From the Guidelines
In patients with polydipsia, urine osmolality is typically less than 200 mOsm/kg, indicating inappropriately diluted urine. This is based on the most recent expert consensus statement on the diagnosis and management of congenital nephrogenic diabetes insipidus, which recommends measuring urine osmolality as part of the initial work-up for suspected diabetes insipidus 1. The detection of inappropriately diluted urine, in combination with high–normal or elevated serum sodium, is pathognomonic for the diagnosis of diabetes insipidus (nephrogenic or central) and warrants early genetic testing if NDI is suspected 1. Key points to consider in the diagnostic approach include:
- Measuring serum sodium, serum osmolality, and urine osmolality
- Suspecting NDI in infants and children who present with polyuria, polydipsia, failure to thrive, and hypernatraemic dehydration, and in adults with unexplained polyuria and polydipsia
- Constructing a comprehensive family history and pedigree to identify family cases In polydipsia, the kidneys are functioning normally but are responding appropriately to the excessive fluid intake by producing dilute urine to maintain fluid balance, resulting in low urine osmolality, usually less than 200 mOsm/kg, as recommended by the international expert consensus statement 1.
From the Research
Urine Osmolality in Polydipsia
- Urine osmolality is an important factor in diagnosing and differentiating polydipsia from other conditions such as central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI) 2, 3, 4.
- In patients with primary polydipsia, urine osmolality is typically low due to the excessive water intake, whereas in CDI, urine osmolality is also low due to the lack of antidiuretic hormone (ADH) 3, 4.
- A study found that desmopressin treatment can be effective in patients with dipsogenic polydipsia, a distinct variety of primary polydipsia, by keeping serum osmolality marginally below the thirst threshold for symptomatic relief 2.
- The diagnosis of polydipsia and other related conditions can be made through a series of steps, including clinical observation, measurement of plasma osmolality and sodium, and a dehydration test to assess urine osmolality 3.
- Urine osmolality can be used to differentiate between primary polydipsia and NDI, with NDI typically having a higher urine osmolality due to the kidney's inability to respond to ADH 3, 4.
- A study found that the measurement of urinary vasopressin and osmolality can be a useful diagnostic tool in differentiating between primary polydipsia and CDI, although it may not be as sensitive as plasma vasopressin measurement 5.
Diagnostic Approaches
- A dehydration test can be used to assess urine osmolality and differentiate between primary polydipsia and other conditions such as CDI and NDI 3, 4.
- The measurement of plasma vasopressin and osmolality can be a powerful diagnostic tool in differentiating between polyuria/polydipsia syndromes 5.
- Desmopressin treatment can be used to diagnose and treat CDI, and can also be effective in patients with primary polydipsia 2, 6.
- Urinary vasopressin measurement can be a useful diagnostic alternative to plasma vasopressin measurement, although it may not be as sensitive 5.
Treatment Options
- Desmopressin treatment can be effective in patients with CDI and primary polydipsia, by keeping serum osmolality marginally below the thirst threshold for symptomatic relief 2, 6.
- The dosage of desmopressin can vary widely between individuals, and individual titration is important to maintain adequate antidiuretic effect 6.