plus, GE Healthcare, Waukesha, WI) with 0.8-sec helical rotation speed, 1.25-mm collimation, and pitch of 3:1. Imaging was initiated 20 seconds after injection of 100 mL of iopamidol (Isovue 370, Bracco Diagnostics, Princeton, NJ) diluted with saline to 120 mL.

CTA results served as the reference standard compared with two groups of D-dimer results: patients whose serum D-dimer measured less than 0.4 µg/mL (the manufacturer's recommended cutoff for a negative exam) and a more liberal cutoff of less than 1.0µg/mL.

The negative predictive value of the D-dimer assay was 100% for both cutoffs, the researchers found. Using the manufacturer’s recommended D-dimer cutoff would have spared 20% of patients from CTA; the higher cutoff would have spared nearly 60%.

The authors cautioned that no standardization exists among the commercially available D-dimer assays. "Cutoff values used for one assay cannot simply be applied to a different assay," they wrote.

Also, because CTA served as the reference standard for the study, rather than traditional angiography, "it is possible that a few patients with both negative D-dimer assays and negative pulmonary CT angiograms could have had pulmonary embolism," the authors noted. However, none of those patients developed clinically apparent pulmonary embolism during the three-month follow-up period, they reported.

"The results of this study have already had a significant impact on the algorithm for pulmonary embolism detection in our hospital," the authors noted.

"Patients whose medical history suggests that their D-dimer level is likely to be elevated (e.g., postoperative patients, patients on anticoagulation, patients with recent trauma) proceed directly to pulmonary CT angiography unless a contraindication is present," they wrote. "All others undergo serum D-dimer evaluation. Patients with a D-dimer value of less than 1.0µg/mL do not undergo pulmonary CT angiography unless a high clinical index of suspicion is present for pulmonary embolism."

However, they concluded, "We believe that until the high negative predictive value of a negative D-dimer assay shown in our study can be confirmed by prospective trials, imaging should be performed in patients with a high pretest probability for pulmonary embolism regardless of the D-dimer results."

By Tracie L. Thompson
AuntMinnie.com staff writer
June 14, 2004

CT assessment of pulmonary embolus predicts prognosis, March 16, 2004

Combined test strategy safely detects pulmonary embolism in outpatients, March 11, 2004

D-dimer testing can eliminate need for ultrasound in patients with suspected DVT, September 25, 2003

Negative CT angiography sufficient to rule out pulmonary embolism, March 9, 2001

Can the utility of V/Q scans for embolism be improved? February, 16, 2000