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.