Differences between Measured Total Nitrogen Losses in Spent Peritoneal Dialysate Effluent and Estimated Nitrogen Losses

Abstract

Objective: Patients treated by peritoneal dialysis (PD) are at increased risk of muscle wasting, and clinical guidelines recommend assessing dietary intake, by calculating protein equivalent of nitrogen appearance (PNA) to assure protein sufficiency. The PNA equations were developed many years ago, and we wished to re-evaluate them by comparing estimated and measured peritoneal nitrogen losses.

Design and methods: This is a cross-sectional observational cohort study. The study setting was an outpatient PD center of a university hospital and the study subjects included 67 patients undergoing PD, in which 61.2% were males, and the median age was 67.3 (53.2-79.4) years. The nitrogen content of 24-hour spent peritoneal dialysate by automated chemiluminescence analyzer was measured and compared with estimates of nitrogen losses based on dialysate urea loss using the Bergstrom, Randerson, and Blumenkrantz equations.

Results: Measured total dialysate nitrogen was more than urea nitrogen equivalent, 5.79 ± 4.07 versus 2.66 ± 1.67 g/day (P < .001). Each equation has an inflation factor to compensate for nonurea protein losses; however, measured nitrogen loss was 27.7 (15.5-59.6) g/day versus Bergstrom, 16.5 (9.8-27.1); Randerson, 16.4 (9.8-27.3); and Blumenkrantz, 12.9 (7.9-25.4) g/day, P < .001. The BlandAltman analysis demonstrated systematic bias with increasing underestimation by these equations with increasing measured nitrogen losses (r = 0.74, P < .001).

Conclusion: Our findings demonstrate that at higher protein losses, the currently used predictive equations underestimate the amount lost. It is important to attempt to compensate the iatrogenic protein loss by recommending the appropriate intake of dietary protein to patients, in an attempt to minimize muscle wasting. This discrepancy may have arisen because of the characteristics of newer PD prescriptions and change in patient demographics. We propose a new equation PNA g/day = 0.31 × (urea loss mmol) + 7.17, which will require prospective validation in additional studies.