Water Loading Test

Norm of Water Loading Test

≥500-mL urine output over 4 hours after water ingestion.
Urine osmolality < serum osmolality or <180 mOsm/kg by 5 hours after water ingestion

 

Usage of Water Loading Test

Diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADHS).

 

Description of Water Loading Test

The water loading test involves administering a large quantity of water and then comparing the osmolality of timed urine and serum collections. In a normal client, increased fluid intake increases urine output and decreases urine osmolality. In clients with SIADHS, however, excess secretion of antidiuretic hormone causes a lower-than-normal urine output in response to the water loading and a urine osmolality that does not decrease below serum osmolality.

 

Professional Considerations of Water Loading Test

Consent form NOT required.

Risks
Fluid overload, congestive heart failure. Complications of nasogastric tube insertion include bleeding, dysrhythmias, esophageal perforation, laryngospasm, and decreased mean pO2.
Contraindications and Precautions
Perform with extreme caution in clients with a history of congestive heart failure.

 

Preparation

1. The baseline serum sodium level should be at least 125 mEq/L before this test is started.
2. Withhold diuretics for 12 hours before the test.
3. Tube: Six red topped, red/gray topped, or gold topped.
4. Also obtain six clean plastic specimen containers.
5. Insert a nasogastric tube if the client will be unable to drink 1 L of water over a short period of time.

 

Procedure

1. Draw a 4-mL blood sample for the baseline serum osmolality. Obtain a 20-mL random urine sample in a clean plastic container for baseline urine osmolality.
2. Have the client drink 1 L of water, or 20 mL/kg of body weight, over 15–20 minutes, or instill it through a nasogastric tube.
3. Document the quantity of urine output, starting with the time of water ingestion andending 5 hours later.
4. Obtain samples for serum and urine osmolality as in step 1 every hour for 5 hours. Label each tube sequentially, and write the collection time on the label.

 

Postprocedure Care

1. Refrigerate the serum samples if they are not tested within 4 hours.
2. Refrigerate all urine samples until they are tested.

 

Client and Family Teaching

1. The client will be asked to drink or have instilled at least 1 L of water within 20 minutes.

 

Factors That Affect Results

1. Diuretics administered within 12 hours before the test invalidate the results.

 

Other Data

1. None.