Lab work collected will consist of serum phosphorus, calcium and intact-parathyroid hormone levels. As these are routinely collected prior to or during a regular CKD clinic visit, the study coordinator will check to see if this data is already available, and will not duplicate this blood work if it is. In addition, all subjects will have blood drawn (30 ml) for assessment of circulating endothelial cells (CECs), a novel marker for vascular damage.