Phosphorus (serum or urine)

Phosphorus is a mineral that builds bones and teeth together with calcium and is also needed by every cell to produce energy. The test shows whether there is too much or too little phosphorus in your blood, most often indicating kidney and parathyroid function and vitamin D stores. Phosphorus is measured in blood and, if necessary, urine.

Who the test is for

• people with chronic kidney disease, in whom phosphorus is monitored regularly;
• when calcium is elevated or low and during investigation of the parathyroid glands;
• in vitamin D deficiency and bone pain of unclear cause;
• in marked muscle weakness, particularly with malnutrition, long-term excessive alcohol intake or after prolonged starvation;
• in recurrent kidney stones, when urine phosphorus is also measured;
• as part of a health check, alongside calcium and magnesium.

What the test reveals

Approximately 85 % of the body’s phosphorus is found in bones and teeth, with the remainder in cells and blood. Blood phosphorus levels are maintained jointly by the kidneys, parathyroid hormone (PTH) and vitamin D: the kidneys excrete excess in urine, PTH increases this excretion, and vitamin D increases absorption of phosphorus from food. Phosphorus is therefore almost always interpreted alongside calcium, and often also PTH, vitamin D and creatinine.

Urine phosphorus shows how much the kidneys excrete. A doctor requests it to clarify low or high blood phosphorus and in kidney stones. The amount in urine strongly depends on diet, because phosphorus is present in almost all protein-rich foods.

The procedure and preparation

For blood phosphorus, blood is taken from a vein, and the draw takes a few minutes. It is best to have blood drawn in the morning, because meals and the time of day change phosphorus levels. Fasting is not essential unless glucose, blood lipids or PT are also being tested from the same blood draw. State which medicines and supplements you take, particularly vitamin D, calcium, antacids, phosphate binders and diuretics, but do not stop them without agreeing with your doctor.

Urine collected over 24 hours is most commonly needed for urine phosphorus.

What your result means

In adults, blood phosphorus is usually approximately 0,79 to 1,42 mmol/L. Growing children and young people have higher values, and exact limits depend on age and the laboratory and are printed on your report. Your doctor interprets the result alongside calcium, PTH, vitamin D and kidney function.

• Low phosphorus most often accompanies vitamin D deficiency, overactive parathyroid glands, malnutrition, excessive alcohol intake or long-term antacid use. Mildly low levels often cause no symptoms, whereas very low levels can cause muscle weakness and bone pain.
• High phosphorus is most often a consequence of chronic kidney disease, when the kidneys can no longer excrete excess. Underactive parathyroid glands, excess vitamin D and phosphate-containing laxatives or enemas also raise it. Higher values may be expected in children and while a fracture is healing.
• An unexpectedly high result may occur if blood cells in the sample have been damaged, so your doctor repeats it.
• Urine phosphorus is interpreted alongside blood phosphorus and diet. High excretion with low blood phosphorus suggests loss through the kidneys, while low excretion suggests low intake or absorption.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Every branch accepts urine samples for phosphorus, also without a referral. Prices for all tests and panels are listed in the price list.

Frequently asked questions

Do I need to fast for phosphorus?

It is not essential unless glucose, blood lipids or PT are also being tested from the same blood draw. However, phosphorus temporarily falls after a meal, particularly one rich in carbohydrates, and changes during the day, so a morning blood draw is best.

What is the relationship between phosphorus and calcium?

They are regulated by the same hormones, PTH and vitamin D. With underactive parathyroid glands, calcium is usually low and phosphorus high, while the reverse occurs with overactivity. Your doctor therefore considers both results together.

Does diet affect blood phosphorus?

Only slightly with healthy kidneys, because they excrete excess. In chronic kidney disease, intake becomes important, particularly from processed meat, processed cheese, cola-type drinks and ready-made foods with phosphate additives.

Does phosphorus show bone condition?

Not directly. Blood phosphorus can be normal even when bones are weakened. Bone density is measured by densitometry, while phosphorus, calcium, vitamin D and PTH help identify the cause of bone changes.