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Difficulty: HardEndocrinology and Metabolism

Match each set of laboratory results from a water deprivation test (followed by desmopressin administration) to the most likely pathophysiologic diagnosis.

  • Baseline serum osmolality: 271 mOsm/kg. Peak urine osmolality after water deprivation: 680 mOsm/kg. Urine osmolality after desmopressin: 690 mOsm/kg.Primary polydipsia
  • Baseline serum osmolality: 298 mOsm/kg. Peak urine osmolality after water deprivation: 130 mOsm/kg. Urine osmolality after desmopressin: 610 mOsm/kg.Complete central diabetes insipidus
  • Baseline serum osmolality: 299 mOsm/kg. Peak urine osmolality after water deprivation: 140 mOsm/kg. Urine osmolality after desmopressin: 145 mOsm/kg.Complete nephrogenic diabetes insipidus
  • Baseline serum osmolality: 292 mOsm/kg. Peak urine osmolality after water deprivation: 340 mOsm/kg. Urine osmolality after desmopressin: 520 mOsm/kg.Partial central diabetes insipidus

Answer

The correct matches are: Profile 1 to Primary polydipsia; Profile 2 to Complete central diabetes insipidus; Profile 3 to Complete nephrogenic diabetes insipidus; Profile 4 to Partial central diabetes insipidus.
Correct matching relies on a stepwise analysis of the water deprivation test. First, baseline serum osmolality differentiates overhydration (primary polydipsia) from dehydration (diabetes insipidus). Second, the response to water deprivation tests the integrity of the entire ADH axis (pituitary release and renal response). Finally, the response to exogenous desmopressin isolates the defect: a robust response means the kidneys work but lack endogenous ADH (central), while no response means the kidneys are resistant (nephrogenic).

Step-by-Step Solution

1
Evaluate the baseline serum osmolality.
Low serum osmolality (<285 mOsm/kg) points to primary polydipsia (water intoxication). High or high-normal serum osmolality (>295 mOsm/kg) points to a form of diabetes insipidus (water loss).
This establishes whether the polyuria is driven by excessive intake or excessive renal loss.
2
Assess the peak urine osmolality after fluid restriction (water deprivation).
If urine concentrates appropriately (>600 mOsm/kg), the ADH axis is intact. If it remains dilute (<300 mOsm/kg), complete diabetes insipidus is present. A submaximal concentration (300-600 mOsm/kg) suggests a partial defect.
Water deprivation stimulates endogenous ADH release. A failure to concentrate urine indicates either lack of hormone or lack of renal response.
3
Evaluate the change in urine osmolality following exogenous desmopressin administration.
A significant increase in urine osmolality (>50%) indicates central DI. A minimal or absent response (<10%) indicates nephrogenic DI.
Providing exogenous ADH bypasses the pituitary gland, directly testing the renal collecting duct's ability to respond to the hormone.

Key Concept

Interpretation of the water deprivation and desmopressin stimulation test in the differential diagnosis of polyuria.
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Topics

Endocrinology and Metabolism

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