nPCR = 0.22 + (.036 * intradialytic rise in BUN * 24)/(intradialytic interval).
For instance, if the pre-dialysis BUN is 70 and the post-dialysis BUN is 18, and the intradialytic interval is 44 hours (e.g., there is an interval of 44 hours from the end of one dialysis until the beginning of the next), then the nPCR is calculated to be 1.24 g/kg/day. As you can see, a large intradialytic rise in BUN is generally indicative of adequate nutrition. Most guidelines specify maintaining the protein intake above 1.0 - 1.2 g/kg/day in dialysis patients, with values less than 0.8 g/kg/day being equated with malnutrition.
If the patient has significant residual renal function, then one must add a term that takes into account endogenous urea clearance. Importantly, these equations are only valid for individuals in steady state, and are not as helpful in the setting of acute illness. Separate equations exist for peritoneal dialysis, since the BUN levels are relatively constant and therefore pre- and post- levels cannot be used to estimate nPCR; rather urea levels in serum and peritoneal filtrate can be compared. Nutritional status can be especially important to assess in patients on peritoneal dialysis, since these patients are at high risk of losing albumin in the peritoneal filtrate. There is a nice discussion of this topic in Up-To-Date (" Protein catabolic rate in maintenance dialysis "), which includes alternative formulas for nPCR that incorporate Kt/V.