What is your approach to timing of the plasma, arterial, and venous urea collections in a patient on hemodialysis for whom you are attempting to calculate arteriovenous fistula recirculation?

I use the two-needle slow blood flow technique to calculate arteriovenous fistula recirculation:

1. After initiation of hemodialysis, turn off ultrafiltration in approximately 30 minutes and then draw urea samples from the arterial and venous ports of the blood lines.
2. Reduce the access blood flow rate...

I typically check within the first 30 minutes of dialysis. At max, 60 min.