Salivary Phosphorus and Level of Kidney Function

Previous studies have demonstrated that salivary phosphate levels are elevated in patients with chronic kidney disease and that there is a direct and linear correlation between serum phosphorus and salivary phosphorus. However, data regarding the relationship between salivary phosphate levels and stage of kidney disease (estimated glomerular filtration rate (eGFR) is lacking; as such, this study seeks to examine that relationship. There is a normal circadian rhythm to serum phosphorus with a mid-afternoon peak at approximately 3 PM and a nadir at approximately 7-11 AM. Furthermore it has been demonstrated that a fasting serum phosphorus level at 8 AM is NOT indicative of 24 hour mean serum phosphorus. Thus the relationship between salivary phosphorous and serum phosphorus may vary over the course of the day. This is a single-center study in which saliva, urine, and serum will be collected at two time points in a minimum of 105 subjects with various degrees of renal function (not on dialysis). Assignment to the renal group will be based upon the Visit 2 estimated glomerular filtration rate value. Enrollment in each group will continue at random until the target of 15 subjects per group is achieved. It is possible that an individual group may exceed the target enrollment number as their assignment will occur retrospectively. All subjects will be required to have their morning assessment conducted between the hours of 7AM and 11AM. Clinical chemistry, serum phosphorus, intact parathyroid hormone(iPTH), plasma fibroblast growth factor 23 (FGF-23),serum 1,25 di-hydroxyvitamin D, serum 25-OH vitamin D, and a spot urine will be collected. A 24-hour urine sample will also be collected. Passive and stimulated salivary collection will occur. The salivary collection must occur in a fasting state with nothing to eat or drink for at least 90 minutes prior to the collection. Subjects will be permitted to leave the facility. They will be instructed to consume a lunch of their choice between the hours of 1100AM and 1200PM. They will be instructed to eat nothing after 1200PM. Subjects will return to the clinic for Visit 3 as directed by the study staff. Visit 3 will be conducted between 1PM and 4PM. Laboratory assessments, salivary collections, and spot urines will be collected at Visit 3. A dietary recall of intake at Visit 2 and 3 will also be completed.