The estimation of protein equivalents of total nitrogen in Chinese CAPD patients: an explanatory study

Objective The protein equivalent of total nitrogen appearance (PNA) formula, based on the urea nitrogen appearance (UNA), is popularly used by stable continuous ambulatory peritoneal dialysis (CAPD) patients to estimate dietary daily protein intake (DPI). However, we found that the estimated DPI was higher than that directly evaluated from the dietary records of most of our CAPD patients. Therefore, in the present study, we tried to determine possible bias in PNA estimation by UNA with a nitrogen balance study of our CAPD patients. Methods Thirty-one CAPD patients with stable clinical conditions were included. Their 3-day dietary records were reviewed by a dedicated dietitian to calculate their energy, protein, and nitrogen intake (NI). The nitrogen removal (NR) from urine and dialysate was measured by the Kjeldahl technique. Then, we calculated the proportion of urea nitrogen appearance (UNA) in total nitrogen appearance (TNA) and analyzed the possible factors that could affect this proportion. Results Among these patients, 17 males and 14 females, the mean age was 64.19 +/- 12.42, and the dialysate drainage volume was 6700 (2540) ml/day. The percentage of UNA in TNA was 63.22 +/- 6.66%. Compared with the other classic nitrogen balance studies in the CAPD population, the protein nitrogen and other nonurea nitrogen losses in this study were all lower. Based on these 31 nitrogen balance studies, we proposed a pair of new equations to estimate PNA by UNA. (1) PNA = 9.3 + 7.73 UNA; (2) PNA = PNPNA + TPL = 6.7 + 7.28 UNA + TPL. Conclusion Our study suggested that the PNA formula generated from previous European studies overestimated DPI in our CAPD patients.