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The Impact of the Introduction of a Rapid D-Dimer Assay on the Diagnostic Evaluation of Suspected Pulmonary Embolism
Nir M. Goldstein, MD;
Marin H. Kollef, MD;
Suzanne Ward, RN;
Brian F. Gage, MD, MSc
Arch Intern Med. 2001;161:567-571.
ABSTRACT
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Background Rapid D-dimer assays are being used in the diagnostic evaluation of
suspected pulmonary embolism (PE). Although this hypothesis is anticipated
to decrease the use of ventilation-perfusion (VQ) scans and other diagnostic
tests for PE, it has not been tested in a "real-world" environment.
Subjects and Methods A randomized prospective trial was conducted on 470 of the 5390 enrolled
patients aged 60 years and older who had previously undergone any diagnostic
tests for PE at an urban teaching hospital. The use of D-dimer as part of
the diagnostic evaluation for PE was promulgated in the 2 randomly chosen
intervention firms. The remaining 2 firms served as controls.
Main Outcome Measures The number of VQ scans, spiral computed tomographic scans, and pulmonary
angiograms performed. Secondary outcomes included mortality and thromboembolic
or bleeding events during 3 months of follow-up.
Results Of the 470 inpatients who underwent evaluation for PE on a per PE workup
basis, fewer VQ scans were performed in the intervention firms (63.8% vs 81.3%; P<.01). However, the number of patients evaluated for
PE nearly doubled in the intervention firms (304 vs 166; P<.01), so that more VQ scans were performed in the intervention
than in the control firms (194 vs 135; P<.01).
Ninety-four patients from the control firms and 160 patients from the intervention
firms were diagnosed and treated for venous thromboembolic disease (P<.01). There were no differences in secondary outcomes
during the 3-month follow-up.
Conclusions The introduction of a rapid D-dimer assay increased the number of VQ
scans performed because the number of patients screened for PE increased.
A larger number of patients in the intervention firms were diagnosed as having
venous thromboembolic disease (PE and/or deep vein thrombosis). There were
no perceived changes in mortality or venous thromboembolic events during the
3-month follow-up.
INTRODUCTION
PULMONARY embolism (PE) is a serious and life-threatening disease that
has an incidence of 23 per 100 000 patient-years and causes 200 000
deaths annually in the United States, with untreated mortality rates as high
as 30%.1 Among treated patients, fatal PE is
a rare event,2 with clinical recurrence occurring
in 8% to 26%.2, 3 Thus, the goal
of treatment is to prevent death from the initial and recurrent PEs and generally
consists of the administration of intravenous heparin followed by at least
6 months of oral anticoagulation.1, 4
Because anticoagulant use can cause serious complications, including fatal
hemorrhage,5 accurate diagnosis of PE is crucial.
With no pathognomonic clinical features, the diagnosis of PE requires
objective testing. Pulmonary angiography is the gold standard technique for
diagnosis of PE6; however, it is costly, invasive,
and not uniformly available. Noninvasive procedures, such as the radioisotope
ventilation-perfusion (VQ) scan and lower extremity duplex ultrasonography,
have simplified the diagnostic approach to venous thromboembolic (VTE) disease.
However lung scans are costly and are diagnostic in only 30% to 50% of patients
with suspected PE,7, 8, 9
while duplex ultrasonography reveals a deep vein thrombosis (DVT) in only
8% to 19% of patients who have a nondiagnostic lung scan.9, 10, 11, 12, 13
Plasma D-dimer (DD) is a cross-linked fibrin degradation product, resulting
from activation of the coagulation and fibrinolysis processes. Plasma DD has
been extensively studied in the setting of suspected PE and DVT.14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Using enzyme-linked immunosorbent assays (ELISAs) at the clinically validated
cutoff value of 500 µg/L, the specificity of DD is low (20%-50%), but
the sensitivity approaches 95%, with a normal DD level virtually excluding
the diagnosis of PE in low-risk patient populations.14, 15, 16, 17, 18, 19
Recently, a whole blood agglutination assay SimpliRED (AGEN Biomedical Ltd,
Queensland, Australia) has been developed as a novel DD assay that is easier
and faster to perform than the ELISA-based tests. We have shown previously
that DD determination by the SimpliRED method has a high sensitivity and negative
predictive value for VTE among patients admitted to a medical intensive care
unit.20 Based on this experience, we performed
an investigation with 2 goals: (1) to determine how the availability of a
rapid DD test affected the diagnostic evaluation of suspected PE and (2) to
assess the safety of using a rapid DD assay in clinical practice.
