The normal serum phosphorus concentration is 3.4 to 4.5 mg/dl (1.12 to 1.45 mmol/L). This fluctuates with age (it is higher in children than adults), dietary intake, and acid–base status. There is a diurnal variation, which reaches its nadir between 8 and 11 a.m.

Your bones and teeth contain most of your body’s phosphorus . However, some phosphorous is in your blood. Your doctor can assess your blood phosphorus level using a serum phosphorus test.