ArticlesValue of assessment of pretest probability of deep-vein thrombosis in clinical management
Introduction
Since the late 1980s, high-resolution real-time B-mode ultrasound has been used for the diagnosis of deep-vein thrombosis.1 Many studies have reported sensitivities and specificities for the various ultrasound imaging modalities to be over 95% for proximal deep-vein thrombosis in symptomatic patients and consequently venous ultrasound imaging is now widely accepted as the non-invasive test of choice for the diagnosis of deep-vein thrombosis. However, ultrasound is relatively insensitive to deep-vein thrombosis isolated to the calf.2 Calf deep-vein thrombosis is usually a self-limited condition with a very low risk of pulmonary embolism, but 20% to 30% of calf deep-vein thrombosis may extend to involve the larger more proximal veins, which carry a much higher risk of pulmonary embolism.3 For this reason it is recommended that patients who are initially negative on ultrasound testing have follow-up (serial) tests over the next 7 to 10 days to exclude proximal extension. Two studies involving over 300 patients showed that it was relatively safe to withhold anticoagulants in outpatients with negative serial ultrasound results over 7 days since only 1 ·3% of these patients developed venous thromboembolic complications over 3-month follow-up periods.4, 5 However, serial testing is inefficient and inconvenient for patients, and costly for the health-care system since most patients do not have deep-vein thrombosis on the serial test. Ultrasound is also limited by false results. In previous studies the positive and negative predictive values of ultrasound for deep-vein thrombosis were about 94%.5, 6
We have previously suggested, on the basis of a large clinical trial, that clinical assessment with a clinical model may overcome the limitations of ultrasound.7 The positive and negative predictive values of diagnostic tests are dependent on prevalence and thus should differ depending on the probability category. In our previous study we demonstrated the high positive predictive value of ultrasound in the patients with moderate and high pretest clinical probability and the high negative predictive value in the patients at low probability. Through logistic regression analysis we simplified the original model but we had not prospectively tested the revised model.8 It was our impression that we could safely assess patients with significantly fewer diagnostic tests than the serial approach requires. In this report the simplified model was used in combination with ultrasound to guide management of patients with suspected deep-vein thrombosis.
Section snippets
Methods
This study was a prospective cohort trial of outpatients with symptoms and suspected deep-vein thrombosis referred to the Queen Elizabeth II Health Sciences Centre, Halifax, Nova Scotia, Canada, or the Ottawa Civic Hospital, Ottawa, Ontario, Canada. The protocol was approved by the research ethics committees of our institutions. Consecutive patients referred to outpatient clinics or the Radiology Departments with pain or swelling of the lower extremity in whom the diagnosis of deep-vein
Results
593 of the 918 patients who were eligible were enrolled. 10 patients refused consent. 315 patients were excluded for the following reasons: 194 because of a previous episode of deep-vein thrombosis or pulmonary embolism; 53 had signs or symptoms suggestive of current pulmonary embolism; 42 were geographically located such that follow-up could not be done; 20 had another disease making life expectancy less than 3 months; and six patients required long-term anticoagulant therapy. The mean age of
Discussion
In an earlier report we validated a clinical model in patients with suspected deep-vein thrombosis7 but we did not use it in a management strategy. The model was simplified after logistic regression analysis8 and in this study the new model was used in a management strategy which decreased the number of diagnostic tests required in patients with suspected deep-vein thrombosis. As with the original model physicians were able to accurately stratify patients with suspected deep-vein thrombosis
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