Dialysis techniques and adequacy

Abstract

<b>Introduction and Aims:</b> Blood uremic toxin measurements are presumed to reflect extracellular fluid, on the assumption of a uniform distribution between its two compartments, plasma and interstitial fluid (ISF). ISF is however separated from plasma by microvascular pores; toxins may not necessarily circulate “freely”. Interstitial uremic composition may be particularly important due to proximity to cell metabolism, and a much larger, often expanded ISF volume in CKD. Little is known about ISF composition in CKD, owing to the difficulty of sampling the compartment. This study aimed to extract ISF and study its uremic composition in CKD patients, in steady states and during dialysis, in comparison to plasma. <b>Methods:</b> A flow-variation microdialysis technique was modified and adapted to simultaneously sample subcutaneous ISF of urea, creatinine, phosphate and urate in CKD patients during, and off haemodialysis (HD). ISF concentrations of these toxins were compared to their plasma levels. Metabolomic and proteomic analyses of were performed on contemporaneous ISF and plasma samples, to characterise their uremic profiles. A novel microneedle was also developed and tested as a new means of sampling ISF in CKD patients. <b>Results:</b> Microdialysis was performed on 24 subjects (4 controls, 13 CKD non HD, 7 HD). Although a good correlation was seen between plasma and ISF urea (r=0.98), creatinine (r=0.94), phosphate (r=0.74) and urate (r=0.82), paired analysis of the whole group showed significant differences between ISF and plasma for urea (p=0.01), creatinine (p=0.02) and urate (p=0.03), with slightly lower individual ISF levels. During HD, the ISF toxin decay curves lagged behind their plasma counterparts in most patients, indicating a disharmony between plasma and ISF toxin clearances. Metabolomics revealed over 6000 metabolites in ISF, with 33 peaks consistent with known small and protein-bound uremic toxins, and several unidentified peaks occurring more in ISF than plasma, and vice-versa. Larger hydrophobic metabolites like sphingolipids did not seem to circulate freely between the two compartments. Proteomics identified 354 proteins in ISF, with the uremic toxins beta-2 microglobulin, cystatin C and complement factor D seen in ISF, but not in plasma of some patients, and several others exhibiting a compartmental behaviour. The novel microneedles were successful in painlessly extracting ISF in 68.8% of oedematous CKD patients. <b>Conclusions:</b> ISF is toxin rich, ideal for metabolomic and proteomic research in uremia, and may reveal earlier accumulation of toxins and possibly a sequestration of some larger toxins within the compartment. Even small toxins may show a differential compartmental distribution, enhanced by dialysis. Microneedles show great promise in minimally invasively sampling ISF, and may bridge the gap to better understanding of ISF uremic composition, with a likely significant impact on treatment delivery in CKD, and the assessment of its efficacy.