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.

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.

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

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.