Phosphate
At a Glance
Why Get Tested?
To evaluate the level of phosphate in your blood and to aid in the diagnosis of conditions known to cause abnormally high or low levels
When to Get Tested?
As a follow-up to an abnormal calcium level, if you have a kidney disorder or uncontrolled diabetes, and if you are taking calcium or phosphate supplements
Sample Required?
A blood sample taken from a vein in your arm, or a fasting or timed urine collection (usually a 24-hour sample)
The Test Sample
What is being tested?
The serum phosphate test measures the amount of inorganic phosphate in your blood plasma (the liquid part of blood).
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. About 70% to 80% of the phosphates are combined with calcium to help form bones and teeth, about 10% are found in muscle, and about 1% is in nerve tissue. The rest is found within cells throughout the body, where it is mainly used to store energy; about 1% of total body phosphate is found within plasma.
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 significant amounts of phosphate. The body maintains phosphate levels in the blood by regulating how much it absorbs from the intestines and how much it excretes or conserves in the kidneys.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. If a timed urine sample is required, you will be asked to save all of your urine over a set time period (usually 24 hours).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
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 digestive system and interfere with the absorption of phosphate, calcium, and magnesium. Testing also can help to diagnose disorders that affect the kidneys and interfere with mineral excretion and conservation 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.
While phosphate levels are most commonly performed on blood samples, fasting or timed urine phosphate measurements also may be used to monitor the removal of phosphate by the kidneys.
When is it requested?
A phosphate test is often requested to help diagnose diseases and conditions that cause problems with the body’s utilization of calcium. The test may help in the diagnosis of problems with hormones, such as parathyroid hormone (PTH) and Vitamin D, which functions as a hormone, that regulate the body’s calcium level and, to a lesser degree, phosphate level.
Although abnormal phosphate levels usually cause no symptoms (with the exception of very low levels), phosphate testing often is performed as a follow-up to an abnormal calcium level and/or related symptoms, such as fatigue, muscle weakness, cramps or bone problems.
Phosphate testing may be requested when symptoms or other tests suggest kidney and/or disorders of the digestive system.
A phosphate test may be requested in people who are malnourished as this can cause the phosphate level in the blood to fall.
If conditions causing abnormal phosphate and/or calcium levels are found, testing for both may be requested at regular intervals to monitor treatment effectiveness.
If you have a kidney disorder, kidney stones, or uncontrolled diabetes, your doctor may monitor phosphate levels to make sure that you are not loosing or retaining excessive amounts.
What does the test 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:
- Hypercalcaemia (high levels of calcium), especially when due to high levels of parathyriod hormone (PTH)
- Overuse of diuretics (drugs that encourage urination)
- Severe burns
- Diabetic ketoacidosis after treatment
- Hypothyroidism
- Hypokalaemia (low levels of potassium)
- Chronic antacid use
- Rickets and osteomalacia (due to Vitamin D deficiencies)
Higher than normal levels of phosphate (hyperphosphataemia) 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
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 young 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
Ask a Laboratory Scientist
Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
