section name header

Introduction

Phosphate (PO4) Level

A phosphate (PO4) level measures the amount of phosphate in the blood. Phosphate is a mineral that contains phosphorous and is required for generation of bony tissue and functions in the metabolism of glucose and lipids, in the maintenance of acid–base balance, and in the storage and transfer of energy from one site in the body to another. Phosphorus enters the red blood cells with glucose and therefore is lowered in the plasma after carbohydrate ingestion or infusion.

Phosphate levels are always evaluated in relation to calcium levels because there is an inverse relation between the two elements. Of the human body’s total phosphorus content, 85% is combined with calcium in the bone and the remainder resides within the cells. Most of the phosphorus in the blood exists as phosphates or esters. When calcium levels are decreased, phosphorus levels are increased, and when phosphorus levels are decreased, calcium levels are increased. An excess of one electrolyte in serum causes the kidneys to excrete the other electrolyte. Many of the causes of elevated calcium levels are also causes of decreased phosphorus levels. As with calcium, the controlling factor is PTH.

Procedure

  1. Obtain a fasting, 5-mL in an SST or red-topped tube, venous blood sample (see Chapter 2 for venous blood collection). Serum is preferred, but heparinized blood is acceptable. Serum should be removed from clot as soon as possible after collection.

  2. Observe standard precautions.

Clinical Implications

  1. Hyperphosphatemia (increased blood phosphorus levels) is most commonly found in association with kidney dysfunction and uremia. This is because phosphate is so minutely regulated by the kidneys. These conditions include the following:

    1. Renal insufficiency and severe nephritis (accompanied by elevated BUN and creatinine) and acute kidney injury

    2. Hypoparathyroidism (accompanied by elevated phosphorus, decreased calcium, and normal kidney function) and pseudohypoparathyroidism

    3. Hypocalcemia

    4. Milk-alkali syndrome (Burnett syndrome; high levels of calcium and metabolic alkalosis)

    5. Excessive intake of vitamin D

    6. Fractures in the healing stage

    7. Bone tumors and metastases

    8. Addison disease

    9. Acromegaly

    10. Liver disease and cirrhosis

    11. Cardiac resuscitation

  2. Hypophosphatemia (decreased phosphorus level) occurs in the following conditions:

    1. Hyperparathyroidism

    2. Rickets (childhood) or osteomalacia (adult) and vitamin D deficiency

    3. Diabetic coma (increased carbohydrate metabolism)

    4. Hyperinsulinism

    5. Continuous administration of IV glucose in a patient without diabetes (phosphorus follows glucose into the cells)

    6. Liver disease and acute alcoholism

    7. Vomiting and severe diarrhea

    8. Severe malnutrition and malabsorption

    9. Gram-negative septicemia

    10. Hypercalcemia of any cause

    11. Prolonged hypothermia

    12. Respiratory alkalosis due to cellular use of phosphorus for an accelerated glucose metabolism

Interventions

Pretest Patient Care

  1. Explain test purpose and blood sampling procedures. The patient should fast before the test.

  2. Note on test requisition if any catastrophic stressful events have taken place that may cause high phosphate levels.

  3. Note time of day test is drawn; levels are highest in the morning and lowest in the evening.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Have patient resume normal activities.

  2. Review test results; report and record findings. Modify the nursing care plan as needed.

  3. Monitor patient as appropriate for calcium imbalances. When phosphate levels rise rapidly, calcium levels drop; watch for arrhythmias and muscle twitching. The signs and symptoms of phosphate depletion may include manifestations in the neuromuscular, neuropsychiatric, GI, skeletal, and cardiopulmonary systems. Manifestations usually are accompanied by serum levels <1 mg/dL (<0.32 mmol/L).

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors

  1. Phosphate levels are normally high in children.

  2. Phosphate levels can be falsely increased by hemolysis of blood; therefore, separate serum from cells as soon as possible.

  3. Drugs can be the cause of decreases in phosphorus.

  4. The use of laxatives or enemas containing large amounts of sodium phosphate will cause increased phosphate levels. With oral laxatives, the blood phosphate level may increase as much as 5 mg/dL (1.6 mmol/L) 2–3 hours after intake. This increased level is only temporary (5–6 hours), but this factor should be considered when abnormal levels are seen, which cannot otherwise be explained.

  5. Seasonal variations exist in phosphate levels (maximal levels in May and June, lowest levels in winter).

Reference Values

Normal

Clinical Alert

Critical Values1.0 mg/dL (0.32 mmol/L