Rule of 4s for Diagnosing Ultrafiltration Failure in PD

In peritoneal dialysis, ultrafiltration failure is defined as the inability of the peritoneal dialysate to maintain euvolemia--and generally speaking, this implies some degree of hypervolemia. Technically, it can occasionally result from trivial explanations--for example, an incorrect PD prescription not properly calibrated to that patient's peritoneal equilibration test, or poor patient compliance with an appropriate PD regimen. But usually when the term ultrafiltration failure is used, it often implies that the peritoneal membrane has evolved into such a high transporter status that net ultrafiltration becomes impossible.

One useful definition for ultrafiltration failure status, based on a simple office test, is the following "Rule of 4's": give the patient a bag of 4.25% dextrose-based PD fluid for 4 hours. If there is not net ultrafiltration of greater than 400 mL over this time period, then the patient has ultrafiltration failure. Ordinarily, PD with 4.25% solution is a reliable way of removing excess fluid, to the point where in the pre-CVVH and inpatient HD days, installing a PD catheter and initating exchanges with 4.25% solution was actually a means of treating acute inpatient pulmonary edema in the ICU.

Ultrafiltration failure often correlates with the loss of residual renal function and the onset of anuria. Strategies to avoid UF failure include using icodextrin periodically for longer dwells, minimizing salt intake in the diet, and using a cycler overnight. If these maneuvers do not succeed, ultrafiltration failure may ultimately signify an inability to proceed with PD and the necessity of transitioning to hemodialysis.