The specific effect of session time versus frequency of therapy on outcomes is more difficult to determine since time and frequency are interrelated.  Nonetheless, various clinical experiences have shed light on the respective roles of these variables.  The Tassin experience is often quoted as an example of how long dialysis session time may favorably affect outcomes¹.  The annual fractional mortality rate is calculated to be a mere 4% over a period of four years for patients receiving an average dialysis time of 8 hours/session.

Comparative mortality rates among various series and registers.  The normalized mortality rates bear little relationship to treatment time.  Based on references 2 and 3.

In reviewing the survival rate of various large series and registry data, one of the closest survivals are those reported with only 2.3 hours thrice weekly by Gotch^2,3.  Albeit this close ranking to the Tassin experience, there is a significant difference in survival.  The actual survival is closer to the series with lower session length.  Thus, session time per se should not be considered the most significant determinant of outcome, but the marked difference between the Tassin and Gotch mortality rates still suggest that session time may be important.

References:

1. Charra B, Calemard E, Ruffet M. Survival as an index of adequacy of dialysis. Kidney Int 41:1286-1291, 1992
2. Gotch FA, Sargent JA, Keen ML.  Wither goest Kt/V? Kidney Int 58 (Suppl 76):S3-S18, 2000
3. Gotch FA, Uehlinger DE.  Mortality rate in U.S. dialysis patients.  Dial & Transplant 20:255-257, 1991