The dialysate flow rate of twice the blood flow rate theory was developed back in the old days of dialysis (early 70’s) when most patients used Scribner Shunts and had a maximum blood flow rate of about 250 ml/min. There wasn’t definitive science behind it, though it was the golden rule of thumb for many years. When we started using kinetic modeling (Kt/V), we learned that you get great improvements in clearance by increasing blood flow, and small improvements by increasing dialysate flow. You do get higher clearances at higher dialysate flows, but you get less bang for your buck.
For example, a shift from a blood flow rate of 250 up to 400 ml/min would give a 30% increase. An shift in DFR from 500 to 800 ml/min might only increase clearance by 10%. Though many centers use an DFR of 800 ml/min, they could get the same additional clearance by running the patient 15 minutes longer.
In SDD, the blood flow rates usually remain high, in the 500-800 ml/min range. In SND, the number of minutes has increase so much that you can reduce the clearance, which is done by reducing both blood and dialysate flow rates. The low clearances per minute make the dialysis much more gentle, while the total clearances are higher.
Curt