Hampson correctly points out that Heijboer et al 1 demonstrated a very low incidence of objectively proved venous thromboembolism (2.2%) in patients with suspected deep vein thrombosis who had normal findings on 3 serial impedance plethysmographs. The tests were performed on days 1, 2, and 8. It is therefore possible that fewer serial impedance plethysmographic evaluations than were used in our study 2 may be adequate. However, in our study, of 16 cases of proximal vein thrombosis detected on serial testing, 7 were detected between days 8 and 14 and 4 were detected between days 10 and 14. In performing our cost analysis, the more conservative approach using 5 serial tests was chosen. The use of 3 as opposed to 5 serial tests would achieve further cost savings. Our more recent report 2 extended the previous study 3 to 1564 consecutive patients with suspected pulmonary embolism. Of the 627 patients
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