Realize this is LONG after the original post. But since it's moderately difficult to find a really complete answer to this question online, wanted to note the Lifeguard status of TWA 800 is explained in detail in a NY Times article dated August 4, 1996: "Medical Cargo on Planes Poses Airline Safety Gap"
Link is
Medical Cargo on Planes Poses Airline Safety Gap - The New York Times
Viewing this article did not require a subscription for me. Hopefully that's true for others also.