Objective: We compared the Bergstrom's and Randerson's formula for PNA determination, and compared the normalization of PNA by ideal body weight (IBW) and standard body weight (SBW) as estimated by the Watson's formula. METHODS. We studied 208 Chinese PD patients. Two 24-h dialysate and urine collections were performed six months apart. Protein nitrogen appearance was determined by the Randerson's formula (PNA-Rand) and Bergstrom's formula (PNA-Berg), the latter used as the gold standard. PNA-Berg was normalized with IBW and SBW, denoted as NPNA-IBW and NPNA-Watson respectively. The change of PNA over six months, denoted as APNA-Rand and APNA-Berg, were calculated. The results were compared by the Bland and Altman's method.
Results: At zero month, the average PNA-Berg was 61.8 +/- 14.8 g/day, and the average PNA-Rand was 58.1 +/- 14.5 g/day. The value of PNA-Rand was consistently lower than the corresponding PNA-Berg. The bias of PNA-Rand was -3.7g/day. The limits of agreement were -9.2 to +1.8 g/day. When NPNA-Watson was compared to NPNA-IBW, the bias of NPNA-Watson, using NPNA-IBW as gold standard, was 0.01 g/kg/day; the limits of agreement were -0.22 to +0.23 g/kg/day. The difference between NPNA-Watson and NPNA-IBW correlated with the body mass index (r = -0.820, p < 0.001) and body weight (r = -0.834, r < 0.001). After six month, there was a significant reduction in urine protein loss. However, total protein loss was only slightly reduced (7.3 +/- 3.0 to 6.9 +/- 2.8 g/day, p = 0.029). The correlation between APNA-Berg and APNA-Rand remained excellent (r = 0.983, p < 0.001). The bias of APNA-Rand was +0.3 g/day; the limits of agreement were -4.7 to +5.2 g/day.
Conclusion: Ideal body weight that is validated for specific ethnic group, rather than the Watson's formula, should be used for normalization of PNA. Although the Randerson's formula under-estimates PNA when compared to the Bergstrom's formula, it is a reliable method for serial PNA monitoring because dialysate protein loss is stable in most patients.