Phosphorus test: what the result means

Phosphorus is a mineral, most of which is stored in the bones and teeth together with calcium, while the rest takes part in the energy metabolism of cells (ATP), in the transmission of nerve signals and in muscle contraction. The amount of phosphate circulating in the blood is only a small share of all the phosphorus in the body, but it reflects how the body regulates mineral metabolism. The amount of phosphorus in the blood is controlled by the kidneys, the parathyroid glands (through parathyroid hormone) and vitamin D — these mechanisms are closely tied to calcium metabolism, which is why phosphorus is often assessed together with calcium and with markers of kidney function. This test helps assess the state of the kidneys, the bones and the parathyroid glands, and it is also used when monitoring uncontrolled diabetes mellitus or disturbances of the acid-base balance, because the amount of phosphate in the blood can change quickly in these conditions.

• When assessing kidney function and monitoring chronic kidney disease
• When bone disease or a parathyroid gland (parathyroid hormone) disorder is suspected
• When monitoring uncontrolled diabetes mellitus, including diabetic ketoacidosis
• When assessing disturbances of the body's acid-base balance
• Together with calcium and vitamin D tests, when a disorder of mineral metabolism is suspected

Raised phosphorus (hyperphosphataemia) is most often linked with late-stage chronic kidney disease or kidney failure, when the kidneys no longer excrete the excess of phosphate. Other causes are hypoparathyroidism, acidosis, diabetic ketoacidosis, and long-term use of steroids, laxatives or phosphate-containing enemas.

• Late-stage chronic kidney disease and kidney failure
• Hypoparathyroidism (insufficient function of the parathyroid glands)
• Acidosis — too much acid in the blood
• Diabetic ketoacidosis
• Liver disease
• Too much vitamin D, or an excess of phosphate from food
• Long-term use of steroids, laxatives or phosphate-containing enemas

Symptoms that can occur with high phosphorus

• Brittle nails, coarse hair
• Rash, itching
• Muscle cramps
• Bone pain
• Seizures
• An irregular heartbeat

What is usually checked next

• Markers of kidney function (creatinine, eGFR)
• Calcium and parathyroid hormone (PTH) tests — they help tell the cause apart
• The level of vitamin D
• A review of the medicines you take, with your doctor — some diuretics, antacids or laxatives can affect the result

Low phosphorus (hypophosphataemia)

Lowered phosphorus (hypophosphataemia) can be linked with increased function of the parathyroid glands (hyperparathyroidism), with vitamin D deficiency, with too little phosphorus from food (a cause rarely encountered in the United States), with heavy alcohol use, or with long-term use of certain antacids and diuretics.

• Hyperparathyroidism (increased function of the parathyroid glands)
• Vitamin D deficiency, rickets or osteomalacia
• Too little phosphorus from food — a rarely encountered cause
• Heavy, long-term alcohol use
• A raised amount of calcium in the blood
• Long-term use of certain antacids or diuretics

• Muscle pain or weakness
• Bone pain
• Confusion, irritability
• Seizures
• In severe cases — loss of consciousness (coma)

What is usually checked next when phosphorus is low

• Parathyroid hormone (PTH) and calcium tests
• The level of vitamin D in the blood
• An assessment of kidney function
• A review of the medicines you take (antacids, diuretics) with your doctor

Why the range for children is different

In children the phosphorus range per MedlinePlus is 4.0-7.0 mg/dl, while for adults the same source gives 2.8-4.5 mg/dl — the children's interval is not only higher but also wider. MedlinePlus explains that phosphorus is needed to build strong bones and teeth and that it takes part in the work of nerves and muscles, but it does not directly explain why the interval for children differs from the one for adults. That is why a result has to be compared with the interval meant for children, and not with the adult range. One thing to keep in mind: the source reports results in the units used in the United States (mg/dl), while laboratories in Lithuania most often use mmol/l. The exact interval, matched to the child's age, should be looked up on the report of the particular laboratory, because children's ranges can differ depending on the age group.

Reading the result

Phosphorus is often assessed together with calcium and with markers of kidney function. The same mechanisms — parathyroid hormone and vitamin D — regulate calcium metabolism as well, which is why these two markers are often looked at side by side. The interval that applies to your result is the one printed on your own laboratory's report, and going over the result together with the medicines and supplements you take is something for your doctor. More on results in general: how to read blood test results. The test is also used when monitoring uncontrolled diabetes mellitus, including diabetic ketoacidosis — the marker followed there is glucose.