Osmolality refers to the concentration of solutes in plasma or serum (particle number) or in urine (number of particles). Osmolality affects the movement of fluids across body membranes and the kidney's ability to concentrate or dilute the urine. Dehydration causes an increase in osmolality, and overhydration causes a decrease. Increased osmolality causes an increase in antidiuretic hormone (ADH) secretion, which results in increased reabsorption of water by the kidneys, increased concentration in urine, and decreased concentration in serum. This lower concentration in serum osmolality normally reduces ADH secretion, which then decreases water reabsorption by the kidneys and excretion of diluted urine. Urine is normally more concentrated than plasma; the ratio of urine to serum osmolality ranges from 1:1 to 3:1 in normal states. A decrease in the 1:1 ratio is seen in fluid overload or in diabetes insipidus, and a ratio that does not rise above 1.2:1.0 indicates a loss of renal concentration function.⁶³

Serum osmolality is mostly used to monitor fluid and electrolyte balance; urine osmolality is used to monitor the concentrating ability of the kidneys and fluid and electrolyte balance. Decreased levels in serum osmolality are seen in fluid excess or overhydration, hyponatremia, and syndrome of inappropriate ADH (SIADH) secretion. Decreased levels in urine osmolality are seen in diabetes insipidus, excess fluid intake or overhydration, hypokalemia, hypercalcemia, and severe renal disease. Increased levels in serum osmolality are seen in dehydration, hypercalcemia, hypernatremia, hyperglycemia, diabetes insipidus, ketosis, severe renal disease, alcohol ingestion, and mannitol therapy. Increased levels in urine osmolality are seen in Addison's disease, SIADH, hypernatremia, shock or acidotic states, and CHF.

• A delay of longer than 10 hours in testing the specimen can affect test results.
• Osmotic diuretics and mineralocorticoids can affect test results.
• Improper technique such as tourniquet in place for an extended time can cause hemostasis.

• Screening for alcohol ingestion revealed by increase in osmolality level as the alcohol level in the blood is increased
• Monitoring fluid and electrolyte balance (especially sodium) and determining a state of dehydration or overhydration
• Evaluating ADH secretion or suppression
• Monitoring IV fluid replacement therapy to prevent fluid excess or overload
• Evaluating the effect of renal dialysis therapy and course of renal failure

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).

• Obtain a history of conditions affecting renal function, a medication regimen, and the results of an electrolyte panel. Any medications that may affect test results should be withheld, although this practice should be confirmed with the person ordering the study.
• Inform the client that a urine specimen can be collected, tested, and compared with the results of the blood test.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis or hemostasis 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 dehydration, such as thirst, dry skin and mucous membranes, and poor skin turgor. Assess relationship to urine osmolality and electrolyte panel. Prepare to increase fluid intake orally or IV.
• Decreased levels: Note and report decreased levels. Assess for symptoms associated with overhydration such as edema, weight gain, dyspnea, cough, and venous distention. Assess relationship to urine osmolality and electrolyte panel. Administer ordered medications such as diuretic therapy.
• Critical values: Notify physician at once of levels less than 240 mOsm or greater than 360 mOsm.