Stremke ER, et al. - Researchers undertook this secondary analysis to investigate whether 24-hour urine phosphorus is reliable and accurate as a biomarker of phosphorus intake and absorption in moderate CKD. The study included eight patients with moderate CKD on a tightly controlled dietary intake. In these subjects, highly variable 24-hour urine phosphorus was detected, which was not associated with dietary phosphorus intake or absorption, rather it was found to have an inverse association with phosphorus retention.

Methods

• In 2-week balance studies, participants were patients with stage 3–4 CKD on tightly controlled phosphorus and calcium intakes.
• Researchers analyzed thirteen 24-hour urine collections per patient in order to assess variability and reliability of 24-hour urine phosphorus and phosphorus-to-creatinine ratio.
• They used a published equation to determine the accuracy of 24-hour urine phosphorus to predict phosphorus intake.
• They determined the associations of 24-hour urine phosphorus with phosphorus intake, net absorption, and retention.

Results

• Within and among subjects, they detected wide day-to-day variation in 24-hour urine phosphorus (coefficient of variation of 30% and 37%, respectively).
• Findings showed two 24-hour urine measures were required to achieve ≥75% reliability.
• Up to 98% underestimation in some patients and overestimation up to 79% in others was found to be resulted from the estimation of dietary phosphorus intake from a single 24-hour urine.
• A negative correlation of 24-hour urine phosphorus with whole-body retention but no relation to net absorption was observed .