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The most widely used clinical method for measuring recirculation in the access device is based on urea dilution. The three simultaneous blood samples required during hemodialysis interrupt the treatment, and results of chemical analysis are often delayed for several days. Alternatively, detecting recirculation by dilution of arterial blood caused by a bolus of normal saline injected into the venous blood line has several advantages. In this study, an ultrasound sensor clamped onto the arterial line entering the dialyzer was used to detect such dilution from a reduction in sound velocity observed in the saline diluted blood. Within the target range, the change in ultrasound velocity (ultrasound dilution) is linearly correlated with the dilution of whole blood by normal saline. The same sensor was also used to measure flow in the blood line using an established ultrasound transit-time method. During 34 hemodialyses in 28 patients, only 3 patients had detectable recirculation measured by ultrasound dilution. To further evaluate the sensitivity of the new method the dialysis lines were reversed during hemodialysis in the 25 patients with no recirculation. After this, all had detectable recirculation ranging from 10 to 60%. The mean error of duplicate measurements was 3.9 +/- 2.8%. Recirculation by ultrasound dilution correlated closely with recirculation measured by urea dilution (r = 0.9156, p < 001). The data suggest that the ultrasound dilution method is both sensitive and accurate. Ease of use and immediate availability of results added to the clinical usefulness of this method for evaluating the integrity of the hemodialysis access.