Plasma oxalate level in pediatric calcium stone formers with or without secondary hyperoxaluria

Association between glomerular filtration rate (measured by high-performance liquid chromatography with iohexol) and plasma oxalate

INTRODUCTION

Secondary hyperoxalemia is a multifactorial disease that affects several organs and tissues in patients with native or transplanted kidneys. Plasma oxalate may increase during renal failure because it is cleared from the body by the kidneys. However, there is scarce evidence about the association between glomerular filtration rate and plasma oxalate, especially in the early stages of chronic kidney disease (CKD).

METHODS

A case series focuses on the description of variations in clinical presentation. A pilot study was conducted using a cross-sectional analysis with 72 subjects. The glomerular filtration rate (GFR) and plasma oxalate levels were measured for all patients. Results: Median (IQR) GFR was 70.50 [39.0; 91.0] mL/min/1.73 m2. Plasma oxalate was < 5.0 µmol/L in all patients with a GFR > 30 mL/min/1.73m2. Among the 14 patients with severe CKD (GFR < 30 mL/min/1.73 m2) only 4 patients showed a slightly increased plasma oxalate level (between 6 and 12 µmol/L).

CONCLUSION

In non-primary hyperoxaluria, plasma oxalate concentration increases when GFR < 30mL/min/1.73 m2 and, in our opinion, values greater than 5 µmol/L with a GFR > 30 mL/min/1.73 m2 are suggestive of primary hyperoxaluria. Further studies are necessary to confirm plasma oxalate increase in patients with low GFR levels (< 30mL/min/1.73 m2).

Erythrocyte oxidative stress in patients with calcium oxalate stones correlates with stone size and renal tubular damage

OBJECTIVE

To investigate how erythrocyte oxidative stress relates to renal tubular damage and calcium oxalate stone size in patients as oxidative stress has been demonstrated to be associated with stone formation in disease progression.

METHODS

The study included 29 controls, 29 patients with kidney stones, and 28 patients with ureteral stones. Venous blood samples were collected to measure the expression and activity of antioxidant enzymes in the isolated erythrocytes. A 24-hour urine sample was collected to measure urinary chemistry. The cellular levels of oxalate and the oxidative stress marker malondialdehyde (MDA) were determined to examine their correlations with stone size and renal tubule damage.

RESULTS

Calcium oxalate stone deposition and high free radical levels in venous blood associated with high levels of urinary oxalate, glutathione S-transferases tubular damage markers, and MDA and low urinary citrate levels. Compared with the erythrocytes of controls, the erythrocytes of stone groups had significantly lower levels and activities of antioxidant proteins, namely, reduced glutathione, catalase, and copper- or zinc-superoxide dismutase. The ureteral stone group also had significantly lower erythrocyte glutathione peroxidase levels and glutathione reductase activity than the controls. Erythrocyte oxalate levels correlated positively with erythrocyte MDA levels and negatively with erythrocyte antioxidant protein activities. Erythrocyte oxidative stress, as indicated by cellular MDA levels, also correlated well with urinary glutathione S-transferases and stone size.

CONCLUSION

These results suggest that oxalate-mediated oxidative stress in erythrocytes might contribute to the tubular damage and stone accumulation in patients with hyperoxaluria.