Differentiating syndrome of inappropriate ADH, reset osmostat, cerebral/renal salt wasting using fractional urate excretion.

OBJECTIVES: Clinical and laboratory data of reset osmostat (RO) and cerebral/renal salt wasting (C/RSW) mimic syndrome of inappropriate antidiuretic hormone (SIADH) and can pose diagnostic challenges because of significant overlapping between clinical and laboratory findings. Failure to correctly diagnose hyponatremia may result in increased mortality risk, longer hospital stay, and is cost-effective. We aim to illustrate clinical and laboratory similarities and difference among patients with hyponatremic disorders and discuss the diagnostic value of factional uprate excretion (FEurate) to differentiate SIADH from RO and C/RSW.

CASE PRESENTATIONS: We report the use of FEurate in the evaluation of three patients with hyponatremia and elevated urine osmolality in the absence of edema or clinical evidence of dehydration to differentiate SIADH from RO and C/RSW.

CONCLUSIONS: Measurement of FEurate may offset in part the diagnostic confusion imparted by the diagnoses of SIADH, RO, and C/RSW.