Oxalate Analysis and its Applications.

Until recently knowledge of oxalate metabolism has been hampered by problems in the measurement of oxalate in urine, plasma and other body fluids. The studies described in this thesis were directed towards the development of simple sensitive assays for oxalate, which could then be used to investigate oxalate metabolism in normal subjects and idiopathic calcium oxalate stone formers. An evaluation was performed of several possible methods. The definitive method developed was a solvent generated ion exchange chromatographic system with electrochemical detection. Acidified urine was pretreated by dilution with neutral phosphate buffer and passage through a C18 cartridge. Stabilised plasma was diluted with neutral acetate buffer and oxalate extracted using a strong anion exchange cartridge. Plasma samples were stabilised with the reducing agent dithiothreitol. There was no significant protein binding of oxalate at physiological pH. Reference ranges were established for urine and plasma oxalate, and also for renal clearance and fractional excretion. Fasting lowered plasma and urine oxalate ranges and renal handling was modified by diet. Studies on normal subjects revealed no diurnal variation in urine oxalate but plasma oxalate decreased overnight. Oxalate loading in normal subjects demonstrated renal adaptation to the load, with net renal reabsorption of oxalate becoming net secretion. These studies suggest that the main site of oxalate absorption is the small intestine. It was found that increasing the load beyond a critical level did not increase the absorption, indicating a limited capacity. Oxalate loading did not effect ketogenesis in normal subjects and loading with ascorbic acid (1g), an oxalate precursor, did not alter urine or plasma oxalate levels. Idiopathic calcium oxalate stone formers showed a 22. 8% incidence of hyperoxaluria, although the frequency of hyperoxaluria within individuals varied. Hyperoxaluria was associated with hyperuricosuria, hypercalciuria and high urinary creatinine output in the fed state, but not on fasting. Hyperoxalurics had a net tubular secretion of oxalate in the fasting state, suggesting a renally mediated loss, delayed gut absorption or increased endogenous production of oxalate