Probing the Dry Weight (DW) by Bioimpedance (BIA): Which is the Gold Standard Between Clinical DW and BIA DW?

Background Probing the dry weight (DW) was largely dependent on clinical subjective estimates 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 tissues 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.

Very recently, a test aimed at assessing DW in hemodialysis (HD) patients has been developed, the "resistance stabilization test" (REST). It is based on the following four items:

1. one or more daily and/or alternate day HD sessions lasting 6 hours with ultrafiltration (UF) rate ≤ 0.5 kg/hour are planned;
2. BIA measurements are determined injecting 800 microAmpere at 50 kilohertz alternating sinusoidal current with a standard tetrapolar technique (BIA 101 Impedance Analyzer). Resistance (R) is recorded starting at the beginning of the HD session (R0) and then, continuously, until the end of the 6-hour session;
3. DW is defined as the weight achieved after flattening of the curve of the ratio R0/Rt (R0 is R at time 0 and Rt is R at a given time t during the HD session) of less than + 1% over 20 minutes in the presence of ongoing UF, indicating no further decline in extracellular volume;
4. if at the end of the 6-hour HD session R stabilization is not attained, a new 6-hour HD treatment with UF rate ≤ 0.5 kg/h is planned until a BIA DW (according to the item 3) is obtained.

Aim of the study A study group is being created (REST/Collaborative Study Initiative) with the aim of verifying if BIA-based DW (BIA DW) control is truly superior to current volume management in HD patients.

Protocol of the study DW determined with clinical methods (Clinical DW) is the gold standard by definition: Clinical DW is determined under strict clinical surveillance by the same attending physician. He will be helped by a clinical score of volume state about symptoms and signs of hypo- or hypervolemia. The physician is asked to adjust the DW of the candidates until their clinical score reaches zero after a given number of HD sessions before the BIA measurement. This Clinical DW will be compared with BIA DW, as obtained after performing REST;

Phases of the study

The protocol study includes three sequential phases:

1. as already mentioned, the Clinical DW is the gold standard by definition. Items of form B must be strictly applied. Form B must be filled in session after session, until score = 0, index of euvolemia, is achieved;
2. The Clinical DW as indicated by the score = 0 becomes the target DW of the next standard HD session, in which BIA measurements are performed: R values are recorded continuously during the session (Form C must be filled in).
3. REST is performed the following dialysis session (all the details have been given in the background section). As per protocol, these dialysis sessions may be one or more than one, until flattening of the curve of the ratio R0/Rt (R0 is R at time 0 and Rt is R at a given time t during the HD session) of less than + 1% over 20 minutes in the presence of ongoing UF, is obtained.