Clinical features and diagnosis of venous thrombosis
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The clinical diagnosis of venous thrombosis is inaccurate because the clinical findings are both insensitive and nonspecific. The sensitivity of clinical diagnosis is low because many potentially dangerous venous thrombi are clinically silent. The specificity of clinical diagnosis is low because the symptoms or signs of venous thrombosis all can be caused by nonthrombotic disorders. For these reasons, a practical approach for the diagnosis of venous thrombosis is important. A current approach to the diagnosis of clinically suspected venous thrombosis favors the use of impedance plethysmography over Doppler ultrasonography as the main test for this disorder. This is because impedance plethysmography is precise and objective, whereas the interpretation of Doppler ultrasonography is subjective and requires considerable skill and experience to form reliable diagnoses. The use of serial impedance plethysmography has been evaluated recently in a prospective study. The rationale of repeated impedance plethysmography evaluation is based on the premise that calf vein thrombi are only clinically important when they extend into the proximal veins, at which point detection with impedance plethysmography is possible. Therefore, by performing repeated examinations with impedance plethysmography in patients with clinically suspected venous thrombosis, it is possible to identify patients with extending calf vein thrombosis who can be treated appropriately. Impedance plethysmography is performed immediately on referral; if it is positive in the absence of clinical conditions that are known to produce falsely positive results, the diagnosis of venous thrombosis is established, and the patient is treated accordingly. If the result of the initial impedance plethysmography evaluation is negative, anticoagulant therapy is withheld, and impedance plethysmography is repeated the following day, again on day 5 to 7 and on day 10 to 14. If impedance plethysmography becomes positive during this time, a diagnosis of venous thrombosis is made and anticoagulant therapy is commenced. Positive impedance plethysmography in the presence of conditions known to produce a false positive result (for example, congestive cardiac failure) should be confirmed by venography. If noninvasive tests for the diagnosis of venous thrombosis are not available, a clinical suspicion of venous thrombosis should be objectively confirmed or excluded by performing ascending venography.
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