Applicability of decision analysis to everyday clinical practice: a controlled feasibility trial

External validation and comparison of recently described prediction rules for suspected pulmonary embolism

PURPOSE OF REVIEW

The assessment of pretest probability, allowing the categorization of patients clinically suspected of having pulmonary embolism in low, intermediate, and high clinical probability, is an essential step in contemporary diagnostic strategies because it permits limiting the number of additional diagnostic tests, especially invasive tests. Clinical probability can be evaluated implicitly or by prediction rules. Two prediction rules for pulmonary embolism have been described: the Canadian prediction rule (the Wells score) and the Geneva prediction rule. Their original descriptions were published in 2000 and 2001, respectively. These prediction rules need to be externally validated, and, ideally, outcome studies should demonstrate that patients may be safely treated on the basis of the assessment of the clinical probability they provide. Therefore, the purpose of this review is to discuss the external validation of these rules, because this particular point has been only recently achieved.

RECENT FINDINGS

Application of both rules in an external setting and in a prospective study have confirmed their validity. A recent study suggests that the best evaluation is probably based on a prediction rule associated with possible clinical override.

SUMMARY

Studies comparing an empiric assessment with explicit assessment, such as the Wells simplified score or the Geneva score, have shown that the three tools show similar accuracy. External validation and use of both rules in prospective management studies have only recently been performed and have confirmed their validity. Some reports suggest that empiric assessment may be influenced by level of training. Objective prediction rules seems to be less influenced by experience and should be preferred by more junior doctors. The tool used for clinical probability assessment is probably less important than the principle of a careful clinical probability assessment in each patient with suspected pulmonary embolism.

The utility of pretest probability assessment in patients with clinically suspected venous thromboembolism

The assessment of pretest probability (PTP), with stratification into low-, intermediate- and high-risk groups is an essential initial step in the current diagnostic management of patients with suspected venous thromboembolism (VTE). In combination with additional information, it reduces the need for initial and supplementary imaging, and allows considerable refinement of the posterior probability of VTE following non-invasive imaging. PTP may be assessed either empirically or by using various decision rules or scoring systems, the best known of which are the simplified Wells scores for suspected deep vein thrombosis (DVT) and pulmonary embolism (PE), and the Geneva score for suspected PE. Each of these approaches shows similar directional and categorical accuracy, and has been validated as facilitating clinically useful classification of the PTP, although an overview of data suggests that fewer patients tend to be classified as low PTP when assessed empirically. This group is the most important to identify, as several outcome studies have shown that imaging and treatment are safely obviated in outpatients with suspected DVT or PE who have a low PTP in combination with negative d-dimer testing, a subgroup accounting for up to half of all patients studied. Hence, while probably not of critical importance, the explicit approach offered by scoring systems might be preferred over empirical assessment, particularly when used by more junior staff.

The natural history of venous thromboembolism: impact on ventilation/perfusion scan reporting

Deep vein thrombosis (DVT) and pulmonary embolism (PE) are separate but related aspects of the same dynamic disease process known as venous thromboembolism (VTE). Recent community studies have shown that VTE is a major health issue for the developed world, with at least 201,000 new cases each year in the United States, comprising 107,000 with DVT and 94,000 with PE. A quarter of PE cases die within 7 days, some so rapidly that treatment or intervention is impossible. Despite the availability of heparin prophylaxis, the annual incidence of VTE has remained constant at 1 event per 1,000 person-years since 1979 but reaches 1 event per 100 person-years for the over-85-year-olds. The most important risk factors for VTE are hemostatic and environmental. The recent discoveries of factor V Leiden, prothrombin 20210A, and high concentrations of factor VIII have highlighted the increasing importance of a genetic predisposition to thrombophilia. Acquired hemostatic factors include pregnancy and the puerperium, oral contraception, hormone-replacement therapy, malignant tumors, and antiphospholipid syndromes. Important environmental risk factors include hospitalization with previous surgery or trauma, confinement in a care facility, neurologic disease or paraplegia after stroke, current or recent central venous catheter or transvenous pacemaker, and long airplane flights. Internists may be confused about the risk of PE after ventilation/perfusion (VQ) imaging. This may well arise from their use of the relative risk of PE after a low-probability category scan rather than the absolute risk obtained by incorporating the PE prevalence for their particular patient in the risk analysis. Ideally, personal communication with an experienced referring physician provides this clinical information for nuclear medicine. Diagnostic tools or checklists can be used as an alternative. A general knowledge of the natural history of VTE will encourage the nuclear medicine physician to provide an appropriate clinical signal to complement VQ categorical analysis. Combination of these 2 dynamic elements of the art and science of VQ scan reporting-the clinical pretest probability of PE and lung scan category-will permit an accurate prediction of the absolute risk of PE posttest.

