Two-in-one protocol: simultaneous small-pore and ultrasmall-pore peritoneal transport quantification

Abstract

Background: Reduced free water transport (FWT) through ultrasmall pores contributes to net ultrafiltration failure (UFF) and should be seen as a sign of more severe functional deterioration of the peritoneal membrane. The modified peritoneal equilibration test (PET), measuring the dip in dialysate Na concentration, estimates only FWT. Our aim was to simultaneously quantify small-solute transport, FWT, and small-pore ultrafiltration (SPUF) during a single PET procedure.

Methods: We performed a 4-hour, 3.86% glucose PET, with additional measurement of ultrafiltration (UF) at 60 minutes, in 70 peritoneal dialysis patients (mean age: 50 ± 16 years; 61% women; PD vintage: 26 ± 23 months). We calculated the dialysate-to-plasma ratios (D/P) of creatinine and Na at 0 and 60 minutes, and the Na dip (Dip(D/PNa60')), the delta dialysate Na 0-60 (ΔDNa(0-60)), FWT, and SPUF.

Results: Sodium sieving (as measured by ΔDNa(0-60)) correlated strongly with the corrected Dip(D/PNa60') (r = 0.85, p < 0.0001) and the corrected FWT (r = 0.41, p = 0.005). Total UF showed better correlation with FWT than with indirect measurements of Na sieving (r = 0.46, p < 0.0001 for FWT; r = 0.360, p < 0.0001 for Dip(D/PNa60')). Corrected FWT fraction was 0.45 ± 0.16. A negative correlation was found between time on PD and both total UF and FWT (r = -0.253, p = 0.035 and r = -0.272, p = 0.023 respectively). The 11 patients (15.7%) diagnosed with UFF had lower FWT (89 mL vs 164 mL, p < 0.05) and higher D/P creatinine (0.75 vs 0.70, p < 0.05) than did the group with normal UF. The SPUF correlated positively with FWT in the normal UF group, but negatively in UFF patients (r = -0.709, p = 0.015). Among UFF patients on PD for a longer period, 44.4% had a FWT percentage below 45%.

Conclusions: Measurement of FWT and SPUF is feasible by simultaneous quantification during a modified 3.86% glucose PET, and FWT is a decisive parameter for detecting causes of UFF in addition to increased effective capillary surface.

Figure 1

— Correlations between (A) the dialysate-to-plasma (D/P) sodium at 60 minutes corrected for diffusion (D/P_{Na60′corrected}) and free water transport with an algorithm correction (FWT_corrected) according to Venturoli and Rippe (10) (Pearson r = -0.80, p < 0.0001); (B) the difference (“delta”) of the initial D/P sodium (D/P_Na0′) and the 60-minute D/P sodium corrected for diffusion (Dip_D/P_{Na60′corrected}) and FWT_corrected according to Venturoli and Rippe (10) (Pearson r = 0.68, p < 0.0001); and (C) the corrected dip and delta sodium (Pearson r = 0.85, p < 0.0001).

Figure 2

— Simultaneous small-pore and ultrasmall-pore peritoneal transport evaluation and its profile with time on peritoneal dialysis (PD). Correlations between (A) total ultrafiltration (UF_240′) and time on PD (Pearson r = -0.272, p = 0.023); (B) free water transport with an algorithm correction (FWT_corrected) according to Venturoli and Rippe (10) and time on PD (Pearson r = -0.253, p = 0.035); and (C) small-pore ultrafiltration (SPUF) with time on PD (Pearson r = -0.243 p = 0.003).

Figure 3

— Small-pore and ultrasmall-pore peritoneal water transport pathways. (A) Free water transport (FWT) and small-pore water transport both contribute to net ultrafiltration in patients without ultrafiltration failure (r = 0.257, p = 0.032). (B) Disproportionate decline in FWT in patients with ultrafiltration failure (r = -0.709, p = 0.015).