To assess the effectiveness of hemodialysis, clinicians must know whether the flow that is moving through a vascular access can actually sustain hemodialysis. Before Transonic hemodialysis surveillance, direct measurement of vascular access flow did not exist. Clinicians had to rely on surrogate measurements such as the blood urea nitrogen (BUN) test to learn a percentage of recirculation in order to assess the hemodialysis. Test results took time. They were not precise.