Human biomarkers for measuring phosphorous intake and status
In the circulation, only a small proportion (~10%) is present as inorganic phosphorus bound to proteins. The remainder is present as inorganic phosphates and it is this fraction that is measured using typical analytical methods.
Serum phosphorus is useful in a clinical setting but is not an ideal biomarker for population studies, because the concentration is efficiently controlled within a relatively narrow range in healthy adults and it is influenced by many factors including age, sex and time since last meal (9). Further, studies show that there is no clear relationship between phosphorus and diet (9).
Serum phosphate may be measured in lithium heparin plasma or serum. EDTA citrate, and oxalate plasma should not be used. Haemolysis should be avoided because of the relatively high phosphorus content of red blood cells. Delay in separation of plasma/serum may increase phosphate levels and therefore samples should be spun within 1 hour of collection.
Serum phosphate is most often measured in the investigation and monitoring of hypophosphataemia, e.g. in critically ill patients, those at risk of re‐feeding syndrome or after treatment for diabetic keto‐acidosis. It is also used in the investigation and monitoring of hyperphosphataemia, e.g. in chronic kidney disease (CKD) or in conditions resulting in cell death, and as part of the investigation of disorders of calcium homeostasis.
Urine samples should be fresh or acidified to reduce the formation of insoluble calcium phosphate complexes. Excretion of phosphorus is via the kidneys, and parathyroid hormone and FGF-12 adjust renal clearance (3). A 24 h collection may be taken to reduce the effect of diurnal variation and diet on phosphate excretion (although is subject to inaccuracy arising from problems with collection) (10).
Urine phosphate is most often measured together with serum phosphate, and urine and serum creatinine, to enable calculation of the renal tubular reabsorption of phosphate (TmP/GFR), which can be used to investigate the cause of hypophosphataemia.
Other phosphorous biomarkers
Recent research cites fibroblast growth factor-23 and Klotho may show promise as biomarkers of phosphorous status and metabolism (8).
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