Measurement of urinary oxalate: an enzymatic and an ion chromatographic method compared

The oxalate level in ultrafiltrate fluid collected from a dialyzer is useful for estimating the plasma oxalate level in hemodialysis patients

BACKGROUND

Patients on chronic hemodialysis are likely to develop secondary hyperoxalemia. It is, however, difficult to measure plasma oxalate levels. To measure plasma oxalate levels, rapid plasma separation, deproteinization, and acidification are essential in preventing the formation of oxalate and the deposition of calcium oxalate within the test tube. The present study was undertaken to examine whether the oxalate level in dialyzer ultrafiltrate is potentially useful for estimating plasma oxalate levels.

METHODS

In nine patients on chronic hemodialysis, the plasma, after deproteinization with a filter, and the ultrafiltrate from the dialyzer before hemodialysis were acidified to a pH level of less than 3, followed by the measurement of oxalate levels by ion chromatography. Also, oxalate levels were compared between acidified and non-acidified ultrafiltrates from the dialyzer. In the second part of the study, seven patients on chronic hemodialysis receiving erythropoietin therapy, in whom the ferritin level was more than 300 ng/ml and transferrin saturation was less than 25%, were intravenously administered ascorbic acid, 100 mg, three times a week, after each dialysis session to facilitate the utilization of stored iron. This treatment was continued until the serum ferritin level decreased to a level below 300 ng/ml (for 3 months, at a maximum). The oxalate level in the dialyzer ultrafiltrate after this treatment was compared with that before treatment.

RESULTS

The mean +/- SE oxalate level in the dialyzer ultrafiltrate was 45 +/- 6 micromol/l, essentially equal to the plasma oxalate level (46 +/- 7 micromol/l). The plasma oxalate level had a significant positive correlation with the dialyzer ultrafiltrate oxalate level (plasma oxalate level = 0.99 x dialyzer ultrafiltrate oxalate level + 1.5; r = 0.95; P < 0.0001). The oxalate level in the acidified ultrafiltrate (45 +/- 6 micromol/l) did not differ significantly from that in the non-acidified ultrafiltrate (45 +/- 6 micromol/l). The mean +/- SE duration of ascorbic acid administration was 64 +/- 13 days. The hemoglobin level remained unchanged at 9.6 +/- 0.4 g/dl, whereas the serum iron level increased significantly, from 34 +/- 2 microg/dl to 43 +/- 4 microg/dl (P < 0.05), and serum ferritin levels decreased significantly, from 645 +/- 219 ng/ml to 231 +/- 30 ng/ml after the treatment (P < 0.05). The oxalate level in the acidified ultrafiltrate showed no significant change after ascorbic acid administration (31 +/- 8 micromol/l vs 47 +/- 7 micromol/l).

CONCLUSIONS

In patients on chronic hemodialysis, the oxalate level in acidified ultrafiltrate from the dialyzer was found to be useful for estimating the plasma level of non-protein-bound oxalate. When administering ascorbic acid to hemodialysis patients, the plasma oxalate level can be monitored using this method.

Urinary saturation and nephrocalcinosis in preterm infants: effect of parenteral nutrition

Urinary lithogenic and inhibitory factors were studied in 27 preterm infants; 16 had total parenteral nutrition (TPN) and 11 had breastmilk with an additional glucose-sodium chloride infusion. Urines were collected for 24 hours on day 2 (period A), day 3 (B), and once between days 4 and 10 (C). Urinary calcium oxalate saturation was calculated by the computer program EQUIL 2. Renal ultrasonography was performed every second week until discharge. The calcium/creatinine ratio increased in infants on TPN (A 0.91; C 1.68 mol/mol) and was significantly higher at period C than that in infants on breastmilk/infusion (A 0.52; C 0.36). The oxalate/creatinine ratio was persistently higher with TPN (203 mmol/mol) than with breastmilk/infusion (98; 137). The citrate/creatinine remained constant with TPN (0.44 mol/mol), whereas it increased significantly with breastmilk/infusion (0.26; 0.49). Calcium/citrate rose considerably with TPN, but decreased with breastmilk/infusion to a significantly lower level than with TPN. The urinary calcium oxalate saturation increased with TPN (2.4; 4.5) and decreased with breastmilk/infusion (2.1; 1.5) to a significantly lower value than with TPN. Nephrocalcinosis developed in two infants on TPN. Mean daily calcium intake was similar in both groups, whereas protein, sodium, and phosphorus intake were significantly higher on TPN. It is concluded that the increase in urinary calcium oxalate saturation observed with TPN is due to the combined effect of an increased urinary calcium excretion and higher urinary oxalate/creatinine and calcium/citrate ratios. The changes observed are likely to be caused by TPN itself, which differs in several respects from breastmilk feeding.

Management of primary hyperoxaluria: efficacy of oral citrate administration

The prognosis of primary hyperoxaluria (PH) is not only related to endogenous oxalate production and the response (if any) to pyridoxine (in type I), but is greatly influenced by extrarenal factors like dehydration. The earlier the diagnosis of PH, the better the chances of improving the prognosis in individual patients. Measures to enhance the solubility of calcium oxalate are important. Besides ensuring at all times a generous fluid intake (> 2 l/m2), administration of alkali citrate (0.15 g/kg), which has not been advocated so far in PH, appears very promising. We studied the effect of sodium citrate in six patients with PH. Mean urinary citrate excretion (mmol/day per 1.73 m2) without oral citrate was very low (0.57) and rose to 2.49 with citrate administration. This was accompanied by a significant decrease in the calcium oxalate saturation (calculated by equil 2) from 11.7 to 6.9 (P < 0.05). Treatment in five patients over 10-36 months resulted in improved (1) or stabilized (4) renal function and reduced passage of stones. Additional measures include restriction of salt and of oxalate-rich food. We conclude that long-term administration of alkali citrate is beneficial in patients with PH.

Dietary fibre: the effectiveness of a high bran intake in reducing renal calcium excretion

Fifteen healthy women were given a standardized calcium-rich diet (1800 mg calcium/day) with or without 36 g bran for 5 days. A similar study was also carried out with rice, soy and wheat bran. Urine samples were also collected 24 h. With all brans renal calcium excretion decreased and renal oxalic acid excretion increased. However, influence of rice bran was statistically significant. After 5 days of consuming 36 g rice bran/day 14 of 15 subjects showed decreased calcium excretion, but increased oxalic acid excretion. Relative supersaturation with calcium oxalate, as a measure for the risk of calcium stone formation, increased after addition of all brans.

Investigations of the efficacy of ascorbic acid therapy in cystinuria

We investigated ascorbic acid therapy for cystinuria in a study of seven healthy control persons and seven cystinuric patients. The study lasted 9 days. During the first period, we collected 24-h urine specimens from all subjects on 3 consecutive days. Starting on day 4, all were given 5 g ascorbic acid/day for a period of 6 days. On the last 3 days, 24-h urine specimens were again collected. Quantitative amino acid determination was performed using an HPLC method described elsewhere. During ingestion of ascorbic acid, the mean excretion of cysteine by the control group increased from 134.1 to 159 mumol/day, whereas the excretion of cystine decreased from 107.1 to 82 mumol/day. The corresponding values for the cystinuric patients increased from 352.4 to 452.1 mumol/day for cysteine and decreased from 4,131.6 to 3,663.2 mumol/day for cystine. Thus, ascorbic acid seems to have only mild reducing properties in respect to cystine.