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[Risk factors of melamine-contaminated milk powder related urolithiasis: a multicenter nested case-control study].

OBJECTIVE
To further confirm and clarify the risk factors of melamine associated urolithiasis.


METHODS
Case control research was performed in 6 centers from 5 provinces/cities in China. Children less than 36 months old were screened for urolithiasis and recruited in the study. The children with urolithiasis were included as cases and those without urolithiasis as controls. The children with congenital abnormality of urinary tract were excluded. According to the case:control ratios of 1:1, we sampled the controls from healthy children screened randomly. Due to the complete missing data on factors of vomiting/diarrhea/fever in control group of Center 4, we analyzed the data from 6 centers and 5 centers respectively. The possible influencing factors for urolithiasis including melamine concentration, birth type, age, feeding style and history of vomiting or diarrhea or fever were analyzed by Logistic analysis.


RESULTS
There were 1 329 cases and 1 317 controls with a mean age of 18.4 months. The analysis of data from 6 centers showed the children fed with high melamine formula were 6.26 times more likely to have stones (P<0.01) than those with non melamine formula. Preterm infants were 2.03 times (P<0.01) more likely to have urolithiasis than term infants. The children aged less than 0.5 year, 0.5 to 1 year, 1 to 2 year, 2 to 2.4 year were 2.78 (P<0.01), 2.61 (P<0.01), 2.09 (P<0.01), 1.57 (P<0.01), 1.44 (P<0.05) times more likely to have stones than those more than 2.5 year. Boys were 1.19 times more likely to have stones than girls. Children fed with formula alone were 1.94 times (P<0.01) more likely to have stones than those with formula and breast milk. The analysis of data from 5 centers showed that children fed with high melamine formula were 4.38 times (P<0.01) more likely to have stones compared with those with non melamine formula. Children aged less than 1 year and 1 to 1.9 year were 2.24 (P<0.01) and 1.31 (P<0.05) times more likely to have stones than those more than 2 year. The children fed with formula alone were 1.67 times (P<0.01) more likely to have stones compared to those with formula and breast milk. The children with any two symptoms of vomiting, diarrhea and fever were 15.21 times (P<0.05) more likely to have urolithiasis. The multiple logistic regression model confirmed that above risk factors were independent risk factors for urolithiasis.


CONCLUSION
We confirm that the children fed with high melamine infant formula, preterm infant, boy, children fed with formula alone, and the children with symptoms of vomiting or diarrhea or fever are more likely to have urolithiasis. We also found the risk for urolithiasis decreased with age.