Phosphate (PO4) blood test: high and low results explained

In the body phosphorus combines with oxygen to form phosphates. Lab Tests Online UK explains that phosphates are vital for energy production, muscle and nerve function and bone growth, form building blocks for cell membranes and DNA, and act as a buffer in acid-base balance. About 85 per cent combines with calcium in bones and teeth, 15 per cent is in soft tissues and cells, and about 1 per cent is found in the plasma.

Most phosphate comes from diet: beans, peas and nuts, cereals, dairy products, eggs, beef, chicken and fish. Phosphate in animal-based foods is more easily absorbed than that in plant-based foods. The body keeps the blood level steady by controlling intestinal absorption and kidney excretion.

It is often a follow-up to an abnormal calcium or to symptoms of abnormal calcium such as fatigue, muscle weakness and cramps. Lab Tests Online UK lists other reasons: kidney disorders, uncontrolled diabetes, calcium or phosphate supplements, nutritional problems and suspected hormone imbalance involving vitamin D or parathyroid hormone. It is also used when treating malnourished people, because feeding can lower phosphate, and in ketoacidosis.

Abnormal levels usually cause no symptoms, except very low levels, so the result often appears as a surprise on a routine panel.

Blood is taken from a vein, sometimes after overnight fasting, because meals can change the level. Phosphate can also be measured in urine, usually as a 24-hour collection, to see how much the kidneys are removing. Tell the clinician about supplements, vitamin D, antacids and diuretics.

Dietary deficiency is rare but may be seen with alcoholism and malnutrition. Lab Tests Online UK associates low phosphate with high calcium, especially from high parathyroid hormone, overuse of diuretics, severe burns, treated diabetic ketoacidosis, hypothyroidism, low potassium, chronic antacid use, rickets and osteomalacia from vitamin D deficiency, and increased insulin production.

Very low levels can cause weakness and, in severe cases, affect breathing and the heart, so they are corrected in hospital, particularly during re-feeding of malnourished people.

High levels may be due to or associated with kidney failure, hypoparathyroidism, low calcium, diabetic ketoacidosis when first seen, phosphate supplementation, cardiovascular disease and vitamin D overuse. In people with kidney failure, phosphate is carefully monitored; Lab Tests Online UK warns that persistently high levels can lead to organ damage, and treatment typically involves diet advice and medicines that bind phosphate in food.

Doctors review calcium, parathyroid hormone, vitamin D, creatinine and eGFR together, and may repeat the test fasting. Kidney disease management, vitamin D treatment, dietary changes or treating a parathyroid disorder follow the cause. Seek urgent help for severe weakness, breathlessness or palpitations; call 999 or attend A&E.