Normal fractional urate excretion identifies hyponatremic patients with reset osmostat.
BACKGROUND
Reset osmostat (RO) occurs in 36% of patients with syndrome of inappropriate antidiuretic hormone secretion (SIADH) and is not often considered when evaluating hyponatremic patients. Patients with RO are not usually treated, but recent awareness that symptoms are associated with mild hyponatremia creates a therapeutic dilemma. We encountered patients with hyponatremia, hypouricemia and high urine sodium concentration (UNa), who had normal fractional excretion (FE) of urate and excreted dilute urines that were consistent with RO. We decided to test whether a normal FEurate in nonedematous hyponatremia irrespective of UNa or serum urate would identify patients with RO.


METHODS
We determined FEurate in nonedematous hyponatremic patients. A diagnosis of RO was made if urine osmolality (Uosm) was <200 mOsm/kg in a random urine. We performed a modified water-loading test in patients with a normal FEurate whose random Uosm was >200 mOsm/kg.


RESULTS
All nonedematous hyponatremic patients with FEurate of 4%-11% had RO, as determined by Uosm <200 mOsm/kg on a random urine collection in 8 patients, or after a modified water-loading test in 6 patients. Plasma antidiuretic hormone (ADH) in 4 patients was undetectable at <1 pg/mL during water-loading. Nine patients had baseline concentrated urine, 12 had UNa >20 mmol/L, 9 were hypouricemic, yet all had a normal FEurate. Comorbidities were similar to those reported in RO.


CONCLUSIONS
RO, a benign form of SIADH, occurs commonly. A normal FEurate in a nonedematous hyponatremic patient is highly suggestive of RO. Determining FEurate is superior to serum urate. The therapeutic dilemma for RO must be resolved.
Department of Medicine, Winthrop-University Hospital, Mineola, New York, USA.
Louis J
LJ
Imbriano
Ekambaram
E
Ilamathi
Nicole M
NM
Ali
Nobuyuki
N
Miyawaki
John K
JK
Maesaka
D000368:Aged; D000369:Aged, 80 and over; D015415:Biomarkers / Q000097:blood / Q000652:urine; D015897:Comorbidity; D006801:Humans; D007010:Hyponatremia / Q000097:blood / Q000175:diagnosis* / Q000453:epidemiology / Q000652:urine; D007177:Inappropriate ADH Syndrome / Q000097:blood / Q000145:classification / Q000175:diagnosis* / Q000453:epidemiology / Q000652:urine; D007671:Kidney Concentrating Ability; D008875:Middle Aged; D009481:Neurophysins / Q000097:blood; D009518:New York / Q000453:epidemiology; D009994:Osmolar Concentration; D011237:Predictive Value of Tests; D011498:Protein Precursors / Q000097:blood; D012964:Sodium / Q000652:urine*; D014527:Uric Acid / Q000652:urine*; D016482:Urinalysis; D014667:Vasopressins / Q000097:blood
C438657:AVP protein, human; D015415:Biomarkers; D009481:Neurophysins; D011498:Protein Precursors; D014667:Vasopressins; D014527:Uric Acid; D012964:Sodium