Deep vein thrombosis post-operatively is diagnosed by :

Venography is the most definitive test for the diagnosis of DVT in both symptomatic and asymptomatic patients. It is the gold standard to which other modalities are compared. This procedure involves placement of a small catheter in the dorsum of the foot and injection of a radiopaque contrast agent. Venography is not routinely used for the evaluation of lower extremity DVT because of the associated complications discussed previously. Currently, venography is reserved for imaging before operative venous reconstruction and catheter-based therapy. It does, however, remain the procedure of choice in research studies evaluating methods of prophylaxis for DVT. Ref : Liem T.K., Moneta G.L. (2010). Chapter 24. Venous and Lymphatic Disease. In T.R. Billiar, D.L. Dunn (Eds), Schwaz's Principles of Surgery, 9e.