SUBJECTS AND METHODS
STUDY LOCATION AND POPULATION
The study was conducted within the inpatient medical wards of Barnes-Jewish
Hospital, St Louis, Mo, a 1200-bed, urban teaching hospital. The inpatient
teaching medical service at Barnes-Jewish Hospital is divided into 4 similar
firms (A, B, C, and D). Patients hospitalized in the medical wards of firms
A, B, C, or D between September 1999 and February 2000 were included in the
study if they underwent any diagnostic testing for clinically suspected PE,
including a VQ scan, pulmonary angiogram, DD test, or spiral computed tomogram.
Group randomization was achieved by making the DD test readily available to
patients admitted to firms B and D (intervention firms) and discouraging its
use in firms A and C (control firms). To enlist house staff cooperation with
our study, we undertook several formal educational talks with the house staff
of all 4 firms, as well as providing house staff and attending physicians
with information on the purpose of this investigation. For the intervention
firms, we presented current literature regarding the applicability of the
SimpliRED assay, including its test characteristics and its utility as a screening
tool in non–high-risk patients. To the control firms, we explained the
study protocol and requested that suspected PE be investigated in the traditional
manner, without using the SimpliRED assay. Using this approach, we planned
to study the impact that the introduction of the DD assay would have on test
ordering and clinical outcomes. Control and intervention firms were randomly
assigned. The Human Studies Committee of Washington University School of Medicine,
St Louis, approved the study protocol and waived the need for patients' written
informed consent.
DATA COLLECTION
A dedicated clinical research nurse (S.W.) prospectively recorded data
from the patients' medical records, bedside computerized nursing station (EMTEK
Health Care Systems, Tempe, Ariz), and the hospital's mainframe computer.
She recorded the following admission variables: age, sex, admission diagnosis,
and preadmission diagnoses of comorbid conditions (malignancy, congestive
heart failure, chronic obstructive pulmonary disease, and the presence of
infection). During the patient's hospitalization, she recorded the ordering
and results of diagnostic tests performed for the evaluation of suspected
PE including: DD assay, VQ scan, spiral computed tomography, and pulmonary
angiography. At 3 months after hospital discharge, the research nurse telephoned
participants to assess for the following outcomes: death, recurrent VTE events,
and bleeding complications.
D-DIMER DETERMINATION
We used a recently developed assay for the rapid detection of plasma
DD (SimpliRED). The test reagent contains a composite antibody formed by the
conjugation of a monoclonal antibody that reacts with high affinity to a site
on the  -chain of DD (3B6/22), with a red cell–binding antibody
(RAT-1C3/86). In the presence of elevated levels of DD, the antibody induces
agglutination of the patient's red blood cells.20
Trained laboratory personnel, who were blinded to the clinical data,
performed all tests. Blood samples for DD determination were requested to
be drawn within 24 hours of the clinical suspicion for PE. For each test sample,
a 10-µL drop of whole blood was instilled by pipette into a reaction
well on an agglutination tray. One drop of test reagent was added to the blood
sample and mixed using gentle rocking of the agglutination tray for 2 minutes.
Although it is possible to grade the degree of red cell agglutination with
the SimpliRED DD test, for the purposes of this study a DD test was defined
as positive if any agglutination was observed and negative if no agglutination
was observed.20
DIAGNOSIS OF VTE
For the purpose of this study, the diagnosis of VTE (PE and/or DVT)
was considered established if the firm physician team instituted a therapeutic
intervention (anticoagulation, thrombolysis, or inferior vena cava filter
placement) following performance and interpretation of appropriate diagnostic
tests (VQ scan, spiral computed tomography, pulmonary angiography, duplex
ultrasonography).
STATISTICAL ANALYSIS
The primary outcome measure was the number of VQ scans, pulmonary angiograms,
and spiral computed tomographic scans ordered in each group for the evaluation
of suspected PE. Secondary outcomes included mortality, duration of hospitalization,
and 3-month incidence of recurrent VTE events or bleeding complications. All
comparisons were unpaired and tests of significance were 2-tailed. Continuous
variables were compared using the t test for normally
distributed variables and the Wilcoxon rank sum test for nonnormally distributed
variables. The  2 or Fisher exact tests were used to compare
categorical variables. The primary data analysis compared patients in control
firms (A and C) with patients in intervention firms (B and D). Logistic regression
analysis was used to test the relationship between DD result and patient age
and comorbidities. The proportion of patients undergoing VQ scanning was compared
using standard methods (Snedecor and Cochran).
