1. as creatinie is excreted in tubules, creatinine clearance in RRF overestimates GFR. Whether or not creatinine clearance or averaged urea and creatinine clearance should be used for adequacy study is debatable, but so far no study used averaged urea and cretinine clearance. Though studies still use renal creatinine clearance, simple addition to peritoneal creatinine clearance does not make sense at all.
2. in CAPD, there is very close correlation between peritoneal Kt/V and creatinine clearance, so one parameter is good enough. Addition of second parameter does not yield better results.
3. For APD, discrepancy between Kt/V and creatinine clearance may exist. So both ISPD and EBPG recommend an additional parameter of minimal Creatinine Clearance 40L/week/1.73m2