Modulation of calcium oxalate monohydrate crystallization kinetics by urine of preterm neonates

Preterm neonates frequently develop nephrocalcinosis (NC). However, the cause has not yet been elucidated. This study focuses on the effects of urine from preterm neonates on crystallization kinetics. Urine samples were collected and renal ultrasound examinations of preterm neonates (gestational age < 32 weeks) were performed during the first weeks of life, at term, and ages 6, 12, and 24 months. The effect of urine on crystallization was determined using a seeded crystal growth system, which measures the square root of solubility product ( radicalLc), percentage of growth inhibition (GI), and agglomeration inhibition ([tm]) of calcium oxalate crystals. Data for preterm neonates in the first weeks of life (n = 19) were compared with those for full-term neonates (n = 17) and healthy adults. Moreover, the correlation between [tm] and urinary (U)citrate level was studied. Mean radicalLc (0.27 +/- 0.1 versus 0.36 +/- 0.08 mmol/L) and mean [tm] (81 +/- 32 versus 143 +/- 97 minutes) were lower and mean Ucalcium-creatinine (2.20 +/- 1.74 versus 0.46 +/- 0.73 mol/mol) and Uoxalate-creatinine ratios (0.39 +/- 0.21 versus 0.16 +/- 0.09 mol/mol) were greater in preterm neonates in the first weeks of life compared with full-term neonates (p < 0.05). Furthermore, [tm] was less than the lower limit for healthy adults for all but one preterm neonate; [tm] increased and Ucalcium-creatinine and Uoxalate-creatinine ratios decreased with age (p < 0.005). There was a correlation between [tm] and citrate excretion (coefficient of 38; P < 0.001). Patients with and without NC at term did not differ statistically in mean radicalLc, percentage of GI, or [tm]. In conclusion, urine from preterm neonates in the first weeks of life is highly supersaturated and has a defective ability to inhibit calcium oxalate crystal agglomeration. This ability improves with age and is citrate mediated. We suggest that both the high level of supersaturation and defective ability to inhibit calcium oxalate crystal agglomeration contribute to the high incidence of NC.