RESULTS
A total of 470 consecutive patients being evaluated for clinically suspected
PE from firms A, B, C, and D were enrolled in the study. During enrollment,
the total number of inpatient admissions were similar among the 2 groups (2700
in the control firms vs 2690 in the intervention firms). At the time of enrollment,
no significant differences were found among the intervention and control firms
regarding sex, ethnicity, presence of chronic obstructive pulmonary disease,
congestive heart failure, malignancy, clinical infection, recent surgery,
immunocompromised state, or receipt of DVT prophylaxis, but patients in the
intervention group were older (Table 1).
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Table 1. Characteristics of Patients With Clinically Suspected Pulmonary
Embolism*
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DIAGNOSTIC TESTS FOR PE
In the intervention firms, significantly more patients were investigated
for possible PE (of the 2690 enrollees: 304 [11.3%] vs 166 [6.2%]; P<.01). Among the patients investigated for PE (Figure 1), the intervention group had significantly more DD assays
performed (191 [62.8%] vs 53 [31.9%]; P<.01) and
a lower rate of VQ scans (194 [63.8%] vs 135 [81.3%]; P<.01) performed.
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Figure 1. Diagnostic tests performed per
clinically suspected episode of pulmonary embolism (n = 470). VQ indicates
ventilation-perfusion; PA-Gram, pulmonary angiogram; and CT, computed tomography.
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From the total inpatient firm pool, the intervention group underwent
significantly more DD assays (191 [7.1%] vs 53 [2.0%]; P<.01) and a greater rate of VQ scans (194 [7.2%] vs 135 [5.0%]; P<.01). There was no difference between the 2 groups
in the use of spiral computed tomography or pulmonary angiography (Figure 2).
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Figure 2. Diagnostic tests performed per
intervention group (n = 2690) and control group (n = 2700). VQ indicates ventilation-perfusion;
PA-Gram, pulmonary angiogram; and CT, computed tomography.
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Among patients having a DD assay (n = 244), those in the intervention
group demonstrated a trend toward a lower likelihood of having a positive
DD assay compared with patients in the control group (89 [46.6%] vs 32 [60.4%]; P = .08). Logistic regression analysis showed that the
presence of chronic obstructive pulmonary disease (adjusted odds ratio [AOR],
3.3; 95% confidence interval [CI], 2.1-5.2; P = .007),
clinical infection (AOR, 2.0; 95% CI, 1.5-2.7; P
= .02), and increasing patient age (1-year increments) (AOR, 1.04; 95% CI,
1.03-1.05; P<.001) were independently associated
with a positive DD assay result. Patients with a positive DD assay were more
likely to undergo a VQ scan than patients with a negative assay (58.7% vs
30.1%; P<.01).
Overall more VTE episodes were diagnosed in the intervention group (160
of 2690 admissions) than in the control group (94 of 2700 admissions) (P<.01). Patients in the intervention group were also
more likely to undergo a VQ scan in the case of a positive DD assay (64.0%
vs 43.8%; P = .046).
The sensitivity, specificity, negative predictive value, and positive
predictive value of the SimpliRED assay, with the 95% CIs, were as follows:
60.0% (51.5%-68.5%), 53.0% (44.0%-62.0%), 50.4% (41.6%-59.2%), and 54.5% (45.7%-63.3%).
SECONDARY OUTCOMES
No statistically significant differences were detected in hospital mortality
or length of stay between the 2 groups. Of the patients in the control and
intervention firms, 45% and 48% , respectively, were successfully reached
for the 3-month follow-up. No differences were detected in total mortality,
recurrent VTE events, or bleeding complications (Table 2).
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Table 2. Outcomes of Patients With Clinically Suspected Pulmonary Embolism*
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COMMENT
Our study demonstrated that the introduction of the SimpliRED DD assay
resulted in an increase in the number of VQ scans performed. The availability
of this rapid, inexpensive, and sensitive test also resulted in an increase
in the number of patients screened for PE, as well as increasing the numbers
of patients diagnosed as having VTE. The prevalence of a negative assay (49.6%)
was fairly high even in a medical inpatient population with multiple comorbidities.
Our study was unique in that it was based in a "real-world" environment
where the ultimate decisions regarding the diagnostic and therapeutic approaches
were determined by individual physicians. Contrary to predictions23, 24, 25, 26, 27, 28
that the introduction of a DD assay would reduce more costly and invasive
tests for PE, the widespread adoption of a DD assay increased resource utilization
by increasing the number of patients screened for PE. The increased screening
resulted in a significant increase in the number of VTE events diagnosed.
