The monitoring of relative blood volume changes (ΔRBV) has been advocated for the prevention of hemodialysis (HD) hypotension. Stand-alone devices (Crit-Line) or devices incorporated into the HD apparatus (blood volume monitor [BVM], Hemoscan) are widely used for this purpose. Comparisons between devices are scarce. The aim of this study was, first, to compare ΔRBV results from these 3 devices with ΔRBV calculated from changes in laboratory-derived hemoglobin (ΔRBV-lab-Hb) and, second, to compare ΔRBV results between the different devices. Fourteen patients received 2 HD treatments in a randomized order: one with the Hemoscan and Crit-Line combination and one with the BVM and Crit-Line combination. ΔRBV-lab-Hb was measured at 2 and 4 hr into the HD session. Bland-Altman analyses showed that ΔRBV results from the 3 devices differed systematically from ΔRBV-lab-Hb, i.e., the difference between the 3 devices and ΔRBV-lab-Hb varied significantly (p<0.05) with the magnitude of the measurement. The interdevice comparison showed considerable differences in ΔRBV results. At the end of the treatment, a significant difference (p<0.05) between ΔRBV measured by the Hemoscan and Crit-Line device (−9.8±2.7% and −11.5±4%, respectively) was found. In most patients, a systematic difference between Crit-Line and Hemoscan and between Crit-Line and BVM was observed. Relative blood volume change measurements by Crit-Line, Hemoscan, and BVM yield results that differ systematically from the results obtained from laboratory-derived Hb changes. Furthermore, there are substantial differences in ΔRBV results between the 3 ΔRBV devices.