Introduction: The 2019 Kidney Disease Outcome Quality Initiative (K/DOQI) guideline recommended evaluating arteriovenous fistula (AVF) malfunction risks primarily based on clinical monitoring, which can be assisted with the value of vascular access flow (Qa). Nevertheless, Qa thresholds recommended by different guidelines vary, ranging from 300 to 500 ml/min. This study investigated the optimal Qa threshold to predict future functional patency in AVFs with Qa <500 ml/min. Methods: Both the clinical indicators of access dysfunction and the Qa value were monitored in patients receiving hemodialysis by the radiocephalic AVF. Routine access flow surveillance was performed by the ultrasound dilution method (HD03, Transonic Inc.). The development of clinically significant indicators of access dysfunction, which necessitated percutaneous transluminal angiography (PTA) to maintain functional patency, was analyzed in this cohort. Results: Among the enrolled 302 patients, Qa of 52 patients was under 500 ml/min. These 52 patients received 2 Qa measurements during the follow-up period. Of these 52 patients, serial Qa of 17 patients varied trivially and their AVF remained functional. Multivariable logistic regression analysis revealed that a low Qa per ideal body weight (IBW) is an independent predictor of AVF functional loss. Receiver operating characteristic curve analysis of Qa/IBW in predicting future AVF functional loss revealed that the best cutoff value of Qa is 7.1 times the IBW. Conclusion: For radiocephalic AVFs with Qa <500 ml/min, the minimally required Qa to maintain access function is associated with individual IBW. The IBW-based Qa threshold assessment would allow more flexibility in the treatment of patients and reduce unnecessary invasive measures.