Applying the stop rule
A water deprivation test is stopped when body weight falls by more than 3% of the starting weight or when serum sodium rises above 145 mEq/L, because either signals significant dehydration. Three percent of 70.0 kg is 2.1 kg, so the test stops once weight falls below 67.9 kg. At hour 6 his weight is 67.7 kg, a loss of 2.3 kg, which is 3.3%. The weight rule is met at hour 6, so the nurse stops the test then, even though sodium is still 145 mEq/L.
Working through each hour
At hour 4, the loss is 1.6 kg, or 2.3%, below the limit, and sodium is 145 mEq/L, which is not above 145. At hour 5, the loss is 2.0 kg, or 2.9%; rounding up to 3% does not make it exceed 3%. At hour 6, the loss of 2.3 kg (3.3%) exceeds the limit. At hour 7, sodium first rises above 145 to 146 mEq/L, but by then the weight rule had already been met an hour earlier.
| Time | Weight | Loss | Sodium | Stop? |
|---|
| Start | 70.0 kg | — | 144 | — |
| Hour 4 | 68.4 kg | 1.6 kg (2.3%) | 145 | No |
| Hour 5 | 68.0 kg | 2.0 kg (2.9%) | 145 | No |
| Hour 6 | 67.7 kg | 2.3 kg (3.3%) | 145 | Yes |
| Hour 7 | 66.9 kg | 3.1 kg (4.4%) | 146 | Already passed |
Why waiting is unsafe
A client with severe polyuria, here about 6 liters a day, continues to lose large volumes of dilute urine during the test because he cannot concentrate it. Dehydration can progress quickly, causing hypotension, tachycardia, and dangerous hypernatremia. Waiting for the sodium to rise lets this progress. Watch out! Rounding is a common trap: 2.9% is not more than 3%, and either criterion alone is enough to stop the test.
Nursing role during the test
The nurse weighs the client accurately on the same scale at set intervals, measures urine volume and specific gravity or osmolality, monitors vital signs and serum sodium, and ensures the client does not drink during the test. Key point! Stop a water deprivation test as soon as either limit is exceeded: weight loss over 3% or sodium above 145 mEq/L.