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Summary. Background: Increasing the threshold to define a positive D-dimer could reduce unnecessary computed tomographic pulmonary angiography (CTPA) for a suspected pulmonary embolism (PE) but might increase rates of a missed PE and missed pneumonia, the most common non-thromboembolic diagnosis seen on CTPA.
Objective: Measure the effect of doubling the standard D-dimer threshold for ‘PE unlikely’ Revised Geneva (RGS) or Wells’ scores on the exclusion rate, frequency and size of a missed PE and missed pneumonia.
Methods: Patients evaluated for a suspected PE with 64-channel CTPA were prospectively enrolled from emergency departments (EDs) and inpatient units of four hospitals. Pretest probability data were collected in real time and the D-dimer was measured in a central laboratory. Criterion standard was CPTA interpretation by two independent radiologists combined with clinical outcome at 30 days.
Results: Of 678 patients enrolled, 126 (19%) were PE+ and 93 (14%) had pneumonia. Use of either Wells’ ≤ 4 or RGS ≤ 6 produced similar results. For example, with RGS ≤ 6 and standard threshold (< 500 ng mL−1), D-dimer was negative in 110/678 (16%), and 4/110 were PE+ (posterior probability 3.8%) and 9/110 (8.2%) had pneumonia. With RGS ≤ 6 and a threshold < 1000 ng mL−1, D-dimer was negative in 208/678 (31%) and 11/208 (5.3%) were PE+, but 10/11 missed PEs were subsegmental and none had concomitant DVT. Pneumonia was found in 12/208 (5.4%) with RGS ≤ 6 and D-dimer < 1000 ng mL−1.
Conclusions: Doubling the threshold for a positive D-dimer with a PE unlikely pretest probability could reduce CTPA scanning with a slightly increased risk of missed isolated subsegmental PE, and no increase in rate of missed pneumonia.
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In the past decade, the D-dimer has become integral to the evaluation of patients with a suspected acute pulmonary embolism (PE). Published clinical guidelines recommend that a PE can be safely excluded with the combination of a normal quantitative D-dimer value and a low pretest probability assessment [1,2]. The first quantitative D-dimer assay to receive marketing clearance by the Food and Drug Administration (FDA) for this indication used a threshold of 500 ng mL−1 (VIDAS; Biomérieux, Durham, NC, USA). In conjunction with a low pretest probability this 500 ng mL−1 threshold produces nearly 100% diagnostic sensitivity for detection of an acute PE, and thus has provided the benchmark for subsequent commercial D-dimer assays . Simultaneously, in the past decade, multidetector-row computerized tomographic pulmonary angiography (CTPA) has become the imaging mainstay in the diagnosis and exclusion of an acute PE. CTPA can also disclose clinically important alternative diagnoses, most often pulmonary infiltrates suggestive of pneumonia [4,5]. However, CTPA can cause direct and indirect adverse effects. Two direct adverse effects of CTPA are an increased lifetime risk of cancer from radiation exposure and the risk of contrast nephropathy [6–9]. An indirect effect of CPTA stems from the increasing resolution afforded by 64-channel CT equipment, which has increased the frequency of PE diagnosis based upon an isolated subsegmental filling defect – a finding that definitely causes clinical uncertainty and in the absence of concomitant DVT, may lead to overtreatment [8–13]. Additionally, at one US hospital, almost 40% of patients who had one CTPA underwent a second CTPA that was negative for PE within the next few years . These concerns prompted the National Quality Forum to issue an imaging efficiency measure (NQF #IEP-005-10) intended to reduce CTPA usage in low-risk and D-dimer negative patients.
Judicious use of D-dimer-based screening can decrease unnecessary CTPA usage, but some clinicians may be reticent to use D-dimer in patients with risk factors for a false-positive D-dimer, including age > 70 years, active malignancy or pregnancy [15–17]. Several authors have proposed increasing the D-dimer threshold based upon the pretest probability, or the patient’s age or comorbidity [18–21]. The central hypothesis of this work stems from the concept that one D-dimer threshold does not fit all patients equally, and that the threshold for abnormal can be safely increased under two circumstances: (i) in patients who have a condition that predict an elevated D-dimer concentration even in the absence of PE, or (ii) those with a low enough pretest probability. Increasing the D-dimer threshold would probably increase the exclusion rate, but potentially at the cost of increasing the false-negative rate. No previous study has enrolled a large, prospective, multicenter sample of inpatients and outpatients who have all undergone CTPA testing, standardized quantitative D-dimer testing and with the methodology to determine how many of these false-negative cases are isolated subsegmental filling defects. Moreover, a sensible study of D-dimer should also acknowledge the potential clinical significance of alternative diagnoses revealed by CTPA, most often infiltrate suggesting pneumonia [4,5,22–25]. The primary goal of the present study was to measure the change in the frequency of PE exclusion, the coincident change in the frequency and size of missed PE, and the posterior probability of pneumonia produced when the D-dimer threshold is doubled for the subset of patients aged ≥ 70 years or for patients with a PE unlikely pretest probability.
