Protein catabolic rate in maintenance dialysis

INTRODUCTION

The protein catabolic rate (PCR), also called the protein equivalent of nitrogen appearance (PNA), is the parameter used in most hemodialysis units to assess dietary protein intake in patients who are in a steady state. Suppose, for example, that a patient has a desirably low predialysis BUN. This finding could represent either a well nourished patient who is adequately dialyzed, or decreased protein intake which is usually a reflection of inadequate dialysis. The PCR will distinguish between these possibilities.

The PCR is determined by measuring the interdialytic appearance of urea in body fluids plus any urea lost in the urine in patients with residual renal function. Retrospective data from a large group of dialysis patients indicates that abnormalities in a number of standard laboratory measurements related to patient nutrition (plasma concentrations of albumin, creatinine, urea nitrogen, and transferrin) are associated with increased mortality [1]; however, only PCR is of value in prospectively predicting morbidity in hemodialysis patients [2]. (See "Assessment of nutritional status in end-stage renal disease".) In the National Cooperative Dialysis Study (NCDS), for example, a PCR greater than 1 g/kg per day and a timed average urea concentration of 50 mg/dL (18 mmol/L) were associated with low morbidity [2]. The latter alone is insufficient, since it can be induced by a low protein intake.

Although PCR is often viewed as a variable that can be manipulated independently, it varies directly with the Kt/V, a measure of dialysis adequacy. (See "Kt/V and the adequacy of hemodialysis".) As examples:

• In one study of a small group of hemodialysis patients, the intensity of dialysis was increased by enhancing dialysis time, blood flow, and/or membrane surface area [3]. As the Kt/V rose from 0.82 to 1.32 over a three month period, there was a concurrent elevation in PCR from 0.81 to 1.02 g/kg per day. The rise in PCR was indicative of increased protein intake (and better nutrition) due, presumably, to improved appetite. A second group in which the dialysis regimen was unchanged had no increase in either Kt/V or PCR.
• In another report, converting 13 patients from conventional dialysis (3 x 4 hours/week) to three times per week nocturnal dialysis (3 x 8 hours/week) led to a significant increase in nPCR (1.39 g/kg per day at baseline to 2.25 g/kg per day at 12 months) [4].

The PCR is a function of protein catabolism and reflects protein intake only if the patient is in a steady state regarding nutrition. Unless there is obvious evidence of poor nutrition (eg, PCR below 0.8 g/kg per day) or underdialysis (eg, Kt/V <1,2), alterations in dialysis prescription should be undertaken only after clear trends in these parameters are apparent. This may require several months of monitoring PCR and Kt/V to ascertain that a significant change has occurred.