Change in thrombus length on venous ultrasound and recurrent deep vein thrombosis.

BACKGROUND Despite successful treatment, deep vein thrombosis may not fully resolve in some patients. Consequently, in patients with new symptoms in a previously affected leg, noncompressibility on ultrasound cannot be assumed to be due to recurrent thrombosis; therefore, we sought to determine interobserver agreement on ultrasound measurements of thrombus length and identify the change in length that is likely to indicate thrombus extension. METHODS Cross-sectional study of patients with previous proximal deep vein thrombosis and residual ultrasound abnormalities, but without symptoms suggestive of recurrence. The patients had a standardized series of measurements of thrombus length independently performed by 2 ultrasonographers. The outcome measure was the agreement between the ultrasonographers on the distance of thrombus margins from 4 anatomical landmarks. RESULTS There was good interobserver agreement on thrombus length when measured from the saphenofemoral junction, mid-popliteal fossa, and mid-patella (93% or more of the variance was accounted for by the paired measurements). Using these landmarks, the 95th percentile for the absolute difference between paired measurements was 8.9 cm for the saphenofemoral junction, 8.5 cm for the mid-popliteal fossa, and 8.4 cm for the mid-patella. CONCLUSIONS When 2 ultrasound examinations are compared, an apparent increase in thrombus length of 9 cm or greater is likely to be supportive of a diagnosis of recurrent deep vein thrombosis. Increases in thrombus length of less than 9 cm appear to be within the bounds of measurement error.