Measurement of hemodialysis (HD) access flow (QA) is a noninvasive approach for arteriovenous graft or fistula surveillance. Flow dilution (FD) and in-line dialysance (DD) are two common methods for measuring QA. In a randomized fashion, we prospectively evaluated QA using FD and DD in 48 HD patients during three separate HD sessions over a span of 3 months. The measurement of QA was similar (1,016 +/- 412 ml/min for FD and 1,009 +/- 425 ml/min for DD, p = 0.44 and 0.79 for the mean and standard deviation, respectively). While FD successfully measured QA >or=2,000 ml/min, DD "saturated" (indicating a QA >or=2,000 ml/min without providing a numerical QA value) (n = 17). The correlation coefficient for QA <or=2,000 ml/min between DD and FD measurements from the same dialysis sessions was high (r = 0.9, p < 0.001) with a straight line slope of 0.997. Access flow values for FD and DD methods were comparable at QA <or=2,000 ml/min and no single method tended to over- or underestimate QA compared with the other. For QA >2,000 ml/min, FD provided a quantitative QA measure, and is therefore a potentially useful tool for QA above this threshold, while DD is not.