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Patients were enrolled from 30 January 2007 to 27 April 2008. Figure 1 shows the flow diagram of patients starting with those subjects screened for inclusion to the point of final outcome with respect to PE diagnosis. Seven hundred and twenty-five patients signed a consent form and 678 had complete data. Table 1 presents the percentages of the 678 patients who were enrolled with complete data who had each inclusion criterion and Table 2 presents the demographic data and additional clinical characteristics of these 678 patients. One hundred and eleven (18%) patients were aged > 70 years.
Table 2. Clinical characteristics of the study population
|Variable||Mean or n||SD or %|
|Vital sign data|
| Pulse rate (beats min−1)||84||18|
| Respiratory rate (breaths min−1)||19||4|
| Pulse oximetry||97||2|
| Systolic blood pressure (mmHg)||128||21|
| Emergency department||334||49%|
| Rehabilitation unit||9||1%|
| Age (years)||55||16|
| Body weight (lbs)||194||56|
| Black race||218||32%|
| Native American race||2||0%|
| Caucasian race||446||66%|
| Asian race||2||0%|
| Other race||10||1%|
| Male sex||258||38%|
|Risk factors for PE|
| Wells’ score||3.0||1.9|
| Revised Geneva Score||6.7||3.4|
| Chronic obstructive pulmonary disease||74||11%|
| Warfarin within past 7 days||89||13%|
| Current smoker||258||38%|
| Sickle cell disease||2||0%|
|Other diagnostic tests done|
| Ultrasound of leg||144||21%|
| Ultrasound of arm||13||2%|
| D-dimer at site||286||42%|
The criterion standard for any filling defect consistent with PE was found in 126/678 patients (19%, 95% CI: 16–22%), including 108 with an acute PE only, nine with an acute and chronic PE and four with a chronic PE only. On follow-up, 60 (9%) patients had a repeat CTPA of which five (8%) showed interval development of a new, acute PE. None of these five had been treated with systemic anticoagulation before the second CTPA. By 30 days follow-up, clinicians treated 124 patients who had any PE observed on CTPA with systemic anticoagulation, withholding anticoagulation in two patients with subsegmental PE, neither of whom developed any thrombotic event within 90 days. Fifty-four of the 126 PE cases (42%) were isolated subsegmental (total pulmonary vascular occlusion < 5%). The criterion standard for pneumonia was found in 93/678 patients (14%, 95% CI: 11–17%). The posterior probabilities of PE and pneumonia in the presence of a normal D-dimer using the conventional cutoff of 500 ng mL−1 was 8/152 (5%) and 16/152 (10.5%), respectively.
To illustrate the relationship between D-dimer and age, Fig. 2 plots the D-dimer concentration vs. age and shows the best fit quadratic equation for D-dimer values for patients with and without PE. The splay in the curves shows widening at 70 years of age, providing support to the decision to increase the D-dimer threshold at this age.
Figure 2. Scatter plot of the plasma D-dimer concentration vs. patient’s age. Red circles represent pulmonary embolism (PE)+ patients and open blue circles represent PE− patients. The curved lines are best fit quadratic equations (PE+: D-dimer = 3096−58*age + 1.13*age2; PE−: D-dimer = 3052−66*age + 0.63*age2).
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To assess the relationship between the size of the PE and D-dimer, Fig. 3A plots the D-dimer concentration as a function of the fraction of pulmonary vascular obstruction for the entire range of D-dimer concentrations. The plot shows no evidence of overall correlation [Pearson’s correlation coefficient (r) value < 0.1], even with multiple attempts at data transformation. However, Fig. 3B, which displays the area shown in the lower left corner of Fig. 2A, does convey important information about the size of PEs in the range of D-dimer values that might be used for exclusion. Dots to the left of the vertical dotted line represent subsegmental PEs. The major point of this plot is that the majority of patients with PE and a D-dimer concentration below 1000 ng mL−1 have subsegmental PEs.
