It is difficult to clinically distinguish SIADH from volume depletion in hyponatremic patients without edema, with correct prediction occurring in only around 40% of cases. The gold standard for diagnosis is the sodium response to fluid challenge. A urine sodium less than 30 mmol/L favors SIADH, but this can miss salt depletion in elderly patients. A fractional excretion of sodium less than 0.5% identifies patients with salt depletion but misclassifies around half of SIADH cases. Combined fractional excretion of sodium less than 0.5% and urea less than 55% may best differentiate SIADH from salt depletion.