Inorganic phosphorus

Phosphorus, Blood

Synonym/acronym: Inorganic phosphorus, phosphate, PO₄.

Common use

To assist in evaluating multiple body system functions by monitoring phosphorus levels in relation to other electrolytes. Used specifically to evaluate renal function in at-risk patients.

Specimen

Serum (1 mL) collected in a gold-, red-, or red/gray-top tube. Plasma (1 mL) collected in green-top (heparin) tube is also acceptable.

Normal findings

(Method: Spectrophotometry)

Age	Conventional Units	SI Units (Conventional Units × 0.323)
0–5 day	4.6–8 mg/dL	1.5–2.6 mmol/L
1–3 yr	3.9–6.5 mg/dL	1.3–2.1 mmol/L
4–6 yr	4–5.4 mg/dL	1.3–1.7 mmol/L
7–11 yr	3.7–5.6 mg/dL	1.2–1.8 mmol/L
12–13 yr	3.3–5.4 mg/dL	1.1–1.7 mmol/L
14–15 yr	2.9–5.4 mg/dL	0.9–1.7 mmol/L
16–19 yr	2.8–4.6 mg/dL	0.9–1.5 mmol/L
Adult	2.5–4.5 mg/dL	0.8–1.4 mmol/L

Values may be slightly decreased in older adults due to dietary insufficiency or the effects of medications and the presence of multiple chronic or acute diseases with or without muted symptoms.

Description

Phosphorus, in the form of phosphate, is distributed throughout the body. Approximately 85% of the body’s phosphorus is stored in bones; the remainder is found in cells and body fluids. It is the major intracellular anion and plays a crucial role in cellular metabolism, maintenance of cellular membranes, and formation of bones and teeth. Phosphorus also indirectly affects the release of oxygen from hemoglobin by affecting the formation of 2,3-bisphosphoglycerate. The reabsorption and excretion of phosphorus is largely regulated by the parathyroid glands and the kidneys. Levels of phosphorus are also affected by dietary intake and are dependent on the presence of activated vitamin D for absorption by the intestines. Calcium and phosphorus are interrelated with respect to absorption and metabolic function. They have an inverse relationship with respect to concentration; serum phosphorus is increased when serum calcium is decreased.

Hyperphosphatemia can result in an infant fed only cow’s milk during the first few weeks of life because of the combination of a high phosphorus content in cow’s milk and the inability of infants’ kidneys to clear the excess phosphorus.

This procedure is contraindicated for

N/A

Indications

Assist in establishing a diagnosis of hyperparathyroidism
Assist in the evaluation of renal failure

Potential diagnosis

Increased in

Acromegaly (related to increased renal absorption)
Bone metastases (related to release from bone stores)
Diabetic ketoacidosis (acid-base imbalance causes intracellular phosphorus to move into the extracellular fluid)
Excessive levels of vitamin D (vitamin D promotes intestinal absorption of phosphorus; excessive levels promote phosphorus release from bone stores)
Hyperthermia (tissue damage causes intracellular phosphorus to be released into circulation)
Hypocalcemia (calcium and phosphorus have an inverse relationship)
Hypoparathyroidism (related to increased renal absorption)
Lactic acidosis (acid-base imbalance causes intracellular phosphorus to move into the extracellular fluid)
Milk alkali syndrome (increased dietary intake)
Pseudohypoparathyroidism (related to increased renal absorption)
Pulmonary embolism (related to respiratory acid-base imbalance and compensatory mechanisms)
Renal failure (related to decreased renal excretion)
Respiratory acidosis (acid-base imbalance causes intracellular phosphorus to move into the extracellular fluid)

Decreased in

(related to decreased circulating calcium in calcium crystal–induced gout; calcium and phosphorus have an inverse relationship)

(related to malnutrition)

(calcium and phosphorus have an inverse relationship)

(insulin increases intracellular movement of phosphorus)

