Peritoneal dialysis kinetics.

Abstract

Traditionally, for patients maintained with peritoneal dialysis (PD), the level of blood urea nitrogen (BUN), creatinine, and other clinical and laboratory data have been used to judge the adequacy of treatment. Because of the relatively high mortality rates in patients maintained with continuous PD, in the United States and the high failure rate of PD itself, concern has been raised about both the adequacy of PD and its measurement. Underdialysis could be a cause of morbidity and mortality that may be masked by a reduction in protein intake that also lowers the BUN, giving a false sense of security. The peritoneal equilibration test (PET) is widely applied to classify an individual patient's peritoneal membrane permeability but when used alone, the PET is not a measure of adequacy. Currently, for both hemodialysis and PD, the near universally accepted method of quantification is Kt/Vurea, an index of small molecule clearance also known as the fractional urea clearance per dialysis. The continuous or near continuous nature of PD requires a different interpretation of Kt/V, which is usually expressed on a weekly basis using a different set of standards. The currently accepted minimum is 1.7 per week for continuous dialysis (CAPD) with somewhat higher values for intermittent PD. Measurement of Kt/V for PD patients is more direct than for hemodialyzed patients but also more tedious and subject to some pitfalls. A representative clearance must be obtained from dialysate collected preferably over a 24-hour period, and an independent measure of the patient's urea volume or total body water is required. The latter is usually estimated from anthropometric data. The average rate of net protein catabolism, an index of protein nutrition, can be calculated from the urea generation rate obtained from the same 24-hour dialysate collection and normalized to the patient's urea volume, similar to PCRn measured in hemodialyzed patients. An allowance for loss of protein in the dialysate is accomplished by application of the Randerson equation, which is similar to the Borah equation used for hemodialysis. Prospective controlled trials are necessary to examine the relationship between nutrition and dialysis dosage. The hope of the future is that prospective studies of dialysis adequacy, including a full clinical assessment of each patient, will help to establish the optimal dose of PD that will eliminate both the short-term and the long-term complications of uremia and allow a proper focus on other common causes of morbidity and mortality that are associated with the loss of kidney function.