That's the answer to your description's question: before or on the 51st ball drawn. But since your heading asks a slightly different question, I'll also answer your heading's question, which is just before 51 balls.
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Compression ultrasound (CUS) is the primary testing procedure used to diagnose proximal deep vein thrombosis (DVT; a blood clot in a deep vein in the thigh or leg), as the method confirms and excludes DVT of the proximal veins (above the knee) but its accuracy for distal vein DVT (below the knee) has been questioned.