Figure 3. (A) Scatter plot of the D-dimer vs. the percentage of pulmonary vasculature obstructed on computerized tomographic pulmonary angiography (CTPA) for all patients with a pulmonary embolism (PE) and (B) shows data for the rectangle at the lower left corner; all red circles to the left of the vertical dotted line represent a subsegmental PE.
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Table 3 presents a matrix designed to systematically examine the posterior probability of PE in the presence of eight increasing D-dimer thresholds according to the age of each patient, stratified into four ranges intervals (< 30, 30–49, 50–69 and ≥ 70 years) and according to the Wells’ and RGS scores, stratified into three intervals (< 2, 2–6, > 6 for the Well’s score and < 4, 4–10 or > 10 for RGS) and then at the single most useful cutoff for each (≤ 4 for Well’s and ≤ 6 for RGS).The purpose of Table 3 was to inspect each cell for the posterior probability to estimate the optimal cutoff in D-dimer and age or pretest probability score, respectively.
Table 3. Posterior probabilities for pulmonary embolism stratified for age, pretest probability at increasing thresholds for a normal D-dimer
|Adjustment criteria||Cutoff for an abnormal D-dimer concentration in ng mL−1|
|500 (%)||750 (%)||1000 (%)||1250 (%)||1500 (%)||5000 (%)||10 000 (%)||25 000 (%)|
|Age||0–29||0/12 (0)||1/18 (6)||1/22 (5)||1/23 (4)||1/23 (4)||8/48 (17)||9/51 (18)||9/51 (18)|
|30–49||5/56 (9)||8/81 (10)||8/88 (9)||9/101 (9)||10/110 (9)||26/169 (15)||28/174 (16)||28/175 (16)|
|50–69||2/69 (3)||6/125 (5)||8/148 (5)||12/187 (6)||18/213 (8)||58/312 (19)||63/323 (20)||67/331 (20)|
|70–100||1/15 (7)||4/34 (12)||4/52 (8)||6/63 (10)||7/77 (9)||13/110 (12)||18/116 (16)||22/121 (18)|
|All ages||8/152 (5)||19/258 (7)||21/310 (7)||28/374 (7)||36/423 (9)||105/639 (16)||118/664 (18)||126/678 (19)|
|Wells’ score||< 2||3/80 (4)||8/139 (6)||9/165 (5)||10/185 (5)||14/212 (7)||36/292 (12)||38/301 (13)||39/304 (13)|
|2–6||3/68 (4)||9/113 (8)||9/134 (7)||15/174 (9)||19/196 (10)||59/316 (19)||68/328 (21)||72/336 (21)|
|> 6||2/4 (50)||2/6 (33)||3/11 (27)||3/15 (20)||3/15 (20)||10/31 (32)||12/35 (34)||15/38 (39)|
|≤ 4||4/106 (4)||11/186 (6)||12/221 (5)||16/260 (6)||21/299 (7)||54/434 (12)||57/445 (13)||60/452 (13)|
|Revised Geneva score||< 4||2/65 (3)||3/98 (3)||3/104 (3)||4/116 (3)||7/126 (6)||18/159 (11)||18/160 (11)||18/160 (11)|
|4–10||4/75 (5)||13/139 (9)||14/178 (8)||19/220 (9)||24/256 (9)||64/398 (16)||72/413 (17)||77/424 (18)|
|> 10||2/12 (17)||3/21 (14)||4/28 (14)||5/38 (13)||5/41 (12)||23/82 (28)||28/91 (31)||31/94 (33)|
|≤ 6||4/110 (4)||10/179 (6)||11/208 (5)||15/241 (6)||19/273 (7)||45/366 (12)||48/373 (13)||51/377 (14)|
Taken together, data in Figs 1 and 2 and Table 3 indicate that posterior probability could be maintained and rule out the rate for PE could be increased if the D-dimer cutoff were increased up to 1000 ng mL−1 for age > 70 years or Wells’ score ≤ 4 or RGS ≤ 6. These cutoffs in D-dimer, age and pretest probability agree with previous studies [18,21,31].