D-dimer testing as the initial test for suspected pulmonary embolism. Appropriateness of prescription and physician compliance to guidelines

OBJECTIVES

Recent studies have shown that strategies for pulmonary embolism diagnosis which have included D-dimer testing have been most cost effective. The objective of this study is to evaluate the impact of a new strategy for pulmonary embolism diagnosis based on D-dimer results.

METHODS

A prospective survey was conducted in the emergency ward and three medical departments of a university teaching hospital. Guidelines for diagnosis of PE were established and implemented through an educational intervention and a specific order form. D-dimer (ELISA) was required for all patients suspected of having PE. A result above 500 ng/ml was to be followed by an a pulmonary imaging procedure. Appropriateness of prescription of D-dimer and non-compliance with guidelines (absence of diagnostic imaging procedure following D-dimer results above 500 ng/ml) were evaluated.

RESULTS

One-hundred sixty patients were studied. D-dimer test was performed in 154 patients (96.3%) suspected of PE during a two-month period. Test results were above 500 ng/ml in 111 cases. PE was confirmed in 20 cases. Twenty percent (31/154) of the D-dimer prescriptions were inappropriate. Among those with D-dimer results above 500 ng/ml, 45% (50/111) of the patients experienced no imaging procedure.

CONCLUSIONS

Despite implementation of clinical guidelines for its use, D-dimer was excessively prescribed. A large proportion of results was not taken in consideration by prescribers. Often new technologies have good experimental results, but behave differently when used routinely in ordinary care settings. It is important that field studies be developed to evaluate the effectiveness of new technologies.

Non-invasive diagnosis of venous thromboembolism in outpatients

BACKGROUND

We designed a simple and integrated diagnostic algorithm for acute venous thromboembolism based on clinical probability assessment of deep-vein thrombosis (DVT) or pulmonary embolism (PE), plasma D-dimer measurement, lower-limb venous compression ultrasonography, and lung scan to reduce the need for phlebography and pulmonary angiography.

METHODS

918 consecutive patients presenting at the emergency ward of the Geneva University Hospital, Geneva, Switzerland, and Hôpital Saint-Luc, Montreal, Canada, with clinically suspected venous thromboembolism were entered into a sequential diagnostic protocol. Patients in whom venous thromboembolism was deemed absent were not given anticoagulants and were followed up for 3 months.

FINDINGS

A normal D-dimer concentration (<500 microg/L by a rapid ELISA) ruled out venous thromboembolism in 286 (31%) members of the study cohort, whereas DVT by ultrasonography established the diagnosis in 157 (17%). Lung scan was diagnostic in 80 (9%) of the remaining patients. Venous thromboembolism was also deemed absent in patients with low to intermediate clinical probability of DVT and a normal venous ultrasonography (236 [26%] patients), and in patients with a low clinical probability of PE and a non-diagnostic result on lung scan (107 [12%] patients). Pulmonary angiography and phlebography were done in only 50 (5%) and 2 (<1%) of the patients, respectively. Hence, a non-invasive diagnosis was possible in 866 (94%) members of the entire cohort. The 3-month thromboembolic risk in patients not given anticoagulants, based on the results of the diagnostic protocol, was 1.8% (95% CI 0.9-3.1).

INTERPRETATION

A diagnostic strategy combining clinical assessment, D-dimer, ultrasonography, and lung scan gave a non-invasive diagnosis in the vast majority of outpatients with suspected venous thromboembolism, and appeared to be safe.

The role of decision analysis in informed consent: choosing between intuition and systematicity

An important goal of informed consent is to present information to patients so that they can decide which medical option is best for them, according to their values. Research in cognitive psychology has shown that people are rapidly overwhelmed by having to consider more than a few options in making choices. Decision analysis provides a quantifiable way to assess patients' values, and it eliminates the burden of integrating these values with probabilistic information. In this paper we evaluate the relative importance of intuition and systematicity in informed consent. We point out that there is no gold standard for optimal decision making in decisions that hinge on patient values. We also point out that in some such situations it is too early to assume that the benefits of systematicity outweigh the benefits of intuition. Research is needed to address the question of which situations favor the use of intuitive approaches of decision making and which call for a more systematic approach.