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Introduction ⬇

Osmolality, a more exact measurement of urine concentration than SG, depends on the number of particles of solute in a unit of solution. More information concerning kidney function can be obtained if serum and urine osmolality tests are run at the same time. The normal ratio between urine and serum osmolality is 3:1. A high urine-to-serum ratio is seen with concentrated urine. With poor concentrating ability, the ratio is low.

Whenever a precise measurement is indicated to evaluate the concentrating and diluting ability of the kidney, this test is done. Urine osmolality during water restriction is an accurate test of decreased kidney function. It is also used to monitor the course of kidney disease; to monitor fluid and electrolyte therapy; to establish the differential diagnosis of hypernatremia, hyponatremia, and polyuria; and to evaluate the kidney’s response to ADH.

Normal Findings ⬆ ⬇

24-hour specimen: 500–800 mOsm/kg of H2O

Random specimen: 50–1200 mOsm/kg of H2O

Urine-to-serum ratio: 1:1–3:1

Procedure ⬆ ⬇

  1. Inform the patient whether the test will be done using a random specimen or 24-hour urine specimen.

  2. For the 24-hour test, follow procedure for long-term, timed urine specimen (2-hour, 24-hour).

  3. Simultaneous determination of serum osmolality may be done. A high urine-to-serum ratio is seen with concentrated urine.

Clinical Implications ⬆ ⬇

  1. Osmolality is increased in:

    1. Prerenal azotemia

    2. Heart failure

    3. Addison disease

    4. Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

    5. Dehydration

    6. Amyloidosis

    7. Hyponatremia

  2. Osmolality is decreased in:

    1. Acute kidney injury

    2. Diabetes insipidus

    3. Hypokalemia

    4. Hypernatremia

    5. Primary polydipsia

    6. Hypercalcemia

    7. Compulsive water drinking (causing increased fluid intake)

  3. Urine-to-serum ratio is:

    1. Increased in prerenal azotemia

    2. Decreased in acute tubular necrosis

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for urine collection, and interfering factors.

  2. To increase sensitivity of the osmolality test, a fasting urine specimen may be collected, which requires a high-protein diet for 3 days before the test. On the day before the test, no liquids should be taken with the evening meal, and no food or liquids should be taken after the evening meal until collection. Check with your laboratory if the patient has diabetes.

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

Posttest Patient Care

  1. Provide patient with food and fluids as soon as the last urine sample is obtained.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings. Monitor appropriately.

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

Interfering Factors ⬆

  1. Intravenous sodium administration

  2. Intravenous dextrose and water administration