Robinson-Cohen et al., Estimation of 24-hour urine phosphate excretion from spot urine collection: development of a predictive equation. Journal of Renal Nutrition, 2014; 24 (3): p. 194-9.
One thing that has always been tricky for doctors and scientists is how to analyze clinical tests. Most clinical tests come from either blood or urine samples, which are tested for the presence and levels of various compounds. First, scientists must demonstrate that these compounds are correlated (either in relatively high or low doses) to a pathology. One this has been determined, the normal/healthy level of the compound needs to be determined. However, just as your heart rate changes throughout the day as you wake up and perform various activities, so too do the levels of different compounds in your system. Thus you can say, most people tend to have a range of values over the course of a day.
Patients with severe chronic kidney disease often suffer from hypophosphatemia. In order to monitor the amount of phosphate intake of a patient, the gold-standard is considered to be a 24-hour urine collection test. However, as Robinson-Cohen et al. point out, doing this is very time consuming and also prone to error. They wanted to check whether spot urine phosphate measurements could be as effective as this “gold standard”.
Phosphatemia is treated through phosphate binders (which aid in phosphate retention). As this would naturally affect the phosphate levels in urine, Robinson-Cohen et al. piggy-backed their experiment onto the Phosphate Normalization Trial. Patients, all with hyperphosphatemia, were administered either a phosphate binder or a placebo. While the main purpose of the trial was to determine the efficacy of the phosphate binders, Robinson-Cohen et al. were mainly concerned with the accuracy of measurement methods used. 143 patients were measured both with spot checks as well as the traditional 24-hour method. Additionally, they used an external replication of the study to validate their results.
Robinson-Cohen et al. found that when certain parameters were applied to the spot checks, they could be indicative of the results shown in the 24-hour test. Notably, corrections had to be made for age, sex, and weight and the creatine concentration. The phosphate:creatine ratio in the spot checks related fairly accurately to the phosphate excretion in the 24-hour samples, once these other corrections were made. Thus, this newer simpler method seems to be an emerging option, although; judging by the tone of the paper, I’m not sure anyone will be calling it the “gold standard” just yet.
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