Improved urea clearance raises the BUN in continuous peritoneal dialysis

Cross-sectional studies in steady state dialysed chronic end-stage renal failure patients show urea clearance (Kt/V) and total urea excretion (protein catabolic rate) correlate positively. However, urea clearance is total urea excretion divided by BUN. Thus urea clearance and BUN relate reciprocally, and so their mathematical product (total urea excretion) is independent of clearance. As such clearance cannot also be a positive correlate of total excretion as demanded by the cross-sectional studies. Furthermore the clearance formula dictates that the positive urea clearance and total urea excretion correlation found in the cross-sectional studies can only occur if the increased urea clearance fails to reciprocally lower the BUN. Thus the relations of urea clearance, urea excretion, and BUN requires further definition. To so define we examine dialysis urea excretion, dialysis urea clearance, BUN, and serum albumin in 13 stabilized chronic uremics with minimal native renal function who are treated by continuous ambulatory peritoneal dialysis (CAPD). Urea clearance and BUN correlate positively (r = 0.62, p < 0.05) and both also correlate positively with dialytic urea excretion and (urea clearance r = 0.912, p < 0.001, BUN r = 0.88, p < 0.001). In addition dialytic urea excretion and serum albumin indexed to body size correlate positively (p < 0.05). Thus in the steady state urea clearance associates with both an increase in BUN and urea output. However the law of conservation of mass makes urea output is a function of protein intake. Thus increased clearance cannot directly increase such output, and so increased clearance must first increase intake but in doing so it increases the retention of the byproducts of enhanced intake, BUN and other protein metabolites, so leading to a paradox, the more removed, the more remains. These observations taken together suggest that in chronic uremia treated by continuous dialysis, elevation of the BUN may be a marker for an adequate restoration of protein metabolism if inadequate dialysis is excluded.