International guidelines recommend using the Villalta score (VS) to diagnose the post-thrombotic syndrome (PTS). However, a high proportion of PTS detected with VS could just reflect the presence of pre-existing primary venous insufficiency (PVI). Furthermore, it is unclear whether the contralateral VS (cl-VS) can be used to assess for pre-existing PVI.
To estimate whether cl-VS can be used to assess for pre-existing PVI, and to assess the proportion of PTS that could be attributable to pre-existing PVI.
Sub-analysis of the SOX multicentre randomized trial focusing on patients with a first unilateral proximal DVT followed for up to 2 years. PVI was defined as a baseline cl-VS>4, and PTS as VS>4 in the leg ipsilateral to DVT starting 6 months after DVT.
Among 680 patients, mean cl-VS remained stable over time: 1.23 (SD 2.49) at baseline and 1.17(2.20), 1.59(2.81), 1.54(2.50), 1.65(2.82), 1.55(2.63) at the 1, 6, 12, 18 and 24 months visits, respectively. Baseline cl-VS and ipsilateral VS measured during follow-up were mildly correlated (Pearson correlation=0.13-0.25). This association disappeared after subtracting from the ipsilateral VS the cl-VS measured at the same visit. Overall, 48.8% of patients developed PTS of whom 12.8% had baseline cl-VS>4.
In our study of patients with a first unilateral proximal DVT, the proportion of patients with PTS who had a cl-VS>4 is modest. However, cl-VS appears to be stable over time. Its assessment could constitute a simple way of documenting preexisting PVI and help to classify patients as having PTS or not.

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