Summary

• Phosphate is important for many essential body functions.
• Most phosphate is stored in your bones and teeth.
• Your bones need both phosphate and calcium for strength.
• The phosphate test helps diagnose disorders that cause unusually high or low levels of phosphate.
• These disorders include kidney disease, bone disorders, gastrointestinal malabsorption disorders and conditions that affect the parathyroid glands.

What is phosphate?

Phosphorus is a mineral that is found in many foods. Your body needs it for many essential functions including building strong bones and teeth, nerve and cell function, energy storage and cell and tissue growth.

You get phosphorous through your diet, and it is absorbed into your body through the small intestine. Once in the body, almost all phosphorus is combined with oxygen, which changes it to phosphate.

Most of the phosphate in your body is stored in your bones and teeth, which need both calcium and phosphate for strength.

How you get phosphate from your diet

Intestines absorb phosphate from your diet. If you are not getting enough phosphate, it may be because there is not enough in your diet, or you are not able to absorb enough, such as if have vomiting or diarrhoea. Conditions in your gut which impact your intestines’ ability to absorb phosphate such as coeliac disease or inflammatory bowel disease (Crohn’s disease and ulcerative colitis) may also affect levels.

How excess phosphate is removed from your body

Kidneys filter excess phosphate from your blood, and it is removed in your urine. If phosphate levels are low, your kidneys adjust them by filtering less into the urine. Abnormal phosphate levels are seen in conditions or diseases that cause too little or too much phosphate to be removed by the kidneys.

Phosphate levels in the body are regulated through a feedback system that also involves calcium and vitamin D. For more on this see Calcium, phosphate and magnesium and Calcium.

Phosphate binds to calcium, lowering the amount of free calcium circulating in your blood. This means that when you have blood tests for calcium and phosphate, the higher the phosphate level, the lower the level of free calcium. As blood calcium levels rise, phosphate levels fall.

Why get tested?

A phosphate test is usually ordered along with other tests such as total calcium, ionised calcium, magnesium, parathyroid hormone (PTH), vitamin D and calcitriol. These tests are used to help investigate a wide range of health conditions.

Kidney disease

Because your kidneys help control the amount of phosphate in your blood by filtering excess phosphate into your urine, a high level of phosphate in the blood is most often caused by a kidney problem. If you have chronic kidney disease you may have regular phosphate tests and if you have kidney failure, your phosphate levels will be closely monitored. When your kidneys are not working properly, the level of phosphate in your blood rises. When phosphate is high in your blood it binds to calcium and causes a build-up of calcium in blood vessels and other parts of the body. This can lead to weak bones and heart disease.

Bone disorders

High phosphate levels can draw calcium out of your bones and weaken them over time. It can also lead to calcification of arteries.

Gastrointestinal disorders

Phosphate testing can be helpful in investigating conditions that affect the gastrointestinal tract (your gut) and prevent the absorption of phosphate, calcium and magnesium. Often this is in someone who is malnourished or is being treated for ketoacidosis.

Parathyroid gland disorders

Parathyroid hormones control the balance of phosphate and calcium in the blood. The parathyroid glands are located behind your thyroid at the base of your neck. They release parathyroid hormone (PTH) to signal to your intestines, bones and kidneys, which all play a part in managing calcium, vitamin D and phosphate. For more on this see Calcium.

A phosphate test can help show how well the parathyroid glands are working. A 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.

Calcium levels

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 (ALP) level. ALP is an enzyme that is found in high levels in the blood when there is a bone disorder.

Other reasons for phosphate testing

• If you are in hospital and on intravenous glucose, your phosphate levels may be monitored often as phosphate levels can fall after a glucose infusion.
• If you are very malnourished, phosphate levels may fall rapidly and require monitoring after you are given food.
• If you have uncontrolled diabetes, you may have phosphate testing.
• Some rare genetic disorders, unusual tumours and some drugs may alter phosphate levels.
• The balance between calcium and phosphate can be disrupted by some infections.
• Phosphate plays an important role in maintaining the body’s acid-base balance. If you have too much or too little acid in your blood you may need to have a phosphate test.

Having the test

Sample

A blood or timed urine sample.

While phosphate levels are most commonly performed on blood samples, fasting or timed urine phosphate measurements may also be used to monitor phosphate elimination by the kidneys.

Any preparation?
None

Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people.
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.
The reference intervals for this test are common reference intervals which means that the majority of laboratories in Australia should be using the same reference intervals.

Any more to know?

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 need swift medical attention.

Soft drinks and pre-packaged food items are high in phosphate, 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 glucose injections. You may be asked to temporarily stop taking medicines including diuretics, that may affect the test.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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