Phosphorus (Inorganic Phosphate)

What is phosphorus?

Phosphorus is the most abundant mineral in the body after calcium, and most of it, together with calcium, forms part of the structure of bones and teeth. The rest is found in the structure of energy-carrying molecules in cells, in cell membranes and in DNA. Dairy products, meat, fish, eggs and legumes are the main dietary sources of phosphorus; some additives in processed foods also contain phosphorus.

The blood phosphorus level is maintained by a balance between the kidneys, intestines and bones on the one hand and parathyroid hormone and vitamin D on the other. Phosphate also helps maintain the body's acid–base balance. The test measures the amount of inorganic phosphate in blood serum, and the result is reported in mg/dL. Because phosphorus and calcium closely affect each other, their results are usually interpreted together.

When is a phosphorus test ordered?

The phosphorus test is usually ordered together with calcium, parathyroid hormone, vitamin D and kidney function tests. In people with chronic kidney disease, phosphorus is monitored at regular intervals, because the level rises when the kidneys cannot excrete excess phosphorus. In dialysis patients, keeping phosphorus under control is an important part of treatment for protecting bone and blood vessel health.

When a person who has been undernourished for a long time starts eating again, phosphorus can drop quickly, so the doctor may monitor this value closely. Phosphorus levels may also be checked when investigating the causes of kidney stone formation. It is also among the monitoring tests for people taking medications that affect bone and mineral metabolism.

• An abnormal calcium level
• Chronic kidney disease and dialysis follow-up
• Muscle weakness, bone pain or frequent fractures
• Suspected parathyroid gland disease
• Malnutrition or long-term heavy alcohol use

How do you prepare for a phosphorus test?

The blood phosphorus level changes during the day and can drop temporarily as phosphorus moves into the cells, especially after carbohydrate-rich meals. To make results comparable, it is preferable to draw blood in the morning after 8–12 hours of fasting. You can drink water while fasting. If you are being monitored regularly because of kidney disease, repeating the checks under similar conditions allows changes to be tracked more accurately.

Phosphate-containing enemas and laxatives, aluminum-containing stomach medications, diuretics and vitamin D supplements can affect the result. If you use any of these medications, inform your doctor before the test and leave the decision about continuing the medication to them. Damage to the blood cells in the tube can also falsely raise phosphorus. If you would rather not go out for this sample, which is best given in the morning on an empty stomach, our nurse can come to your home or workplace.

What does a phosphorus result mean?

High phosphorus is most often seen in chronic kidney disease, in which the kidneys cannot adequately excrete excess phosphorus. Conditions such as low parathyroid hormone secretion, excess vitamin D, the use of phosphate-containing medications or severe muscle breakdown can also raise the value. Phosphorus that stays high for a long time can combine with calcium and lead to deposits in blood vessels and tissues; in the long term, these deposits can harm heart and blood vessel health.

Low phosphorus, by contrast, can be seen with malnutrition, alcohol use, vitamin D deficiency, excess parathyroid hormone secretion, certain medications and during the treatment of very high blood sugar. A markedly low level can cause muscle weakness, fatigue and bone pain. Because of bone growth, values in children are higher than in adults; reference ranges therefore differ by age, laboratory and sex, and this page shows the range of the laboratory that performs the analysis. To make sense of your phosphorus result, your doctor considers your symptoms and your other test results, such as calcium, together.