Table 4 presents the main findings of the present study, including the exclusion rate and posterior probability for eight different D-dimer-based screening strategies. Table 4 first shows the performance of the quantitative site D-dimer that was ordered as part of standard care in 286 patients. Curiously, the site D-dimer was negative in 40 patients (6% of the cohort) who underwent CTPA, only one of whom had a PE, suggesting either an uncoupling of test ordering from subsequent behavior or that clinicians may order the CTPA to diagnose other conditions besides PE. Table 4 then shows the diagnostic performance of the study D-dimer at a standard threshold (< 500 ng mL−1) and at < 1000 ng mL−1 alone and under three assumptions: age > 70 years, a Wells’ score ≤ 4 or an RGS ≤ 6. With any of these three strategies, the net rule out rate could have been substantially increased by doubling the D-dimer threshold, and with minimal cost in terms of increased posterior probability of segmental or larger PE. For example, for patients with a Wells’ score ≤ 4 and the standard D-dimer threshold, the exclusion rate was 16% (95% CI: 13–19%) and the posterior probability was 4/106 or 3.8% (95% CI: 1–9.3%) whereas with a Wells’ score ≤ 4 and a D-dimer < 1000 ng mL−1 the exclusion rate increased to 33% (95% CI: 29–36%), and the posterior probability of PE was 12/221 or 5.4% (95% CI: 2.8–9.3%). Likewise, for patients with an RGS ≤ 6 at standard D-dimer threshold, the exclusion rate was 110/678 (16%, 95% CI: 14–19%) with a 4/110 or 3.6% (95% CI 1–10%) posterior probability whereas for an RGS ≤ 6 and D-dimer < 1000 ng mL−1, the exclusion rate increased to 208/678 (31%, 95% CI 27–44%) with a posterior probability of 11/208 (5.3%, 95% CI 3.0–9.2%). The same four patients with PE were missed by the Wells’ score ≤ 4, and the RGS ≤ 6 and D-dimer < 500 ng mL−1, and three of these four had subsegmental PE and one had a PE with 19% obstruction.
Table 4. Diagnostic indexes produced by screening strategies for a pulmonary embolism (PE) that include an increased D-dimer threshold
|Screening strategy||TP||TN||FN||FP||n||Exclusion rate (%)||Sens (%)||Spec (%)||LR (−)||Posterior probability (%)||FN with Subseg*||CIN†|
|Site D-dimer, standard threshold*||35||39||1||211||286||6||97.2||15.6||0.18||2.5||0||0|
|D-dimer < 500 ng mL−1||118||144||8||408||678||22||93.7||26.1||0.24||5.3||7||3|
|D-dimer < 1000 ng mL−1||105||289||21||263||678||46||83.3||52.4||0.32||6.8||17||11|
|D-dimer < 500 or < 1000 if Age > 70||116||175||10||377||678||27||92.1||31.7||0.25||5.4||9||6|
|Wells ≤ 4 and D-dimer < 500 ng mL−1||122||102||4||450||678||16||96.8||18.5||0.17||3.8||4||2|
|Wells ≤ 4 and D-dimer < 1000 ng mL−1||114||209||12||343||678||33||90.5||37.9||0.25||5.4||10||10|
|RGS ≤ 6 and D-dimer < 500 ng mL−1||122||106||4||446||678||16||96.8||19.2||0.17||3.6||4||2|
|RGS ≤ 6 and D-dimer < 1000 ng mL−1||115||197||11||355||678||31||91.3||35.7||0.24||5.3||10||9|
We then examined the size of the filling defects of patients present when the patient had a PE unlikely pretest probability and a D-dimer < 1000 ng mL−1. Among the 12 false-negative cases produced by the Wells’ score ≤ 4 and a D-dimer threshold of 1000 ng mL−1, 10 of these 12 false-negative PEs were isolated, subsegmental filling defects and no patient with a D-dimer < 1000 ng mL−1 was diagnosed with concomitant DVT and one had active malignancy. Among the 11 patients with an RGS ≤ 6 and a D-dimer < 1000 ng mL−1, 10 of these 11 had isolated, subsegmental filling defects, and none had DVT and one had active malignancy. Seven of the patients with false-negative PEs (12 with Wells’ ≤ 4 and 11 with RGS ≤ 6) had a lower extremity ultrasound and three others underwent lower extremity CT venogram which was negative for DVT. The only PE that was larger than subsegmental that was missed by both strategies was the same PE with 19% obstruction missed by the D-dimer < 500 ng mL−1.
