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Introduction ⬇

Osmolality is the measure of the number of dissolved solute particles in solution. It increases with dehydration and decreases with overhydration. In general, the same conditions that reduce or increase serum sodium affect osmolality.

The water-load test is used as an evaluation of water and electrolyte balance. It is helpful in assessing hydration status, seizures, liver disease, ADH function, and coma, and it is used in toxicology workups for ethanol, ethylene glycol, isopropanol, and methanol ingestions.

Procedure ⬆ ⬇

  1. Determining osmolality:

    1. Obtain a 5-mL venous blood sample in an SST or red-topped tube. Serum or heparinized plasma is acceptable. Observe standard/universal precautions. Label samples with the patient’s name, date, and test(s) ordered.

    2. Collect a 24-hour urine specimen (see Chapter 3) concurrently and keep refrigerated.

    3. Determine osmolality in the laboratory using the freezing point depression methodology for both serum and urine.

  2. Determining water-loading ADH suppression:

    1. The ideal position during the testing period is the recumbent position because the response to water loading is reduced in persons in the upright position.

    2. One hour before testing, the patient is given 300 mL of water to replace fluid lost during the overnight fast. Do not count this water as part of the test load.

    3. Have the patient drink a test load of water (20 mL/kg body weight) within 30 minutes.

    4. After the test load of water is consumed, collect all urine for the next 4–5 hours, and check each voiding for volume osmolality and specific gravity. Obtain hourly blood samples for osmolality and check the entire volume of urine obtained for osmolality.

  3. Remember that normal values for water-loading ADH suppression test are excretion of >90% (>0.90) of water load within 4 hours. Urine osmolality falls to <100 mOsm/kg (<100 mmol/kg). Specific gravity falls to 1.001.

  4. Determine plasma ADH at hourly intervals.

Clinical Implications ⬆ ⬇

Decreased Kidney Function

  1. With decreased kidney function, <80% of fluid is excreted, and urine specific gravity may not fall below 1.010. This phenomenon occurs in the following conditions:

    1. Adrenocortical insufficiency

    2. Malabsorption syndrome

    3. Edema

    4. Ascites

    5. Obesity

    6. Hypothyroidism

    7. Dehydration

    8. Heart failure

    9. Cirrhosis

  2. Disorders with increased ADH secretion (SIADH) give an inadequate response; <90% of water is excreted, and urine osmolality remains >100 mOsm/kg H2O (>100 mmol/kg H2O). Plasma ADH measured at 90 minutes confirms diagnosis of SIADH.

Hyperosmolality and Hypoosmolality

  1. Increased values (hyperosmolality) are associated with the following conditions:

    1. Dehydration

    2. Hypercalcemia

    3. Diabetes, hyperglycemia, diabetic ketoacidosis

    4. Hypernatremia

    5. Cerebral lesions

    6. Alcohol ingestion (ethanol, methanol, ethylene glycol)

    7. Mannitol therapy

    8. Azotemia (high levels of nitrogen-containing compounds; e.g., urea or creatinine)

    9. Inadequate water intake

    10. Chronic kidney disease

  2. Decreased values (hypoosmolality) are associated with the following conditions:

    1. Loss of sodium with diuretics and low-salt diet (hyponatremia)

    2. Kidney disease

    3. Adrenocortical insufficiency

    4. Inappropriate secretion of ADH, as may occur in trauma and lung cancer

    5. Excessive water replacement (overhydration, water intoxication)

    6. Panhypopituitarism

    7. DI (central or nephrogenic)

    8. Pyelonephritis

Osmolal Gap

  1. Abnormal levels (>10 mOsm/kg H2O or >10 mmol/kg H2O) can occur in the following conditions:

    1. Methanol

    2. Ethanol

    3. Isopropyl alcohol

    4. Mannitol

    5. Severely ill patients, especially those in shock, lactic acidosis, and acute kidney injury

  2. Ethanol glycol, acetone, and paraldehyde have relatively small osmolal gaps, even at lethal levels.

Interventions ⬆ ⬇

Pretest Patient Care

Decreased Kidney Function

  1. Explain the test purpose and procedure. The test takes 5–6 hours to complete.

  2. Do not allow food, alcohol, medications, or smoking for 8–10 hours before testing. No muscular exercise is allowed during the test.

  3. The patient may experience nausea, abdominal fullness, fatigue, and desire to defecate.

  4. Discard first morning urine specimen.

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

Hyperosmolality, Hypoosmolality, and Osmolar Gap

  1. Explain test purpose and procedure.

  2. Ensure that no alcohol is ingested during the 24 hours before the test.

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

Posttest Patient Care

Decreased Kidney Function

  1. Observe for adverse reactions to water-loading test such as extreme abdominal discomfort, shortness of breath, or chest pain.

  2. Remember that if water clearance is impaired, the water load will not induce diuresis, and maximum urinary dilution will not occur.

  3. Accurate results may not be obtained if nausea, vomiting, or diarrhea occurs or if a disturbance in bladder emptying is present. Note on chart if any of these effects occur.

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

Clinical Alert

In patients with impaired ability to tolerate the water-loading test, seizures or fatal hyponatremia may occur

Hyperosmolality, Hypoosmolality, and Osmolar Gap

  1. Review test results; report and record findings. Modify the nursing care plan as needed. A patient receiving IV fluids should have a normal osmolality. If the osmolality increases, the fluids contain relatively more electrolytes than water. If it falls, relatively more water than electrolytes is present.

  2. Remember that if the ratio of serum sodium to serum osmolality falls below 0.43, the outlook is guarded. This ratio may be distorted in cases of drug intoxication.

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

Interfering Factors ⬆ ⬇

Osmolal Gap

  1. Decreases in osmolal gap are associated with altitude, diurnal variation with water retention at night, and some drugs.

  2. Some drugs also cause increases in osmolal gap.

  3. Hypertriglyceridemia and hyperproteinemia cause an elevated osmolal gap.

  4. X-ray contrast medium within 3 days.

Reference Values ⬆

Normal

Clinical Alert

Critical Values
Serum osmolality:

  • <240 or >321 mOsm/kg H2O (<240 or >321 mmol/kg H2O)

  • >385 mOsm/kg H2O (>385 mmol/kg H2O) is seen with symptoms of stupor in hyperglycemia.

  • 400–420 mOsm/kg H2O (400–420 mmol/kg H2O) is associated with grand mal seizures.

  • >420 mOsm/kg H2O (>420 mmol/kg H2O) is often fatal.