In suspected SIADH vs DI, which pattern would distinguish each condition?

• A. SIADH has low serum osmolality with inappropriately concentrated urine (high urine osmolality) and high urine sodium; DI has high serum osmolality with dilute, low urine osmolality that fails to concentrate.
• B. DI has high serum osmolality with dilute, low urine osmolality that fails to concentrate.
• C. SIADH has normal serum osmolality and concentrated urine.
• D. SIADH has high urine osmolality; DI has high urine osmolality.

Answer: (A)

The key idea is how ADH affects water reabsorption in the kidneys. In SIADH there’s too much ADH, so the body reabsorbs water, diluting the serum and driving the serum osmolality down. The kidneys still make concentrated urine because ADH is present, and the urine sodium is typically high due to euvolemic natriuresis. In DI, there isn’t enough ADH or the kidneys don’t respond to it, so water isn’t reabsorbed effectively. This raises the serum osmolality, while the urine remains dilute because the collecting ducts can’t concentrate it. You’d see high serum osmolality with dilute, low-osmolality urine that fails to concentrate, alongside high urine output.

So the described pattern fits: SIADH shows low serum osmolality with inappropriately concentrated urine and high urine sodium; DI shows high serum osmolality with dilute urine that fails to concentrate. The other patterns don’t match because they either imply the wrong serum osmolality relationship or the wrong urine concentrating ability for SIADH vs DI.