If i have crohn's disease and polycythemia rubra vera is a negative venous us and d dimer good enough to rule out a clot? have leg swelling lower leg

Nearly: The gold standard to definitively rule out an deep vein clot is venography. However, this is rarely if ever necessary given that compression ultrasonography has a sensitivity and specificity each of around 95% compared to venography. I would recommend further evaluation if you have had an injury in that leg or a previous DVT. Otherwise it is unlikely that you have a thrombus with a negative US.