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Water deprivation test


Also called: Dehydration Test; Concentration Test




Specimen or type of test:


Urine



Purpose of the test


The water deprivation test is performed to diagnose diabetes insipidus (DI) and to assess the kidney’s ability to concentrate urine based on extracellular fluid load.



Basics the nurse needs to know


Normally, as fluid intake is withheld, blood osmolality increases, urine output decreases, and urinary osmolality increases. The increase in serum osmolality causes an increase in antidiuretic hormone (ADH) secretion. In patients with DI, a normal response to increased plasma osmolality does not occur; instead, little or no increase in ADH occurs, resulting in little or no change in urinary output or osmolality.


DI may be caused by a defect in production, release, or use of ADH. If DI is a result of a problem in production (hypothalamic) or release (pituitary) of ADH, it is called neurogenic or central DI. If DI is caused by a failure of the kidney to respond to ADH, it is called nephrogenic DI. The water deprivation test supports the diagnosis of diabetes insipidus.


As part of the water deprivation test, a vasopressin stimulation test or ADH stimulation test may be performed to distinguish between neurogenic and nephrogenic DI. This distinction is important in determining appropriate treatment plans.




REFERENCE VALUES


Osmolality: >400 mOsm/kg or SI: >400 mmol/kg



How the test is done


During the test, the patient is deprived of fluid intake, and periodic urine specimens are obtained to determine osmolality and the specific gravity. The urine is collected in separate clean containers and placed on ice or refrigerated. Strict urinary output measurements are maintained.


If a vasopressin stimulation test is included, hypertonic saline or nicotine is given to stimulate ADH release. If complete neurogenic DI is present, no change is noted in urinary output or osmolality. If partial neurogenic DI is present, only minor changes occur.


If desired, a vasopressin test may be performed. After vasopressin is given, no change will occur in urinary output or osmolality if nephrogenic DI is present. With central DI, the urine osmolality will increase and the urinary output will decrease.



Significance of test results



If no change in urine osmolality exists, the diagnosis of DI is supported.

If the urine osmolarity increased by more than 10% after vasopressin is given, the diagnosis of central/neurogenic DI is supported.


Interfering factors



• Noncompliance with fluid restrictions

• Inability to complete test because of hypovolemia

• Glucosuria

• Administration of radiopaque dyes within 7 days



NURSING CARE


Nursing actions related to timed urine collection and pediatric urine collection procedures are presented in Chapter 2, with the following additional measures.



Pretest



• The nurse assesses the patient’s hemodynamic status. If a vasopressin test is planned, check for a history of coronary artery disease because vasopressin may cause coronary artery spasm.

• Obtain a history of previous 24-hour periods of fluid intake. Report/document if adequate hydration did not take place.

• Obtain baseline serum and urine specimens to determine osmolality.

• Baseline weight is obtained before the evening meal on the day before testing.


During the test



• The nurse observes for early signs of hypovolemia (tachycardia, orthostatic hypotension).

• Observe the patient to ensure compliance with nothing-by-mouth status.

• Obtain a urine specimen every 2 hours. Label each specimen with the time and the amount obtained.

• The nurse weighs the patient every 2 to 4 hours. Maintain the patient on nothing-by-mouth status until 2% to 5% of the patient’s weight is lost (this takes approximately 6 to 12 hours).

• After 2% to 5% of patient body weight is lost and urinary output continues with urinary osmolality plateauing, an ADH stimulation test may be performed by administering hypertonic saline (3% sodium chloride) or nicotine, as ordered.

• If a vasopressin test is to be performed, check the patient’s blood pressure and document it; notify the physician if the patient is hypertensive. Aqueous vasopressin is given subcutaneously or intravenously. One hour after vasopressin administration, collect a urine specimen for amount and osmolality.





image Nursing response to critical values


Hypovolemia may occur with the water deprivation test. Patients with DI will continue to put out urine, even though they have no intake. If a vasopressin test is performed, the administration of vasopressin may produce the complications of high blood pressure or coronary artery spasms, or both.



Hypovolemia.

Tachycardia, restlessness, poor skin turgor, and hypotension are indications of hypovolemia. If these occur, the nurse notifies the physician and anticipates the test being discontinued and fluids being given.



Coronary artery spasms.

With spasm of the coronary artery, perfusion will be affected. The patient may complain of chest pain. The physician is informed and the nurse anticipates that the order for vasopressin to be discontinued.



Water-loading test




Specimen or type of test:


Plasma, Urine



Purpose of the test


The water-loading test is performed to diagnose the syndrome of inappropriate secretion of the antidiuretic hormone (SIADH).



Basics the nurse needs to know


Review the discussion of Plasma Vasopressin (pp. 624). Normally, with an increase in fluid intake, urinary output will increase to maintain a normal plasma osmolality. As the urine volume increases, its osmolality decreases. However, patients with syndrome of inappropriate antidiuretic hormone (SIADH) will not respond to increasing fluid intake.




REFERENCE VALUES


Urinary output increases, and plasma and urinary osmolality decrease.



How the test is done


With the water-loading test, the patient orally ingests a water load of 20 to 25 mL/kg of body weight. Hourly serum and urine osmolality and urine outputs are recorded for 4 hours.



Significance of test results


Little or no change in urinary output or plasma and urine osmolality readings supports the diagnosis of SIADH.



Interfering factors



• Patient is unable to drink the required volume of fluid.

• Medications such as demeclocycline, diuretics, and lithium carbonate.



NURSING CARE



Pretest



• The nurse assesses the patient for hyponatremia.

• Obtain the patient’s cardiac history, the results of which may require that the test be canceled.

• Weigh the patient.


During the test



• Instruct the patient to drink the required fluid.

• Obtain hourly output measurements and send blood and urine to the laboratory to determine the osmolality. On the requisition slip, indicate the hour the specimen was taken, with zero hour being the time the patient ingested the fluid (see discussion of Osmolality, Plasma [p. 464] and Osmolality, Urine [p. 465] for the nursing procedures associated with the collection of these samples).

• The nurse observes the patient for water intoxication.


Posttest



• Weigh the patient.





image Nursing response to critical values


When patients with SIADH undergo the water-loading test, their output is not increased. The increased fluid load in the extracellular fluid can cause dilutional hyponatremia, also called water intoxication.



Dilutional hyponatremia.

Indications of water intoxication are hyponatremia, lethargy, confusion, muscular twitching, stupor and, eventually, convulsions and death. The nurse will observe closely for changes in mental status and report these to the physician. The nurse should anticipate an order for a serum sodium level. If the sodium level is less than 120 mEq/L, the nurse would notify the physician immediately.



White blood cell count


Also called: WBC, Leukocyte Count, White Count




Specimen or type of test:


Blood



Purpose of the test


The white blood cell count indicates the possible presence and severity of infection or inflammatory response. This test may be part of the complete blood count, performed as a routine screening test or it may be performed separately to evaluate a specific problem. The leukocyte count is a general indicator of infection, tissue necrosis, inflammation, and bone marrow activity.

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Feb 18, 2017 | Posted by in NURSING | Comments Off on W

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