[Probing the dry weight in hemodialysis: is bioimpedance the solution?]

Probing the dry weight (DW) was largely dependent on clinical subjective estimate until recently. New bedside non-invasive tools have been developed with the aim of providing more objective information on volume status and guiding physicians in the quest for DW. Among them, bioimpedance (BIA) appears to be very promising in the achievement of this goal. Resistance (R) and capacitance of tissue are the two basic properties in BIA. However, although impedance is an electrical property of tissues that can be directly used in body composition analysis, it is commonly embedded in predictive equations that are derived by correlation with criterion measures of body compartments. In recent studies continuous regional calf multifrequency BIA spectroscopy and wrist-to-ankle whole-body single-frequency BIA were used during dialysis sessions to assess DW. The latter was defined as the weight achieved after flattening of the R0/Rt slope (R0 is R at time 0 and Rt is R at at a given time t during the HD session) for a period of at least 20 minutes despite ongoing ultrafiltration, indicating no further decline in extracellular volume. Two are the main advantages of these methods: they use directly R, with no need for predictive equations; they do not depend on comparison with healthy subjects but mainly use patients as their own control to define DW as stabilization of the R0/Rt slope. In conclusion, clinical methods are fundamental in probing the DW. They must be supported by strict BIA protocols. BIA appears to be a (the) brilliant solution in solving the old problem of DW in hemodialysis patients.