Summary

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

Phosphorus 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.

Phosphate levels in the body are regulated through a feedback system that also involves calcium and vitamin D, which are essential for bone formation and repair.

Most of the phosphate in your body is stored in your bones and teeth, which need both calcium and phosphate for strength. Phosphate binds to calcium, lowering the amount of free (ionised) calcium 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) and vitamin D. These tests are used to help investigate a range of health conditions.

Kidney disease

Your kidneys help control the amount of phosphate in your blood by filtering excess phosphate and passing it into your urine. A high level of phosphate in someone’s blood is most often caused by a kidney problem. Anyone with chronic kidney disease may have regular phosphate tests and if someone has kidney failure phosphate levels are closely monitored.

Bone disorders

High phosphate levels can draw calcium out of 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, interfering with 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. These are small glands behind your thyroid at the base of your neck which release parathyroid hormone to signal to your intestines, bones and kidneys, which all play a part in managing calcium, vitamin D and phosphate. 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 level.

Other reasons for phosphate testing

• In hospitalised patients who are on intravenous glucose, levels are monitored often as phosphate levels can fall after glucose infusion in severely ill patients.
• In very malnourished people phosphate levels may fall rapidly and require monitoring after they are given food.
• Someone with uncontrolled diabetes may also 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.
• As phosphate plays an important role as a buffer, helping to maintain the body’s acid-base balance, someone with too much or too little acid in their blood may also have phosphate testing.

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.

Low levels of phosphate (hypophosphataemia)

Dietary deficiencies in phosphate are rare but may be seen with alcoholism and malnutrition.

Low levels of phosphate may also be due to or associated with:

• Hypercalcaemia (high levels of calcium)
• Overuse of diuretics (drugs that encourage urination)
• Severe burns
• Diabetic ketoacidosis after treatment
• Chronic antacid use
• Rickets and osteomalacia (due to Vitamin D deficiencies)

Higher than normal levels of phosphate (hyperphosphataemia)

High levels of phosphate may be due to or associated with:

• Kidney failure
• Hypoparathyroidism (underactive parathyroid gland)
• Hypocalcaemia (abnormally low levels of calcium)
• Diabetic ketoacidosis when first seen
• Phosphate supplementation

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 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. 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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