Utility of a Urine Screening Tool for Vitamin D Deficiency in Infants and Toddlers

Background:

Insufficient circulating levels of vitamin D is a common problem in both developed and developing countries; it is estimated that over one billion people have vitamin D deficiency at this time. In children, vitamin D deficiency leads to nutritional rickets, which is considered the most common non-communicable disease of children worldwide. Rickets causes significant short term morbidity and mortality through hypocalcemic tetany and seizures, as well as long term morbidity from growth delay and skeletal deformities. Recent data also suggest that children who have had nutritional rickets are three times more likely to develop type 1 diabetes.

There is wide agreement that screening for vitamin D deficiency would be highly beneficial. However, there is debate on how best to do this. Most screening efforts have included serum levels of 25-hydroxy-vitamin D and parathyroid hormone (PTH). However, these assays are complex and are neither available nor affordable for many of the most vulnerable populations.

Calcium excretion in the kidney is a tightly regulated process involving metabolites of vitamin D and PTH. Renal calcium excretion, in the absence of primary renal or parathyroid gland disease, is a sensitive indicator of total body calcium balance. We have shown that renal calcium excretion correlates with blood levels of 25-hydroxy-vitamin D levels in older children and urine calcium-to-creatinine ratios have been shown to distinguish children with active rickets from healthy controls in one Nigerian study. This biomarker is noninvasive and the assays are straight forward and inexpensive. We believe that the urine calcium-to-creatinine ratio has excellent potential as a screening tool for vitamin D deficiency.

Objective and Hypotheses. The objective of this study is to evaluate the urine calcium-to-creatinine ratio as a biomarker of vitamin D deficiency. Our hypotheses are 1) that urine calcium-to-creatinine ratio correlates with serum 25-hydroxy vitamin D level in infants and toddlers at risk for vitamin D deficiency and 2) that the urine calcium-to-creatinine ratio can be used to screen children for vitamin D deficiency. These hypotheses will be tested through two specific aims. Specific Aim 1 is to determine the correlation between the urine calcium-to-creatinine ratio with 25-hydroxy vitamin D levels and with biomarkers of rickets in infants and toddlers at increased risk for vitamin D deficiency. Specific Aim 2 is to determine the sensitivity, specificity, and positive and negative predictive values of one random urine calcium-to-creatinine ratio in relation to biochemical evidence of vitamin D deficiency.

Design. This will be an observational study of infants and toddlers at increased risk for vitamin D deficiency. We will recruit 150 healthy children, 6 to 36 months of age, with risk factors for vitamin D deficiency (premature birth, breast feeding, dairy product avoidance, etc.). Based on previous studies in the US, we anticipate about 12% of these subjects will have vitamin D deficiency. Subjects will have a physical exam targeted to signs of rickets. Laboratories will include a random urine sample for calcium-to-creatinine ratio and blood for calcium, creatinine, alkaline phosphatase, phosphorus, magnesium, intact PTH, and 25-hydroxy vitamin D. Subjects with biochemical evidence of rickets will have x-rays done of the left wrist and knees that will be scored according to previously published rickets scores.

Potential Impact. Vitamin D deficiency is increasingly recognized as a common source of morbidity in both developed and undeveloped countries. The validation of a noninvasive and inexpensive screening tool for vitamin D deficiency would allow for large scale screening of at risk populations and detect this problem early, preventing the morbidities of rickets.