Table 1.
Pathophysiologic findings in hypo-osmolar hyponatremia: Differentiation of dilutional versus depletional mechanisms.
| Hypo-osmolar hyponatremia | Blood urea nitrogen (mg/dL) | Urine osmolality (mOsm/kg H20) | Urine sodium (mEq/L) | AVP (ng/L) | |
|---|---|---|---|---|---|
| Dilutional | |||||
| • Polydipsia | <15 | <100 | Variable | <1.0 | |
| • SIADH | <15 | >100 | >30 | >1.0 | |
|
| |||||
| Depletional | >15 | >300 | <30 | <30 | |
The underlying mechanism of hypo-osmolar hyponatremia can be assessed according to laboratory criteria. Dilutional hyponatremia due primarily to polydipsia produces a maximally dilute urine with a variable (Na+) and suppression of AVP secretion. Dilutional hyponatremia under conditions of SIADH results in a concentrated urine with a sodium value >30 mEq/L and an inappropriate urinary response to hypo-osmolality. Depletional hyponatremia associated with dehydration [BUN >15 mg/dL] leads to concentrated urine with a sodium value <30 mEq/L and volume-dependent stimulation of AVP secretion. AVP – arginine vasopressin; SIADH – syndrome of inappropriate antidiuretic hormone secretion. (Adapted from: Siegel AJ: Is urine concentration a reliable biomarker to guide vaptan usage in psychiatric patients with symptomatic hyponatremia? Psychiatry Res, 2015; 226: 403–4).