Inorganic Phosphate

The serum phosphorus test measures the amount of phosphate in your blood. Normally, only a very small amount of phosphate is present in the blood.

In the body, phosphorus exists as a mineral; it 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 phosphorus is found within plasma (the liquid part of blood).

Most phosphorus 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 phosphorus. The body maintains phosphorus/phosphate levels in the blood by regulating how much it absorbs from the intestines and how much it excretes from or conserves in the kidneys.


You should have nothing to eat or drink after midnight the day of the test or do as instructed. 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).


Normal values depend on many factors. Your lab will provide a range of normal values with your test results.

  • Adult: 3-4.5 mg/dl or 0.97-1.45 mmol/L (SI units)

  • Elderly: values slightly lower than adult

  • Child: 4.5-6.5 mg/dl or 1.45-2.1 mmol/L (SI units)

  • Newborn: 4.3-9.3 mg/dl or 1.4-3 mmol/L (SI units)

Possible critical values: less than 1 mg/dl


Your caregiver will go over your test results with you and discuss the importance of this test. Reference values are dependent on many factors, including patient age, gender, sample population, and testing method. Numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test.

Dietary deficiencies in phosphorus are rare but may be seen with alcoholism and malnutrition. Low levels of phosphorus (hypophosphatemia) may also be due to or associated with:

  • Hypercalcemia (high levels of calcium)

  • Overuse of diuretics (drugs that encourage urination)

  • Severe burns

  • Diabetic ketoacidosis (after treatment)

  • Hypothyroidism

  • Hypokalemia (low levels of potassium)

  • Chronic antacid use

  • Rickets and osteomalacia (due to Vitamin D deficiencies)

Higher than normal levels of phosphorus (hyperphosphatemia) may be due to or associated with:

  • Kidney failure

  • Hypoparathyroidism (underactive parathyroid gland)

  • Hypocalcemia (abnormally low levels of calcium)

  • Diabetic ketoacidosis (when first seen)

  • Phosphate supplementation


  • Make sure you receive the results of your test. Ask how these results are to be obtained if you have not been informed. It is your responsibility to obtain your test results.

  • Your caregiver will provide further instructions or treatment options if necessary.