Sodium (Na, Na⁺) is the most abundant cation in extracellular fluid and, along with its accompanying chloride and bicarbonate anions, accounts for 92 percent of serum osmolality. Sodium plays a major role in maintaining homeostasis through a variety of functions, which include (1) maintenance of osmotic pressure of extracellular fluid, (2) regulation of renal retention and excretion of water, (3) maintenance of acid-base balance, (4) regulation of potassium and chloride levels, (5) stimulation of neuromuscular reactions, and (6) maintenance of systemic blood pressure. Serum sodium levels may be affected by a variety of disorders and drugs (Table 5-22) and are evaluated in relation to other serum electrolyte and blood chemistry results. Tests of urinary sodium and osmolality also may be necessary for complete interpretation. Note that falsely decreased serum sodium levels may occur with elevated serum triglyceride levels and myeloma proteins.

• Elevated serum triglyceride levels and myeloma proteins may lead to falsely decreased levels.
• Adrenocorticosteroids, methyldopa, hydralazine, reserpine, and cough medicines may lead to increased levels.
• Lithium, vasopressin, and diuretics may lead to decreased levels.

• Routine electrolyte screening in acute and critical illness
• Determination of whole body stores of sodium, because the ion is predominantly extracellular
• Known or suspected disorder associated with altered fluid and electrolyte balance (see Table 5-22)
• Estimation of serum osmolality, which is normally 285 to 310 mOsm/kg, by using the following formula, where BUN stands for blood urea nitrogen:
  Serum osmolality =2(Na⁺) +(glucose/20) +BUN/3
  • Note: If the value for serum osmolality is greater than 2.0 to 2.3 times the value for serum sodium, then hyperglycemia, uremia, or metabolic acidosis should be suspected.
• Evaluation of the effects of drug therapy on serum sodium levels (e.g., diuretic therapy)

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 sodium 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 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. Assess for symptoms associated with hypernatremia, such as fluid deficit with thirst; dry mucous membranes and skin; poor skin turgor; or fluid excess with edema, weight gain, or elevated blood pressure. Administer fluid replacement at an ordered rate and time if the client is dehydrated or if diuretics are administered for fluid excess. Instruct client in a low-sodium diet.
• Decreased levels: Note and report decreased levels. Assess for symptoms associated with hyponatremia, such as oliguria, rapid pulse, abdominal cramping, fluid retention, weight gain, edema, or elevated blood pressure. Administer ordered sodium replacement via IV or dietary intake. Instruct client in sodium intake (food, salt tablets) to replace and maintain sodium, especially if this electrolyte is lost because of vomiting, diarrhea, perspiration, or use of diuretics.
• Critical values: Notify the physician at once of levels less than 120 mEq/L or greater than 160 mEq/L.