Relating protein intake to nutritional status in haemodialysis patients: how to normalize the protein equivalent of total nitrogen appearance (PNA)?

BACKGROUND The protein equivalent of total nitrogen appearance (PNA) is assumed to be a reliable estimate of dietary protein intake in haemodialysis patients. Protein requirements are related to body size. In order to standardize PNA to individual differences in body size, PNA is normalized to various terms related to the patient's body weight. It is not clear which is the most appropriate method to normalize PNA. METHODS We calculated five commonly used variants of normalized PNA and related them to indices of nutritional status in 57 stable chronic haemodialysis patients, 57 +/- 15 (mean +/- SD) years of age. PNA, determined by direct dialysate quantification, was normalized to actual post-dialysis dry body weight (DBW), normal body weight (DBWnormal), lean body mass (LBM), normal lean body mass (LBMnormal), and 'normalized' body weight (N). Nutritional status was assessed using an index of nutrition composed of anthropometry derived parameters and plasma albumin concentration. RESULTS PNA(DBW) (0.85 +/- 0.14 g/kg/d) tended to be higher than PNA(DBWnormal) (0.81 +/- 0.14 g/kg/d). PNA(LBM) (1.17 +/- 0.19 g/kg/d) did not differ from PNA(LBMnormal) (1.19 +/- 0.21 g/kg/d). PNA(N) (1.06 +/- 0.14 g/kg/d) was significantly higher than PNA(DBW) and PNA(DBWnormal), but lower than PNA(LBM) and PNA(LBMnormal). Actual PNA (61 +/- 13 g/d) correlated significantly with DBW (r=0.52) and LBM (r=0.63) indicating that large patients eat more protein. Interestingly, actual PNA correlated with plasma albumin (r=0.33) and with the overall index of nutrition (r=0.27) as well. PNA(DBW) correlated negatively with relative DBW (r=-0.32), expressed as a percentage of normal values, indicating that PNA(DBW) is relatively high in underweight patients. In contrast, PNA(DBWnormal) correlated positively with all nutritional parameters as well as with the overall index of nutrition (r=0.33). PNAN and PNA(LBM) did not correlate with the nutritional status, but PNA(LBMnormal) correlated positively with relative DBW (r=0.50) and with overall nutritional status (r=0.34). PNA(DBWnormal) and PNA(LBMnormal) in well-nourished patients showed overlap with the values in patients with evident malnutrition, despite the positive correlation of the normalized PNA values with nutritional status. CONCLUSIONS Normalizing PNA by DBWnormal and LBMnormal appeared to be the most appropriate method to standardize protein intake in haemodialysis patients. Since actual PNA is the purest estimate of protein intake that correlated with nutritional status, we recommend to evaluate actual PNA as well in studies that relate protein intake to patient outcome.