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Phosphates

This test is used to measure serum levels of phosphates, the primary anion in intracellular fluid. About 85% of the body's phosphates are found in bone. The intestines absorb most phosphates from dietary sources; the kidneys excrete phosphates and serve as a regulatory mechanism. Abnormal concentrations of serum phosphates usually result from improper excretion rather than faulty ingestion or absorption from dietary sources.

Normally, calcium and phosphates have an inverse relationship; if one is increased, the other is decreased.

Purpose

• To aid diagnosis of renal disorders and acid-base imbalance.
• To detect endocrine, skeletal, and calcium disorders.

Patient preparation

• Explain to the patient that this test is used to measure phosphate levels in the blood.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes.
• Inform him that he needn't restrict food or fluids before the test.
• Check the patient's medication history for drugs that alter phosphate levels, such as vitamin D, anabolic steroids, androgens, phosphatebinding antacids, acetazolamide, insulin, and epinephrine.

Procedure and posttest care

• Perform a venipuncture, without using a tourniquet if possible, and collect the sample in a 7-ml red-top or red-marble-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Handle the sample gently to prevent hemolysis.

Reference values

• Normally, serum phosphate levels in adults range from 2.5 to 4.5 mg/dl. Children have higher serum phosphate levels than adults. Phosphate levels rise during periods of increased bone growth.

Abnormal findings

Because serum phosphate values alone are of limited diagnostic use (only a few rare conditions directly affect phosphate metabolism), they should be interpreted in light of serum calcium results.

Decreased phosphate levels (hypophosphatemia) may result from malnutrition, malabsorption syndromes, hyperparathyroidism, renal tubular acidosis, and treatment of diabetic ketoacidosis. In children, hypophosphatemia can suppress normal growth.

Increased levels (hyperphosphatemia) may result from skeletal disease, healing fractures, hypoparathyroidism, acromegaly, diabetic ketoacidosis, high intestinal obstruction, and renal failure. Hyperphosphatemia is seldom clinically significant, but it can alter bone metabolism in prolonged cases.

Interfering factors

• Venous stasis due to tourniquet use.
• Sample obtained above an I.V. site that's receiving a solution containing phosphate
• Excessive vitamin D intake or therapy with anabolic steroids or androgens (possible increase)
• Use of acetazolamide, insulin, epinephrine, or phosphatebinding ant­acids; excessive excretion due to prolonged vomiting or diarrhea; vitamin D deficiency; extended I.V. infusion of dextrose 5% in water (possible decrease)
• Hemolysis of the sample (false-high).