Clinical Laboratory Strategies: October 29, 2009

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Implementing D-dimer POC Testing in the Emergency Department
Use of Rapid Whole Blood Assay Associated with Shorter Lengths of Stay
By Genna Rollins

The D-dimer assay is an important first-line test in the work-up of patients with suspected venous thromboembolic disease and can assist physicians in determining whether further radiologic tests, which can be both expensive and time-consuming, need to be carried out. However, when performed in a central lab, total turnaround time for D-dimer testing can be 2 hours or longer. In the context of over-crowded emergency departments (ED) with long waits, initiatives to improve ED efficiency without compromising patient care are particularly valuable. This issue of Strategies explores the experience of one institution in implementing a point-of-care whole blood D-dimer assay to support ED operations.

Negative D-dimer test results in patients with a low pre-test probability of either venous thromboembolism (VTE) or pulmonary embolism (PE) can effectively rule out both conditions and obviate the need for further radiologic tests such as ultrasound or pulmonary CT angiography. As such, the D-dimer assay is a valuable tool in evaluating patients with suspected VTE or PE. This is especially the case given the high demand for emergency department (ED) resources. Now, in a recently published study researchers at Massachusetts General Hospital (MGH) in Boston report that implementing a rapid whole blood D-dimer test in the ED reduced the mean ED length of stay and the number of hospital admissions (Am J Clin Pathol 2009;132:326-331).

The investigators had specific reasons for assessing the utility of a point-of-care D-dimer test in support of ED operations, according to senior author Kent Lewandrowski, MD, associate chief of pathology and director of clinical services for anatomic and clinical pathology at MGH. "For about eight years we’ve had a rapid POC testing satellite lab in the ED which performs a menu of tests, most notably cardiac markers and urine testing for drugs of abuse. So we’re always looking for ways to leverage that operation, and to continue to assist the ED in managing their efficiency and patient flow," he explained. "The D-dimer test came to mind because it can be used to rule-out venous thromboembolic disease, or to decide whether positive results need to be confirmed with much more extensive and less-timely radiologic studies." Lewandrowski also is associate professor of pathology at Harvard Medical School.

At the study onset, turnaround time for D-dimer testing in MGH’s central lab using the VIDAS D-dimer test (bioMerieux) was approximately 2 hours. The average length of stay (LOS) in the hospital’s ED was 9.1 hours for patients subsequently admitted as inpatients, 5.3 hours for patients admitted to the ED observation unit, and 5 hours for patients discharged. The researchers selected the Biosite Triage assay to introduce D-dimer testing to the ED satellite lab for several reasons, including previous analyses which showed that it performed as well as the VIDAS assay, and the fact that a Biosite platform already was being used in the ED satellite lab.

The study involved patient chart reviews on 252 patients before and 211 patients after implementation of the POC test who presented to the ED with suspected PE or VTE. For the 211 patients enrolled after the POC assay was put into operation, paired blood samples drawn simultaneously were evaluated using the

POC and central lab tests, respectively. Use of the POC test dramatically cut D-dimer turnaround times, to 25 minutes from about 2 hours, which was an approximate 79% decrease. In this group of patients there also were significant reductions in mean and median ED LOS, with the mean declining to 7.14 hours from 8.46 hours, and the median decreasing to 5.88 hours from 6.2 hours. In contrast, there was no significant difference in the average LOS for the entire ED population over the same period of time.

The relationship between use of a rapid D-dimer test and reduced ED LOS is particularly noteworthy, according to Rajan Gupta, MD, medical instructor of radiology at Duke University Medical Center. "The study demonstrates that a POC solution for D-dimer has value especially for ED colleagues. That will be critical moving forward as the thrust continues towards shorter ED visits and a quicker disposition for patients. This paper will certainly have an impact on that," he observed. Gupta’s research has focused on the efficacy of clinical risk algorithms and D-dimer assays in evaluating patients with suspected PE (CLN 2009;10:22).

The researchers also found differences in ED admission and discharge patterns after implementation of the rapid D-dimer test. There was a 13.8% decrease in patients admitted to the hospital, but increases in both the percentage of patients discharged from the ED (7.3%), and admitted for observation only (6.4%). "It appears that with availability of the test, the physicians were able to send more patients home or admit them to the observation unit rather than a full medical admission," said Lewandrowski. One metric that did not change was utilization of radiology services. "One possible explanation for that is that the ED physicians already had D-dimer tests available through the central lab, so all we did was make the results faster. When clinicians went through the diagnostic process, the D-dimer results were either positive or negative and they either ordered or didn’t order radiologic studies as a consequence. Simply having the results faster wouldn’t change the number of patients who needed a CT or ultrasound," explained Lewandrowski.

The rapid D-dimer test performed as well as the one in MGH’s central lab, with sensitivity and negative predictive value of both tests at 100%, respectively. The POC assay had a slightly higher specificity of 73.3% in comparison to 67.9% for the central lab assay. Both tests had a poor positive predictive value of 8.3% (POC) and 7% (central lab). MGH used different positive cut-offs for the two assays, 400 ng/mL for the POC test, and 500 ng/mL for the central lab test, so it chose not to report actual values for the POC test in this study cohort. "We don’t actually give a number. We call it positive, borderline, or negative. The reason we do that is because if you have two tests in a hospital that have different cutoffs then confusion can reign," explained Lewandrowski.

The researchers calculated that the POC test was slightly more expensive than the central lab assay, $23.92 per test versus $20.92. However, they concluded that implementing the test in the satellite ED lab still made economic sense by virtue of the increase in ED efficiency. Consideration of lab and ED resources and requirements is a strength of the study, according to Gupta. "Collaborations such as this are pretty essential. There are a lot of papers coming out right now involving D-dimer testing, and the best ones are the ones being done with this cross-departmental collaboration. More of this type of research needs to be done to determine the place of POC testing in busy ED departments."

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