In both study groups, venous compression ultrasonography was performed at each presentation (HDI 5000, UM9 HDI, linear array, 4-7 MHz; ATL, Bothell, Wash; and Sonoline, linear array 7.5 MHz; Siemens, Erlangen, Germany) using a standardized examination protocol.15 To avoid a biased ultrasonographic assessment, ultrasonography was performed by a second physician (T.S., W.O., K.H., or J.B.), and the sonographer (T.S., W.O., K.H., or J.B.) was masked to case or control status of the participants. With the patient supine, the following venous segments were examined with B-mode compression ultrasonography: common femoral vein, proximal part of the great saphenous vein, deep femoral vein, proximal superficial femoral vein, and distal superficial femoral vein. After this, the study participant had to sit up with the legs hanging down. In this position, the following segments were examined with B-mode compression ultrasonography: popliteal vein down to the trifurcation, proximal part of the small saphenous vein, confluence segment of the posterior tibial veins and peroneal veins, proximal and distal parts of the tibial posterior and peroneal veins, lateral and medial gastrocnemius muscle veins, lateral and medial soleal sinusoids of the proximal and distal calf, and distal parts of the great and small saphenous veins. If thrombosis was clinically suspected, Doppler modalities were used for examination of the iliac veins and the inferior caval vein. The diagnostic criterion for thrombosis was the lack of compressibility of 1 or more segments of the veins of the lower extremities.