Diagnostic tests for deep vein thrombosis. Clinical usefulness depends on probability of disease
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We review the general principles that govern the clinical utility of diagnostic tests, particularly with respect to the diagnosis of deep vein thrombosis (DVT). We stress the importance of clinical probability of disease, which strongly influences the positive predictive value (true-positive rate) and negative predictive value (true-negative rate) of all diagnostic tests. In selecting a diagnostic procedure for DVT, the physician must first consider the clinical probability of disease and then the local accuracy of the test employed and its cost-effectiveness. In 75% to 80% of patients suspected to have DVT, clinical management can be based on the results of noninvasive tests, such as ultrasonography or impedance plethysmography (IPG), rather than venography. Ultrasonography has clear advantages over venography with respect to cost and patient comfort, and it defines the anatomic extent of the thrombus. It should be considered the new diagnostic standard for symptomatic DVT. Despite recent reports of lower sensitivity than previously reported, IPG remains an acceptable alternative to ultrasonography for symptomatic DVT in selected patients. Even if the recently reported lower sensitivity proves to be accurate, the probability of adverse clinical outcomes as a result of overlooked disease is still extremely low in patients with a low probability of DVT. The negative predictive value of IPG under these circumstances approaches 99%. Impedance plethysmography is also useful in patients with a high probability of DVT, in whom the positive predictive value may be as high as 97%. When the findings of IPG (or ultrasonography) are at variance with a strong clinical impression, venography should be considered, especially when there is a high clinical probability of disease and a negative noninvasive test result.
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