**1. Introduction**

The role of residual renal function (RRF) in the health and quality of life of both pre-dialysis and dialysis patients is equally important and now well established (Termorshuizen, Korevaar et al, 2003).

RRF plays an important role in maintaining fluid balance, phosphorus control, and removal of uremic toxins in dialysis patients. The importance of RRF in hemodialysis (HD) patients is less well appreciated and it is believed that RRF declined rapidly in HD patients (Morduchowicz, Winkler et al, 1994; Wang, Woo, et al, 2005). Decline of RRF also contributed significantly to anemia, inflammation, and malnutrition in end-stage renal disease (ESRD) patients (Wang, Sea et al, 2001; Pecoits-Filho, Heimburger et al, 2003; Pecoits-Filho, Heimburger et al, 2002; Wang, Wang et al, 2004). More importantly, RRF has also been shown to be a powerful predictor of mortality, especially in patients on peritoneal dialysis (PD) (Bargman, Thorpe et al, 2001; Brener, Thijssen et al, 2011; Maiorca, Brunori et al, 19951).

Glomerular filtration rate (GFR) measured by isotope clearance is considered to be the standard measure of renal function. Other tests, such as serum creatinine, creatinine clearance, urea clearance, an average of the creatinine and urea clearances, and urine volume have been used to assess RRF in chronic kidney disease (Levey, 1990). Despite its limitations, urine volume, the simplest measure of RRF, has been correlated to GFR in studies and most authors defined loss of RRF as urine volume < 200 ml/24 hours (Moist, Port et al, 2000). Urine collections (24 hours for PD, interdialytic for HD) to measure urea and/or creatinine clearance usually done at beginning of chronic dialysis and every 1-3 months in patients with RRF.

In this chapter, we will review available data that have shown a positive impact of RRF on the survival and quality of life of dialysis patients, and outline the current strategies to preserve RRF in PD and HD patients.

Residual Renal Function in Hemodialysis Patients 257

contribution of RRF to the overall survival of HD patients: each 1-unit increase in renal Kt/V resulted in 66% decrease in relative risk of death. Moreover, in patients with preserved RRF, increasing dialysis dose did not result in improved patient outcomes. The international prospective observational DOPPS study has also recently reported the diuretic use and presence of RRF was associated with a better survival in prevalent HD patients (Bragg-Gresham, Fissell et al, 2007). Diuretic use declined after the start of dialysis (9.2% in Europe versus 21.3% in the United States). Patients with RRF on diuretics had almost twice the chances of retaining RRF after 1 year with 7% lower all-cause and mortality and 14% lower cardiac-specific mortality compared to patients not receiving diuretics. All these and other observational studies suggest that preservation of RRF has an important role in the

RRF has been found to be important in maintaining fluid balance of dialysis patients, especially in patients on PD. Suboptimal fluid removal in PD patients is associated with greater rates of all-cause hospitalization and mortality (Ates, Nergizoglu et al, 2001). In the CANUSA Study, urine volume was a strong independent predictor of survival. Every 250 ml/imin urine output was associated with a 36% reduction in overall mortality ((Bargman, Thorpe et al, 2001). RRF may reduce or avoid the need for fluid restriction. Loss of RRF is independently associated with suboptimal blood pressure control, likely a result of chronic volume expansion (Ates, Nergizoglu et al, 2001; Konings, Kooman et al, 2003). The severity of left ventricular hypertrophy (LVH), a strong independent predictor of mortality in dialysis patients, inversely correlates with the presence of RRF Pecoits-Filho, Heimburger et al, 2002; Wang, Wang et al, 2004). In addition, loss of RRF is associated with more severe anemia, hypoalbuminemia, and higher arterial pressure (Pecoits-Filho, Heimburger et al, 2003), all of which are important risk factors for cardiac hypertrophy in dialysis patients. Extracellular fluid (ECF) volume has been also reported to be associated with hypertension

Middle molecule clearance is one of the most widely recognized benefits of RRF. Patients with significant RRF are shown to have lower ß2-mocroglobulin (ß2M) levels (McCarthy, Williams et al, 1994; Montenegro, Martinez et al, 1992; Amici G, Virga et al, 1993) and thus are less prone to dialysis-associated amyloidosis (Copley JB, Lindberg et al, 2001). Preserved RRF is also associated with lower blood levels of uric acid, potassium (Morduchowicz, Winkler et al, 1994), and aluminium (Altmann , Butter et al, 1987), and higher levels of hemoglobin (Pecoits-Filho, Heimburger et al, 2002), presumably due to increased levels of

Hyperphosphatemia is prevalent in dialysis patients (Yavuz, Ersoy et al, 2008; Wang, Woo et al, 2004) and has been linked to vascular calcification and increased cardiovascular mortality in both HD and PD patients (Block, Hulbert-Shearon et al, 1998; Wang AY, Lai et al, 2006). RRF plays a major role in improving phosphate balance in both PD and HD

Inflammation is highly prevalent in dialysis patients (Arici M, Walls et al, 2001) and established to be a strong predictor of mortality in dialysis patients. Loss of RRF was

and left ventricular hypertrophy in HD patients (Fagugli, Pasini et al, 2003).

survival of both HD and PD patients.

**2.3 RRF and metabolic control** 

endogenous erythropoietin.

**2.4 RRF and inflammation** 

patients ((Morduchowicz, Winkler et al, 1994).

**2.2 RRF, volume control and cardiac hypertrophy** 
