How is dialysis adequacy measured in Peritoneal Dialysis
What is dialysis adequacy, how is it measured in PD, and what are the current minimum targets in PD?
Accumulation of uremic wastes not removed by the failing kidneys contributes to poor outcome in patients with ESKD, although this may be a simplistic view, given a number of other neurohormonal changes that accompany loss of kidney function. Ideally the removal of a threshold or adequate amount of waste with dialysis would result in improved outcome, with any incremental amount removed above this amount having little impact on improving clinical outcome. Previous and more recent studies have primarily focused on the relationship between clearance of only small solutes, in particular urea and creatinine, and outcome. It is important to recognize the limitations of this conventional and “convenient” assumption, however. Many important uremic toxins with much larger molecular weight and water solubility behave quite differently than urea and creatinine do and are therefore not removed effectively by standard dialysis, either HD or PD. Despite this, the use of urea and creatinine removal by dialysis has remained the primary metric of dialysis adequacy, with expectation that increased removal of these using augmented dialysis treatment intensity will result in improved clinical outcomes. To date, results of randomized controlled interventional trials comparing distinct tiers of urea removal have failed to demonstrate this. Conversely these studies have shown that poor outcomes can occur if minimum amounts of urea are not removed. Based on these results, both the National Kidney Foundation Kidney Disease Outcomes Quality Initiative and the International Society of Peritoneal Dialysis guidelines for PD recommend that adequacy be assessed by total (peritoneal and kidney) clearance of urea (termed “Kt”), normalized to its volume of distribution (V urea ). Assessment of this requires collection and measurement of urea present in 24-hour collections of both drained PD effluent and urine. Although the current weekly standardized Kt/V urea target recommended by both the Kidney Disease Outcome Quality Initiative (KDOQI) and the International Society for Peritoneal Dialysis (ISPD) is 1.7, both guidelines recognize the importance of clinical assessment of the individual patient in determining need for more dialysis. In recent years, there has been doubt cast on the validity of Kt/V urea as a valuable metric of clinical outcome, much less a payment-required “reportable” one. Other factors possibly important in patient outcome include volume status, sodium, phosphate, and middle molecule (e.g., beta-2-microglobulin) removal, as well as inflammation. The means to address these are currently being considered.