differential diagnosis for metabolic alkalosis: uworld question

[Oh_Gee]

spoiler alert for Renal question in Uworld:

i just did a uworld question where there's a patient w/ metabolic alkalosis and the question was what lab study you should do next. answer was check urine chloride. explanation says urine chloride helps you figure out if you can correct the alkalosis w/ saline or not but does not explain the physiology.

can someone explain the physiology behind this? i could not find it in costanzo.

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[Money Moniker]

It's because urine chloride tells you about what is going on with electrolytes in the body. If it is low, then serum chloride is probably also low, meaning you could have vomiting for example, which incidentally would also cause metabolic alkalosis. You could obviously correct such an alkalosis with saline.

If on the other hand urine chloride is high, you can exclude vomiting and other etiologies of metabolic alkalosis that would necessarily lead to hypochloremia and instead focus on possibilities like Cushing disease that would present with normal or elevated Chloride levels. In Cushing disease you would not be able to correct the alkalosis with saline. There are other possibilities as well but that's the broad strokes.