2 Table 1: Noninvasive methods of surveillance for AV access stenosis. Clinical monitoring Access recirculation Flow studies Pressure studies Dynamic Static Direct visualization Sodium, urea, glucose, diﬀerential conductivity, inline dialysance, ultrasound dilution and thermal methods Doppler ultrasonography Magnetic resonance angiography Debate also exists surrounding surveillance for AVFs on eventual thrombosis or ﬁstula longevity. Measurement of vascular access ﬂow (Qa) is the recommended method of surveillance for AVFs, and much of this data is based on the observational studies in AVGs [6]. There have been numerous observational studies with historical control groups evaluating the beneﬁt of Qa surveillance in AVFs, and results have been mixed [8–13]. Few of these studies [10–13] showed decreased thrombosis rates, while others [8, 9] showed no improvement [14]. Evaluating the available randomized controlled trials can better assess the question of whether the use of surveillance leads to a signiﬁcant decrease in the rate of AVF thrombosis and improves long- term survival. 2. Randomized Controlled Trials To date, there are four randomized trials that evaluated surveillance of AVF for stenosis and two of these evaluated the eﬀect of prophylactic angioplasty on thrombosis rates and long-term outcomes (Table 2). In the most recent study, investigators randomly assigned 137 patients with AVFs to two groups: a group of monthly surveillance with Qa and clinical criteria and a control group with clinical criteria alone [15]. The clinical criteria consisted of an increase in dynamic venous pressures, decrease in blood ﬂow (Qb), excessive bleeding from venopuncture sites or unexplained reduction in URR. The patients in the Qa surveillance arm were referred to angiography if their Qa was <500 mL/min or if it fell by >20% once the access ﬂow was <1,000 mL/min. Each group was referred for any changes in clinical criteria. The primary end point was time to detection of a stenotic lesion that was ≥50%. Although thrombosis rates and AVF longevity were not reported in this study, the patients in the Qa surveillance arm were twice as likely to have stenosis detected compared to the control group. The more relevant outcome of thrombosis rates were evaluated in a separate trial [16]. This study [16] included 103 patients of whom 68 had AVFs and 35 had AVGs. They were randomized into two major groups: one with monthly surveillance by Qa or static venous pressure and a control group. All patients underwent color ﬂow Doppler ultrasound International Journal of Nephrology every 6 months. Patients with Qa < 750 mL/min or static venous pressure ratios ≥0.5 were referred for angiography. Using Doppler ultrasound, AVFs with >50% stenosis, Qa < 600 mL, or greater than 25% decline in Qa were also referred for angiography. If there was evidence of a stenotic lesion ≥50% of the vessel diameter, the patients underwent angioplasty. The primary end point was AVF thrombosis. Access longevity was not studied, but patients with AVFs followed with monthly surveillance by Qa and/or static venous pressure had a lower total thrombosis rate than the control patients (16.8 versus 27.1 per 100 patient years; P < 0.05). These studies revealed that surveillance using Qa detects early stenosis and leads to decreased thrombosis rates, but is there any long-term beneﬁt for early intervention in patients with subclinical stenotic AVFs? This was evaluated in a trial [17] that included 60 patients with a stenotic lesion of >50% by angiogram who were randomized into a treatment group that underwent percutaneous transluminal angioplasty (PTA) or a control group with no intervention. Patients were initially screened with measurements of blood ﬂow (Qb decrease by >30 mL/min on two consecutive hemodialysis sessions), access ﬂow (Qa < 850 mL/min) or urea-based access recirculation. If there was an abnormality, these patients were sent for ﬁstulography, and if >50% stenosis was found, they underwent randomization. All the AVFs with stenotic lesions were considered to be functional if they were providing adequate dialysis. The study end point was thrombosis or surgical revision due to inadequate dialysis. Impressively, the median functional failure-free AVF survival was 84 months (51.8 to 116.2 months) in the PTA group and 21 months (9.8 to 32.2 months) in controls (P < 0.001). A total of 6 patients in the PTA group had thrombosis compared to 14 patients in the control group (P = 0.029). However, the proportion of patients undergoing elective surgery was comparable. Therefore, early intervention in stenotic AVFs led to decreased time to and rates of thrombosis. Surveillance of AVFs and early intervention has been shown to be successful in detecting stenosis and preventing thrombosis, but does identifying a signiﬁcant stenosis lead to an increase in longevity of the access? A follow-up study from the same institution was performed shortly after in 2004 [18]. This study design was similar to the prior study. It included 79 patients with a signiﬁcantly stenotic lesion who were randomized into treatment (PTA or surgical revision) or control group. The stenotic lesion was identiﬁed as in the above study with the only diﬀerence now being a Qa < 750 mL/min. One unique variation in the study protocol was that the treatment and control group were further divided into a “functional” (Qa > 350 mL/min) or “failing” (Qa ≤ 350 mL/min) group. The primary end point was primary patency as deﬁned by the interval from stenosis to access failure. The primary patency rates were higher in the treatment groups compared to the control for both functional (P = 0.021) and failing subgroups (P = 0.005). Access survival rates were signiﬁcantly higher in the treatment group than the control group (P = 0.050). Interestingly, within the treatment group, survival rates