Middle molecular uremic substances retention itself might influence the bioincompatibility of PD solution.

To the Editor: It was demonstrated that a higher peritoneal membrane solute transport rate is associated with a higher mortality risk and a trend to higher technique failure. It has been suggested that the bioincompatible nature of conventional peritoneal dialysis (PD) solutions contributes to the structural peritoneal membrane changes that lead to deterioration in solute transport characteristics and loss of ultrafiltration. However, Fan et al. reported that the clinical outcomes (residual kidney function, peritoneal membrane function, technique survival, and peritonitis rates) were the same, irrespective of standard or biocompatible PD solutions. In the HEMO dialysis study, it became apparent that serum b2 microglobulin (b2M) levels, not KT/V, were the strong predictors of mortality. It was reported that the clearance of b2M by hemodialysis (when high-flux membranes were used) was higher than that by PD. We stained peritoneal tissue from continuous ambulatory peritoneal dialysis (CAPD) patients (n1⁄4 19) for b2M (rabbit anti-human b2M; DAKO, Glostrup, Denmark). In almost all the patients, deposits of b2M were found in the compact zone (Figure 1). We previously investigated the associations of b2M clearance with PD and residual renal function, and serum b2M levels with duration of dialysis in 20 subjects on continuous ambulatory peritoneal dialysis and found that serum b2M levels increased with duration of dialysis. As for b2M clearance by residual renal function per se, it was significantly lower in subjects with long duration of dialysis even though no difference was observed in the total daily b2M clearance. 6 We also reported that b2M is useful as a screening test for the onset of encapsulating peritoneal sclerosis and that the accumulation of both b2M and middle molecular uremic substances may be related to the pathophysiology of encapsulating peritoneal sclerosis. b2 Microglobulin may have a pathogenic role, which is unproven; however still, if middle molecular uremic substance retention itself influences the bioincompatibility of PD solution, it might be worthwhile to attempt combined continuous ambulatory peritoneal dialysis and hemodialysis treatment in patients with high b2M levels for whom sufficient clearance cannot be maintained with continuous ambulatory peritoneal dialysis alone, but this also requires further study. We should not pay attention only to PD solution but also to middle molecular uremic substance retention.