Diagnostic Approach for Diabetes Insipidus

Initial Evaluation

The diagnostic approach for diabetes insipidus requires measurement of serum sodium, serum osmolality, and urine osmolality as initial biochemical work-up, with inappropriately dilute urine (urine osmolality <200 mOsm/kg H₂O) combined with high-normal or elevated serum sodium being pathognomonic for diabetes insipidus 1
Suspect diabetes insipidus in patients presenting with polyuria, polydipsia, failure to thrive (in children), and hypernatremic dehydration 2, 1
Obtain a comprehensive family history and construct a pedigree to identify potential familial cases 2
Initial laboratory tests should include serum sodium 2 and urine osmolality 2, 1

For suspected nephrogenic diabetes insipidus: morning urine osmolality test after overnight fluid avoidance 3
Concentrations above 600 mOsm/L rule out diabetes insipidus 3
Indicated for patients urinating >2.5 L per 24 hours despite attempts to reduce fluid intake 3

Strongly recommended for suspected nephrogenic diabetes insipidus, especially in children 2, 1
Can provide early and definitive diagnosis with important implications for management 2
Recommended genetic testing panel should include at least AQP2, AVPR2, and AVP genes 2

Genetic testing can help avoid unpleasant, challenging, and potentially harmful diagnostic procedures like water deprivation test 2
Early diagnosis prevents prolonged, unrecognized periods of severe hypertonic dehydration 2

The American Academy of Nephrology recommends measuring plasma copeptin instead of direct AVP in adults, as copeptin is released in equimolar 1:1 ratio with AVP and is more stable, with baseline plasma copeptin >21.4 pmol/L being diagnostic for nephrogenic diabetes insipidus in adults 4
Direct AVP measurement is technically challenging because the hormone is unstable and the assay is cumbersome, and for adults, measuring plasma copeptin is recommended as an alternative, with plasma copeptin <21.4 pmol/L suggesting central diabetes insipidus or primary polydipsia and requiring further testing with hypertonic saline or arginine infusion 4
Genetic testing is now strongly recommended as first-line for suspected nephrogenic diabetes insipidus, particularly in children, as it can avoid the need for challenging and potentially harmful diagnostic procedures like water deprivation testing 4