Italian to English Medical Medical (general) Biochemistry Lab result

This collocation occurs in a list of otherwise recognizable Lab results, sodium, potassium etc. as you'd expect from a request for basic biochemistry. The full context is "CLORURO [S/U/DU]" and the result, as you'd expect comes up in mmol/L. It can be found in Google, but always as this abbreviation, and there is never an expansion to explain what the individual letters stand for. I am at a loss and am wondering if anyone has come up against this before. I would be very grateful for some clarification...