A 58-year-old man with small-cell lung cancer presents with severe hyponatremia (Na+ 118 mEq/L), low serum osmolality (245 mOsm/kg), and urine osmolality of 580 mOsm/kg. Clinical examination shows no signs of volume depletion or edema. Which investigation is most appropriate to confirm the diagnosis of syndrome of inappropriate antidiuretic hormone (SIADH)?