We were unable to show a difference in mortality or in the occurrence of recurrent
VTE events at 3-month follow-up. However, follow-up was incomplete and the
study was not designed or powered to detect such potential differences.
Given the excellent reported negative predictive value of DD assays
in non–high-risk patients, several investigators have predicted that
the use of DD would decrease the number of VQ scans performed in patients
with suspected PE.23, 24, 25, 26, 27, 28, 29
Ginsberg and colleagues24 showed that a normal
DD result by SimpliRED was useful in excluding PE in patients who had a low
clinical suspicion for PE, with a negative predictive value of 99%. Because
44% of the participants had a low pretest probability, they hypothesized that
introduction of a DD assay would reduce the number of VQ scans significantly.
De Moerloose and colleagues29 evaluated a rapid
ELISA method in 195 emergency department patients with suspected PE; 29% of
these patients had a negative DD result and none of them experienced thromboembolic
events during 6 months of follow-up. The authors thus speculated that 29%
of the emergency department patients could have avoided further testing. Oger
and coworkers28 studied 386 patients in the
emergency department with suspected PE using a rapid ELISA (Liatest). They
demonstrated a sensitivity of 100% for PE, with 21% of their patients having
a negative test result. This study also speculated that DD would reduce further
investigations and shorten hospital stay. However, the impact of introducing
a DD assay on physicians' ordering patterns had never been tested previously.
We found that the introduction of SimpliRED had the effect of increasing use
of VQ scans due to the increase in the number of patients screened for VTE.
This increased utilization of a DD assay is precedented by other situations
in which the introduction of safer, quicker, inexpensive, or more sensitive
tests have increased the number of patients screened for disease. Examples
include the impact of prostate-specific antigen determinations on the frequency
of prostatic biopsies,30 as well as diagnostic
tests for colorectal cancer following the introduction of stool occult blood
testing.31 Similarly, a readily available assay
for DD such as SimpliRED, which is inexpensive and noninvasive, may allow
greater numbers of patients to be screened for VTE, resulting in more diagnoses
of VTE as well as greater overall diagnostic test utilization.
Our study has several limitations. First, it was performed within a
single institution using patients cared for by house staff who had been educated
about the usefulness and limitation of DD testing, and as such may not be
generalizable to some settings. Second, the study was limited to medical inpatients
and is not applicable to nonmedical specialties (eg, surgical or obstetrical
patients) or to outpatients. Third, despite discouraging its use, 32% of patients
in the control group had DD tests performed. Fourth, 3-month follow-up was
incomplete with approximately 50% of patients being available for follow-up.
Finally, we did not dictate the utilization of diagnostic tests or treatment
for VTE. Therefore, comparisons with other institutions using different diagnostic
and therapeutic approaches is problematic.
The introduction of the SimpliRED DD assay in medical inpatients with
clinically suspected PE resulted in a significant increase in the number of
patients diagnosed as having VTE, at the expense of a significant increase
in VQ scans ordered. Despite the overall increase in the screening for PE,
the utilization of VQ scans per individual workup was reduced. The introduction
of the SimpliRED assay did not result in any changes in mortality or recurrent
VTE at 3 months' follow-up. Despite advanced age and multiple comorbidities,
half of the medical inpatients in this study had a negative DD assay. We conclude
that the unrestricted use of the SimpliRED DD assay in medical inpatients
resulted in an increase in the number of VTE events diagnosed at the cost
of increased VQ scanning with no significant change in mortality or recurrent
VTE events at 3 months. Larger mortality and cost-effectiveness studies are
needed to evaluate whether the increase in VTE events diagnosed with the availability
of DD assays justifies the increased utilization of VQ scanning.
AUTHOR INFORMATION
Accepted for publication September 14, 2000.
We thank Sam Santoro, MD, for his assistance in establishing the SimpliRED
assay in the Barnes-Jewish Hospital Laboratory, as well as Charles E. Trunk,
RPh, MBA, and Scott R. Early, RPh, for their assistance in tracking VQ scans.
From the Pulmonary and Critical Care Division (Drs Goldstein and Kollef
and Ms Ward), Division of General Medical Sciences (Dr Gage), and Department
of Internal Medicine (Drs Goldstein, Kollef, and Gage and Ms Ward), Washington
University School of Medicine, St Louis, Mo.
Corresponding author: Marin H. Kollef, MD, Campus Box 8052, Washington
University School of Medicine, 660 S Euclid Ave, St Louis, MO 63110 (e-mail: kollefm{at}msnotes.wustl.edu).
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