Phosphorus (P), the dominant intracellular anion, is measured in serum as phosphate (HPO₄^{- -}, H₂PO₄^-). Results are reported as inorganic phosphorus (Pi). Phosphates are vital constituents of nucleic acids, intracellular energy storage compounds, intermediary compounds in carbohydrate metabolism, and various regulatory compounds, including that which modulates dissociation of oxygen from hemoglobin. Phosphorus also aids in regulation of calcium levels and functions as a buffer in the maintenance of acid-base balance. It contributes to the mineralization of bones and teeth, promotes renal tubular reabsorption of glucose, and, as a component of phospholipids, aids in fat transport.

As with calcium, phosphorus ions undergo continuous turnover, with bone serving as the major reservoir. Serum contains a relatively small amount of phosphorus at any given time. Phosphorus levels are largely regulated by the parathyroid glands and vitamin D, and they are normally reciprocal to those of serum calcium. The equilibrium between serum phosphate levels and intracellular stores is affected by carbohydrate metabolism and blood pH. When persons with diabetic ketoacidosis are treated with insulin, for example, phosphate enters the cells along with glucose and potassium. Phosphate excretion is controlled by the kidneys. Disorders and drugs associated with altered phosphorus levels are listed in Table 5-28. Note that several disorders associated with decreased phosphorus levels are the same as those causing elevated serum calcium levels (e.g., hyperparathyroidism).

Note: Phosphorus is measured in terms of phosphate; the results cannot be expressed in milliequivalents because different phosphate groups have different valences.

• Phosphate levels are higher in children because of bone growth and active bone formation.
• Values vary diurnally, being higher at night than in the morning.
• A number of drugs may alter serum phosphate levels (see Table 5-28).
• Hemolysis of the sample may cause falsely elevated values resulting from release of phosphate from red blood cells.

• Support for diagnosing disorders associated with altered phosphorus/phosphate levels, especially bone disorders, parathyroid disorders, renal disease, and alcoholism (see Table 5-28)
• Monitoring of the effects of renal failure on phosphorus levels, which are usually increased in the disorder
• Support for identification of the cause of growth abnormalities in children
• Monitoring for the effects of various drugs on serum phosphate levels (see Table 5-28)

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

• Because many drugs may alter serum phosphorus/phosphate levels, a medication history should be obtained. It is recommended that any drugs that may alter test results be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis, which may falsely elevate levels, and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any medications withheld before the test.
• Increased levels: Note and report increased levels (hyperphosphatemia) and associated calcium levels. Instruct client to avoid foods high in phosphorus (milk and dairy products, poultry, fish, grain cereals).
• Decreased levels: Note and report decreased levels (hypophosphatemia) in relation to calcium levels. Instruct client to include foods and fluids high in phosphorus, as already listed.
• Critical values: Notify the physician at once of levels less than 1 mg/dL.