Serum Phosphorus


The phosphorus blood test measures the amount of phosphate in the blood.

Alternative Names

Phosphorus - serum; HPO4-2; PO4-3; Inorganic phosphate; Serum phosphorus

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

Your health care provider may tell you to temporarily stop taking medicines that may affect the test. These medicines include water pills (diuretics), antacids, and laxatives.

DO NOT stop taking any medicine before talking to your provider.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

Phosphorus is a mineral the body needs to build strong bones and teeth. It is also important for nerve signaling and muscle contraction.

This test is ordered to see how much phosphorus is in your blood. Kidney, liver, and certain bone diseases can cause abnormal phosphorus levels.

Normal Results

Normal values range from 2.4 to 4.1 mg/dL.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

A higher than normal level (hyperphosphatemia) may be due to many different health conditions. Common causes include:

  • Diabetic ketoacidosis (life-threatening condition that may occur in people with diabetes)
  • Hypoparathyroidism (parathyroid glands do not make enough of their hormone)
  • Kidney failure
  • Liver disease
  • Too much vitamin D
  • Too much phosphate in your diet
  • Use of certain medicines such as laxatives that have phosphate in them

A lower than normal level (hypophosphatemia) may be due to:

  • Alcoholism
  • Hypercalcemia (too much calcium in the body)
  • Hyperparathyroidism (parathyroid glands make too much of their hormone)
  • Too little dietary intake of phosphate
  • Very poor nutrition
  • Too little vitamin D, resulting in bone problems such as rickets (childhood) or osteomalacia (adult)


Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling lightheaded
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)


Leone KA. Calcium, magnesium, and phosphorus. In: Adams JG, ed. Emergency Medicine: Clinical Essentials. 2nd ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 166.

Yu SLA. Disorders of magnesium and phosphorous. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 119.

Review Date: 11/1/2015

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