There is a significant amount of data available regarding the utilization of serum P as a biomarker of intervention aimed at phosphorus load reductions; however, minimal information regarding the timing of phosphorus measurements (i.e. when serum P is assessed during a 24-hour period) is available. Wolf et al, have demonstrated that despite an 80% reduction in urinary P with dietary P restriction and treatment with P binders over a 2-week period, there was no change in serum P or in FGF-23. However, both biomarkers were sampled once every 3 days over a 2-week period without regard to the time of day. This suggests that single time point measurements of serum P in patients with CKD do not accurately reflect phosphate load. As such, it is plausible that mean 24-hour serum P, or mean 24-hour serum FGF-23 are better markers of phosphorus load in CKD than single measurements.