English translation: electrólitos sanguíneos (séricos)
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Don't worry, I don't guess when it comes to translation, just trying to illustrate my point with that word. It will most likely come down to a note about a vague/unknown acronym.
Thank you for the tip on the web site!
In my research I did find several listings of lab tests in acronyms from health care providers with "ES" included, but none of them defined it. I have a feeling it is semi-standard, but just haven't found it yet.
You can go to www.cosnautas.com (created by Dr. Fernando Navarro) and look up almost any existing Spanish medical acronym. He presents 32 different possible meanings of "ES", including the one I've included in my answer.