How to calculate the normal filtered and excreted load of urinary calcium?
The serum calcium concentration is normally about 10 mg/dL. The kidney filters complexed and free calcium, which make up 60% of the total, or 6 mg/dL. The normal glomerular filtration rate (GFR) is 120 mL/min. Thus, the filtered load of calcium is 6 mg/100 mL × 120 mL/min × 1440 min/day = 10,368 mg/day. Because the kidney reabsorbs 98% of the filtered calcium, only 2% is excreted. Thus the kidney normally excretes about 200 mg of calcium/day (10,368 mg/day × 0.02 = 207 mg/day). If the excreted calcium level increases to 5%, the urinary calcium level increases to 500 mg/day.