(parathyroid hormone [PTH] increases renal excretion)

(decreases return of phosphorus to general circulation)

(related to insufficient intestinal absorption of phosphorus)

(related to deficient intake)

(evidenced by hypophosphatemia)

(PTH increases renal excretion)

(related to decreased renal absorption)

(related to vitamin D deficiency)

(related to excessive loss)

(related to vitamin D deficiency, which reduces intestinal and renal tubular absorption of phosphorus)

Critical findings

Adults
Less than 1.0 mg/dL (SI: Less than 0.3 mmol/L)
Greater than 8.9 mg/dL (SI: Greater than 2.9 mmol/L)
Children
Less than 1.3 mg/dL (SI: Less than 0.4 mmol/L)
Greater than 8.9 mg/dL (SI: Greater than 2.9 mmol/L)
Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.
It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.
Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.
Interventions including IV replacement therapy with sodium or potassium phosphate may be necessary. Close monitoring of both phosphorus and calcium is important during replacement therapy.

Interfering factors

Drugs that may increase phosphorus levels include anabolic steroids, β-adrenergic blockers, ergocalciferol, furosemide, hydrochlorothiazide, methicillin (occurs with nephrotoxicity), oral contraceptives, parathyroid extract, phosphates, sodium etidronate, tetracycline (occurs with nephrotoxicity), and vitamin D.
Drugs that may decrease phosphorus levels include acetazolamide, albuterol, aluminum salts, amino acids (via IV hyperalimentation), anesthetic agents, anticonvulsants, calcitonin, epinephrine, fibrin hydrolysate, fructose, glucocorticoids, glucose, insulin, mannitol, oral contraceptives, pamidronate, phenothiazine, phytate, and plicamycin.
Serum phosphorus levels are subject to diurnal variation: They are highest in late morning and lowest in the evening; therefore, serial samples should be collected at the same time of day for consistency in interpretation.
Hemolysis will falsely increase phosphorus values.
Specimens should never be collected above an IV line because of the potential for dilution when the specimen and the IV solution combine in the collection container, thereby falsely decreasing the result. There is also the potential of contaminating the sample with the substance of interest if it is present in the IV solution, thereby falsely increasing the result.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in a general evaluation of body systems.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s endocrine, gastrointestinal, genitourinary, and musculoskeletal systems; symptoms; and results of previously performed laboratory tests and diagnostic and surgical procedures.
Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

Potential complications: N/A
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
Nutritional Considerations: Severe hypophosphatemia is common in elderly patients or patients who have been hospitalized for long periods of time. Good dietary sources of phosphorus include meat, dairy products, nuts, and legumes. To decrease phosphorus levels to normal in the patient with hyperphosphatemia, dietary restriction may be recommended. Other interventions may include the administration of phosphate binders or calcitriol (the activated form of vitamin D).
Nutritional Considerations: Vitamin D is necessary for the body to absorb phosphorus. Educate the patient with vitamin D deficiency, as appropriate, that the main dietary sources of vitamin D are cod liver oil and fortified dairy foods such as milk, cheese, and orange juice. Explain to the patient that vitamin D is also synthesized by the body, in the skin, and is activated by sunlight.
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family. Educate the patient regarding access to nutritional counseling services. Provide contact information, if desired, for the Institute of Medicine of the National Academies (www.iom.edu).
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include biopsy bone, blood gases, BUN, calcitonin, calcium, calculus kidney stone panel, carbon dioxide, chloride, collagen cross-linked N-telopeptides, CBC WBC count and differential, creatinine, fecal analysis, fecal fat, FDP, glucagon, glucose, GH, insulin, lactic acid, lung perfusion scan, lung ventilation scan, osmolality, osteocalcin, PTH, parathyroid scan, phosphorus urine, potassium, US abdomen, and vitamin D.
Refer to the Endocrine, Gastrointestinal, Genitourinary, and Musculoskeletal systems tables at the end of the book for related tests by body system.