Effects of different sampling methods for measurement of post dialysis blood urea nitrogen on urea kinetic modeling derived parameters in patients undergoing long-term hemodialysis.

A simple and reliable sampling method for measuring post-dialysis blood urea nitrogen (C2) has not been well established. Therefore, the effects of different methods of sampling C2 on calculating urea kinetic modeling (UKM) derived parameters were studied in patients on hemodialysis. At first, C2 values were sampled at the end of hemodialysis at a blood flow rate of 300 ml per min (blood flow rate (Qb) 300 ml per min). They were then divided into two groups. In group 1 (n = 11), C2 samples were taken after slowing down Qb to 100 ml per min for 3 min (C2-100); the blood pump was then stopped for 1 min, and C2 was again sampled (C2-100-stop). Finally, C2 was sampled with Qb restored to 100 ml per min and the venous line clamped for another 12 sec (C2-100-clamp). In group 2 (n = 11), C2 samples were collected after Qb was slowed down to 50 ml per min for 3 min (C2-50), and C2-50-stop was then measured. Finally, C2-50-clamp was measured. Using C2-clamp as a reference standard, UKM parameters were calculated using a variable volume, single pool UKM. The authors found that Kt/V calculated by C2-300, C2-100, and C2-50 overestimated the reference Kt/V by 8.4-9.1%, 5.1%, and 3%, respectively. In contrast, protein catabolic rate and time-averaged BUN were affected only minimally. Therefore, a low flow technique with a Qb of 50 ml per min for 3 min can be used for the routine estimation of C2 and UKM-derived parameters with reasonable accuracy.