[Oxalic acid--important uremic toxin]

Oxalic acid is thought to be a significant uremic toxin that participates in the pathogenesis of uremic syndrome. AIM OF THE STUDY was to summarise results which we obtained during the study ofoxalic acid in biological fluids (plasma, saliva, urine and dialysate) in patients suffering from chronic kidney diseases (CKD), stage 3-5 and after renal transplantation.

In the retrospective study were investigated 28 healthy subjects, 112 CKD stage 1-4 patients, 39 haemodialysis patients and 27 CAPD patients. Besides 21 patients were investigated after renal transplantation. We used the following therapeutic methods: maximal water diuresis, diet with low (2g/day) and high (15g/day) sodium chloride intake, administration intravenous furosemide (20mg) and renal replacement therapy [CAPD, haemodialysis (HD), haemofiltration (HF) and postdilution haemodiafiltration (HDF)] and renal transplantation. Oxalic acid was determined by spectrophotometric method using oxalate oxidase which is free from vitamin C interference. Vitamin C was determined by spectrophotometric method.

In CKD patients and those after renal transplantation direct relationships between plasma oxalic acid and serum creatinine were found (r = 0.904 and 0.943, respectively, P < 0.001). Despite of high plasma oxalic acid in uremic patients (23.1 +/- 10 micromol/l), there was no significant difference in salivary oxalic acid between control subjects (126.5 +/- 18 micromol/l) and CKD stage 3-4 patients (133.9 +/- 23.7 micromol/I). The urinary excretion of oxalic acid during maximal water diuresis in healthy subjects (n = 15) (from 37.5 +/- 17.4 to 110.2 +/- 49.3 micromol/4 hours) and after intravenous furosemide (CKD stage 3-4, n = 15) (from 34.5 +/- 5.5 to 66.7 +/- 8.1 micromol/3 hours) increased significantly, but was not affected by high intake of NaCI in diet (CKD stage 3-4, n = 12). One tablet of Sorbifer Durules containing 100 mg Fe2+ and 60 mg vitamin C did not lead to further increase of uremic hyperoxalemia in haemodialysis patients. Four-hour HD, H F and HDF led to the significant decrease of plasma oxalic acid, but the most significant decrease was observed during HDF (63.3%).

The results of this study indicate, that renal replacement therapy is not effective for permanent reduction of elevated plasma levels of oxalic acid--important uremic toxin.