The "gold standard" method to evaluate the mass balances achieved during dialysis
for a given solute remains total dialysate collection (TDC). However, since handling
over 100 liter volumes is unfeasible in our current dialysis units, alternative methods
have been proposed, including urea kinetic modeling, partial dialysate collection
(PDC) and more recently, monitoring of dialysate urea by on-line devices. Concerned
by the complexity and costs generated by these devices, we aimed to adapt the simple
"gold standard" TDC method to clinical practice by diminishing the total volumes to
be handled. We describe a new system based on partial dialysate collection, the continuous
spent sampling of dialysate (CSSD), and present its technical validation. Further,
and for the first time, we report a long-term assessment of dialysis dosage in a dialysis
clinic using both the classical PDC and the new CSSD system in a group of six stable
dialysis patients who were followed for a period of three years. For the CSSD technique,
spent dialysate was continuously sampled by a reversed automatic infusion pump at
a rate of 10 ml/hr. The piston was automatically driven by the dialysis machine: switched
on when dialysis started, off when dialysis terminated and held during the by pass
periods. At the same time the number of production cycles of dialysate was monitored
and the total volume of dialysate was calculated by multiplying the volume of the
production chamber by the number of cycles. Urea and creatinine concentrations were
measured in the syringe and the masses were obtained by multiplying this concentration
by the total volume. CSSD and TDC were simultaneously performed in 20 dialysis sessions.
The total mass of urea removed was calculated as 58038 and 60442 mmol/session (CSSD
and TDC respectively; 3.1 +/- 1.2% variation; r = 0.99; y = 0.92x -28.9; P < 0.001).
The total mass of creatinine removed was 146,941,143 and 150,071,195 mumol/session
(2.2 +/- 0.9% variation; r = 0.99; y = 0.99x + 263; P < 0.001). To determine the long-term
clinical use of PDC and CSSD, all the dialysis sessions monitored during three consecutive
summers with PDC (during 1993 and 1994) and with CSSD (1995) in six stable dialysis
patients were included. The clinical study comparing PDC and CSSD showed similar urea
removal: 510 +/- 59 during the first year with PDC and 516 +/- 46 mmol/dialysis session
during the third year, using CSSD. Protein catabolic rate (PCR) could be calculated
from total urea removal and was 1.05 +/- 0.11 and 1.05 +/- 0.09 g/kg/day with PDC
and CSSD for the same periods. PCR values were clearly more stable when calculated
from the daily dialysate collections than when obtained with urea kinetic modeling
performed once monthly. We found that CSSD is a simple and accurate method to monitor
mass balances of urea or any other solute of clinical interest. With CSSD, dialysis
efficacy can be monitored at every dialysis session without the need for bleeding
a patient. As it is external to the dialysis machine, it can be attached to any type
of machine with a very low cost. The sample of dialysate is easy to handle, since
it is already taken in a syringe that is sent directly to the laboratory. The CSSD
system is currently in routine use in our unit and has demonstrated its feasibility,
low cost and high clinical interest in monitoring dialysis patients.