Efficacy of dialysis in peritoneal dialysis: utility of bioimpedance to calculate Kt/V and the search for a target Kt. 
To calculate Kt/V, volume (V) is usually obtained by Watson formula, but bioimpedance spectroscopy (BIS) is a simple and applicable technique to determinate V, along with other hydration and nutrition parameters, in peritoneal dialysis (PD) patients.  Dialysis efficacy can also be measured with Kt, but no experience exists in PD, so there is no reference/target value for Kt that must be achieved in these patients to be considered adequately dialyzed.  We evaluated the efficacy of PD with Kt/V using Watson formula and BIS for V calculation, assessed hydration status in a PD unit by data obtained by BIS, and attempted to find a reference Kt from the Kt/V previously obtained by BIS.  In this observational prospective study of 78 PD patients, we measured V using BIS (V bis) and Watson formula (V w) and calculated weekly Kt/V using both volumes (Kt/V bis/V bis and Kt/V w).  With the BIS technique, we obtained and subsequently analyzed other hydration status parameters.  We achieved a reference Kt, extrapolating the value desired (weekly Kt/V 1.7) to the target Kt using the simple linear regression statistical technique, basing it on the results of the previously calculated Pearson's linear correlation coefficient.  Volume was 1.8 l higher by Watson formula than with BIS (p < 0.001).  Weekly Kt/V bis was 2.33 ± 0.68, and mean weekly Kt/V w was 2.20 ± 0.63 (p < 0.0001); 60.25 % of patients presented overhydration according to the BIS study (OH >1.1 l).  The target value of Kt for the reference weekly Kt/V bis (1.7) was 64.87 l. BIS is a simple, applicable technique for calculating V in dialysis that can be especially useful in PD patients compared with the anthropometric formulas, by the abnormally distributed body water in these patients.  Other parameters obtained by BIS will serve to assess both the distribution of body volume and nutritional status in the clinical setting.  The target Kt value obtained from Kt/V bis allowed us to measure the efficacy of PD in a practical way, omitting V measurement.