How can I make this medical mystery as realistic as possible in my story?

Setting is a college in a small American town in New Jersey.

I’m writing this juvenile mystery series where the main character utilizes the labs in his school to solve mysteries.

In this first mystery, a student is found passed out. Everyone thinks it’s alcohol poisoning but the main character is the only one who witnesses the guy foam at the mouth and coughed up blood. In reality, he was poisoned but survived (he also drank a lot at a party and the pepretator took advantage of that to mask the fact that he was poisoned). I may alter this in any way as I decide what poison to use and learn more about poisons.

The paramedics take the victim away and then later he’s in a coma. But I’m split as to whether or not to put the victim in a coma.

How can I make this so that the doctors CAN’T figure out the victim was poisoned despite doing blood tests while the victim is in a coma? If the doctors would find a way no matter what, then I probably won’t put the victim in a coma and everyone, including him, thinks he just drank too much.

(any suggestions on what poison to use is also appreciated)