This test measures the level of phosphate in your blood to help diagnose disorders that cause abnormally high or low levels of phosphate. These can be grouped as those that cause poor absorption of minerals from the intestines or interfere with the process for getting rid of minerals by the kidneys. Blood levels of calcium, magnesium, parathyroid hormone (PTH), and vitamin D may also be measured at the same time.
What is being tested?
The majority, (70–80 per cent) of phosphate exists in the bones, complexed with calcium as hydroxyapatite, and a small fraction, 1 per cent is in the blood mainly as inorganic phosphate and phospholipids. The rest is found within cells throughout the body, where it is mainly used to store energy. This small blood fraction is under hormonal control by PTH, vitamin D and fibroblast growth factor 23 (FGF23).
Absorption of dietary phosphate occurs in the small intestine utilising pathways which may involve vitamin D, and FGF23 mobilises phosphate from bone, to maintain plasma phosphate levels for energy use. Ten to twenty percent of phosphate is excreted via the kidneys which help maintain steady phosphate levels in the blood via tubular reabsorption from urine.
In the body, phosphorus is combined with oxygen to form a variety of phosphates (PO4). Phosphates are vital for energy production, muscle and nerve function, and bone growth. They also play an important role as a buffer, helping to maintain the body’s acid-base balance.
Most phosphate in the body comes from dietary sources. A variety of foods, such as beans, peas and nuts, cereals, dairy products, eggs, beef, chicken and fish contain small amounts of phosphate.
How is it used?
Phosphate testing can be helpful in people who are malnourished or who are being treated for ketoacidosis. Phosphate testing is used to help diagnose and evaluate the severity of conditions and diseases that affect the gastrointestinal tract, interfering with the absorption of phosphate, calcium and magnesium. Testing also can help to diagnose disorders that affect the kidneys, interfering with mineral excretion and conservation, and phosphate levels are carefully monitored in people with kidney failure.
When a person has a known problem that affects their phosphate and/or calcium levels, phosphate levels may be monitored regularly to determine the effectiveness of treatment. Usually, it is done with other tests.
While phosphate levels are most commonly performed on blood samples, fasting or timed urine phosphate measurements also may be used to monitor phosphate elimination by the kidneys.
When is it requested?
A phosphate test is often requested to help interpret abnormal blood calcium levels which may be due to either over or under-activity of the parathyroid glands or due to vitamin D deficiency.
To follow up patients with chronic renal disease as high phosphate found in renal disease contributes to bone disease and calcification of arteries.
In very malnourished people phosphate levels may fall rapidly and require monitoring after they are given food.
In hospitalised patients who are on intravenous glucose, levels are monitored frequently as phosphate levels can fall after glucose infusion in severely ill patients.
Some rare genetic disorders, unusual tumours and some drugs may alter phosphate levels.
What does the result mean?
Dietary deficiencies in phosphate are rare but may be seen with alcoholism and malnutrition.
Low levels of phosphate (hypophosphataemia) may also be due to or associated with:
Higher than normal levels of phosphate (hyperphosphataemia) may be due to or associated with:
Is there anything else I should know?
Abnormally high levels of phosphate can lead to organ damage due to calcification (calcium phosphate deposits in organs, such as the kidneys).
Phosphate levels are normally higher in children than in adults because their bones are actively growing. Low phosphate levels in children can inhibit bone growth. Very low levels of phosphate are rare but require swift medical attention.
Soft drinks and pre-packaged food items are high in phosphate content, sometimes as phosphoric acid, which some nutritionists believe contributes to over consumption of phosphorus.
Test results may be affected by the use of enemas and laxatives containing sodium phosphate, excess vitamin D supplements, and by intravenous glucose administration.
Common questions
Abnormal phosphate levels are usually detected because of the relationship with and effect on calcium. If you have abnormal calcium levels, your doctor usually will check your phosphate and alkaline phosphatase level.
Yes, but only about 50% of the phosphate in plant sources such as beans, lentils, grains, peanuts and almonds is available to the body because we lack the enzymes to process it. An exception to this is yeast breads because yeast provides the necessary enzyme.
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