To assess the rate of contrast-induced nephropathy, we examined data from 224 patients who had a serum creatinine measured before CTPA and again at 2–7 days post enrollment as part of standard care. Of the 224 patients with paired serum creatinine measurements, 37 (17%, 95% CI: 12–22%) developed contrast-induced nephropathy, using the definition of a 50% relative increase in creatinine value. In the last column on the right, Table 4 presents the number of cases of CIN that could have been prevented with each screening strategy. Ten patients with CIN had a Wells’ score ≤ 4 and a D-dimer < 1000 ng mL−1 and nine with CIN had RGS ≤ 6 and D-dimer < 1000.
When the data were restricted to inpatients only, we found a trend towards reduced protocol efficiency. For example, among inpatients, the combination of a Wells’ score ≤ 4 and a D-dimer < 500 ng mL−1 would have allowed a rule out rate of 12% producing a posterior probability of PE of 6.8% whereas a Wells’ score ≤ 4 and a D-dimer < 1000 ng mL−1 would have ruled out 26% producing a posterior probability of PE of 7.7%.
Because active cancer is known to elevate the D-dimer and sharply reduce the test specificity, we examined the impact of D-dimer at a threshold at 1000 ng mL−1 for patients aged > 70 years or with active cancer and 500 ng mL−1 for all others . This strategy would have led to 195 (28.7%) patients with a negative screen, with 12 (6.1%) false negatives, and thus provided a minimal incremental increase in rule out rate.
Regarding potential alternative diagnoses on CTPA, radiologists found at least one pathologic finding on 425/552 CTPA scans that did not have PE. Both radiologists agreed that infiltrate was present in 93 patients who received antibiotics within 24 h and had a discharge diagnosis that included pneumonia. Thus, pneumonia was present in 14% (95% CI: 11–17%) of patients who underwent CTPA for a suspected PE, and pneumonia accounted for most clinically important acutely life-threatening, alternative diagnoses seen on CTPA. Other diagnoses seen on CTPA included aortic dissection (9), thoracic aneurysm (10) and pneumothorax (3). Table 5 shows the diagnostic indexes for each putative strategy for pneumonia. The primary findings in Table 5 was that the strategy of doubling the D-dimer for age < 70, was not an efficient screening tool for pneumonia because, whereas the combination of an RGS ≤ 6 and a D-dimer < 1000 led to the lowest posterior probability of pneumonia. Plain film chest radiographs were ordered on 565 patients (83%) and among these, 200 (35%) were interpreted as normal and 58 (10%) were interpreted as positive for infiltrate. Surprisingly, only 21 of these 58 were interpreted as positive for infiltrate on CTPA. These data suggest substantial discordance between plain film chest radiographs and the CTPA.
Table 5. Diagnostic indexes for pneumonia using the proposed D-dimer-based screening strategies
|Screening strategy||TP||TN||FN||FP||n||Sensitivity (%)||Specificity (%)||LR(−)||Posterior probability (%)|
|D-dimer < 500 ng mL−1||77||136||16||449||678||82.8||23.2||0.74||10.5|
|D-dimer < 1000 ng mL−1||66||283||27||302||678||71.0||48.4||0.60||8.7|
|D-dimer < 500 or < 1000 if Age > 70||75||167||18||418||678||80.6||28.5||0.68||9.7|
|Wells ≤ 4 and D-dimer < 500 ng mL−1||83||96||10||489||678||89.2||16.4||0.66||9.4|
|Wells ≤ 4 and D-dimer < 1000 ng mL−1||76||204||17||381||678||81.7||34.9||0.52||7.7|
|RGS ≤ 6 and D-dimer < 500 ng mL−1||84||101||9||484||678||90.3||17.3||0.56||8.2|
|RGS ≤ 6 and D-dimer < 1000 ng mL−1||81||196||12||389||678||87.1||33.5||0.39||5.8|
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The present study examines the hypothesis that selected use of an elevated threshold for a normal D-dimer concentration can decrease the need for CTPA scanning for PE. As has been reviewed recently, extensive previous literature has been published to suggest that D-dimer can be elevated in the elderly, in patients with certain comorbidities and in patients with a low pretest probability . The present study was designed specifically to address this question, after years of planning, and using methods designed to enhance the generalizability of results. We enrolled a sample of patients who represent the whole population of patients undergoing 64 channel CTPA at four large hospitals, including a balance of ED and hospitalized patients . We used a rigorous criterion standard inasmuch all CTPA scans were read by two independent radiologists and the central radiologist graded the size of PE; we also included clinical follow-up. D-dimer concentrations were measured in a central laboratory using a well-studied D-dimer assay that was the first cleared by the FDA as an aid to exclusion of PE. Recognizing the possibility that in clinical practice, some physicians eschew the D-dimer and order a CTPA to help them evaluate non-thromboembolic pathology, we also examined the ability of D-dimer to exclude pneumonia, the most common important alternative diagnosis seen on CTPA scans [4,5].
