Urine Osmolarity in SIADH

Urinary Na excretion

In SIADH, urinary loss of Na⁺ continues despite significant hyponatremia. In these patients, as in healthy patients, urinary Na⁺ excretion is a reflection of Na⁺ intake and, therefore, usually is greater than 20 mmol/L. However, in the setting of Na⁺restriction in patients with SIADH or in patients with volume depletion due to extrarenal losses, the urinary Na⁺ concentration may be very low.

Urine Osmolarity
Patients with hyponatremia should turn off ADH and have a urine that is maximally dilute (ie, 50-100 mOsm/kg); however, in patients with SIADH, the urinary osmolality is usually submaximally dilute (ie, >100 mOsm/kg). One of the more common errors in recognizing SIADH is the failure to realize that the urine’s osmolality must be only inappropriately elevated and not necessarily greater than the corresponding serum osmolality.


Formative Quiz 2 has a case of SIADH with a urine osmolarity of 850.  I think this is incorrect for reasons state above.