In SIADH, which urine osmolality finding supports the diagnosis?

In SIADH, which urine osmolality finding supports the diagnosis?

• A. Urine osmolality <100 mOsm/kg
• B. Urine osmolality around 150 mOsm/kg
• C. Urine osmolality >300 mOsm/kg
• D. Urine osmolality equal to serum osmolality

Answer: (C)

In SIADH, ADH is secreted inappropriately, so the kidneys keep reabsorbing water even though the blood is becoming dilute. This makes the urine remain concentrated despite hyponatremia and low serum osmolality. Urine osmolality measures how concentrated the urine is. With SIADH, the urine stays concentrated because ADH is driving water reabsorption. A urine osmolality above about 300 mOsm/kg is the typical finding that supports this diagnosis, reflecting the inappropriately concentrated urine in the face of hyponatremia. Low urine osmolality (very dilute urine) would point away from SIADH and toward conditions where ADH is suppressed or water intake is excessive (like primary polydipsia). A value around 150 is not as strongly suggestive of SIADH, whereas urine osmolality equal to serum osmolality would imply isosthenuria, not the typical concentrating pattern seen in SIADH.

In SIADH, ADH is secreted inappropriately, so the kidneys keep reabsorbing water even though the blood is becoming dilute. This makes the urine remain concentrated despite hyponatremia and low serum osmolality.

Urine osmolality measures how concentrated the urine is. With SIADH, the urine stays concentrated because ADH is driving water reabsorption. A urine osmolality above about 300 mOsm/kg is the typical finding that supports this diagnosis, reflecting the inappropriately concentrated urine in the face of hyponatremia.

Low urine osmolality (very dilute urine) would point away from SIADH and toward conditions where ADH is suppressed or water intake is excessive (like primary polydipsia). A value around 150 is not as strongly suggestive of SIADH, whereas urine osmolality equal to serum osmolality would imply isosthenuria, not the typical concentrating pattern seen in SIADH.