In order to distinguish between FGF23-dependent and FGF23-independent hypophosphatemia, the renal tubular reabsorption of phosphate has to be calculated by Tmp/GFR (i.e., the maximum rate of tubular phosphate reabsorption divided by the glomerular filtration rate, as assessed by serum and urinary phosphate and creatinine determined in fasting urine and serum samples) [15]. This evidence concerns the gene FGF23 and hypophosphatemia.