I can't see how "agent" can be translated from "Das Mittel"... does this translation make sense?
This sentence can be interpreted in two ways, I think. Agent can be a person such as an insurance agent. It can also be a healing agent such as a medicine. The latter would not be a typical use but might make more sense in a research setting.
It does make sense if "agent" is used like "Agens". But I, being a German, can't remember ever having overheard or read it anywhere.
"Agent" is a pharmaceutical word for "medicine".
agent means a chemical or substance that make other substances change.
To avoid confusion, the sentence should read "..is kept at the hospital"