These resultant data allow several clinically important observations. First, the data in Table 4 show that physicians sometimes order a D-dimer at the site and then disregard the result. In fact, the site D-dimer was negative in 40 patients who underwent CTPA, and only one of whom had a PE. Second, most patients with a PE in the presence of D-dimer concentrations below 1000 ng mL−1 had very small PEs. Had a high sensitivity D-dimer been ordered in all patients regardless of pretest probability and using the standard threshold of 500 ng mL−1, 152 (22%) of CTPA scans could have been obviated. Importantly, these 152 patients included 8 (5.3%) possible false-negative D-dimer patients who had filling defects on CTPA suggestive of PE, and only 1 of these 8 patients had a filling defect in an artery that was larger than subsegmental. Had a more liberal definition of 1000 ng mL−1 been used, the exclusion rate would have increased to 310 (46%), but at a cost of a false-negative rate of 21/310 (6.8%) including four patients with a filling defect larger than subsegmental. This rate would appear unacceptably high, and underscores the need for additional criteria. The third main observation was that increasing the normal D-dimer threshold to < 1000 ng mL−1 for patients with a ‘PE unlikely’ pretest probability, based upon either the Wells’ score ≤ 4 or an RGS ≤ 6, would potentially obviate one-third of CTPA scans with isolated subsegmental filling defects accounting for most of the false negatives. In addition to preventing radiation exposure, this strategy would have potentially prevented at least 10 cases of contrast induced nephropathy, using a very strict definition .The potentially simpler strategy of elevating the D-dimer threshold to 1000 ng mL−1 only for patients over 70 years had similar results, but adding active malignancy had a marginal value. Our data also show some support for the D-dimer as a screening tool for pneumonia, particularly for an RGS ≤ 6, and a D-dimer < 1000 ng mL−1 the combination of which significantly lowered the posterior probability of pneumonia from a baseline of 14% (95% CI: 11–17%) to 5.8% (95% CI 3–10%). The final salient observation was the remarkably high rate of subsegmental PE (42% of PEs), which raises a question as to whether the sample may have spectrum or severity bias, or if this reflects the effect of 64 channel multi-row detector CTPA and a criterion standard of PE seen by either the site radiologist or the IRR radiologist.
The present study must be in interpreted with caution because of the absence of clear data on the significance of an isolated subsegmental PE, which comprised almost all of the false-negative cases, and most of these patients were treated with anticoagulation. Observational studies suggest the outcomes of patients with isolated subsegmental PE who are not treated have similar outcomes to those who are treated with anticoagulation, although the literature lacks a randomized trial to address this question [8,12,13,35]. Indeed, the present study only tests the hypothetical contribution of elevating the threshold to define an abnormal D-dimer concentration, as opposed to examining outcomes of patients in a management study. Thus, the next step should be an outcomes trial that randomizes low pretest probability patients to one of two groups, both of which would have anticoagulation withheld: D-dimer at a standard threshold or D-dimer ‘at twice the local threshold’. The rational for stating the D-dimer ‘at the local threshold’ as opposed to 500 ng mL−1 is that thresholds for abnormal vary with many quantitative D-dimer assays. Indeed, in the present study, each of the four hospitals used a different local D-dimer assay, each with a different threshold for abnormal. Another limitation involves the interpretation of the significance of the D-dimer as a screen for pneumonia, because we assume that some clinicians are motivated to order CTPA to evaluate for alternative diagnoses such as pneumonia, a point that is based upon experience of the authors. Lastly, in the interest of brevity, we did not use all potentially valid pretest probability methods .
In conclusion, a screening strategy that doubles the threshold for an abnormal D-dimer concentration for patients over 70 years old or patients with a Wells’ score ≤ 4 or an RGS ≤ 6 could significantly reduce the rate of CTPA with no net increase in the rate of missing a larger than subsegmental PE and no increase in the rate of missed pneumonia.