Persistent withdrawal occlusion is a specific catheter malfunction characterized by inability to withdraw blood through the device while infusion is maintained. The main causes are fibroblastic sleeve and tip malposition (associated or not to venous thrombosis around the tip). All current guidelines recommend infusing vesicant/antiblastic drugs through a central venous port only after assessment of blood return. In PWO, blood return is impossible. We have recently started to assess the intravascular position of the tip and the delivery of the infusion in the proximity of the cavo-atrial junction utilizing transthoracic/subxiphoid ultrasound with the ‘bubble test’. We found that this is an easy, real-time, accurate and safe method for verifying the possibility of using a port for chemotherapy even in the absence of blood return, as it occurs with persistent withdrawal occlusion.