"I was assuming that at one point and I got a lot of shrugged shoulders and shakes of the head, so I don't know if that's a yes or a no. No one will tell me and I don't know how this is going to go," Heigl said, noting series creator Shonda Rhimes' passion for plot secrecy.
"I don't know if I live or die. I don't know how Izzie fares," said the actress, who's played the character since the show debuted in 2005.
"I'm there" if Izzie remains part of "Grey's Anatomy," Heigl said. She called the set "one of my favorite places to be" and said her colleagues are also friends.
She and fellow cast members, including Ellen Pompeo (Dr. Meredith Grey) and Patrick Dempsey (Dr. Derek Shepherd), were on hand for the celebration that included a cake decorated with scalpels and syringes made out of icing.
Rhimes, who's also an executive producer for the show, called the milestone a "huge accomplishment" and thanked the cast and crew for their efforts.
In an interview, she dropped a tantalizing clue about the episode airing May 7.
"The only thing I'll tell you about the 100th is that it takes place on Meredith and Derek's wedding day," Rhimes said. "That's big ... and that's all I'm going to say."
On Thursday's episode, Izzie was seen surviving brain surgery. There's been speculation that Heigl wants to pursue her blossoming film career ("Knocked Up," the upcoming "The Ugly Truth") full-time, and that her character might be killed off.
Rhimes declined comment on what would happen to Heigl's character.
Last month, fellow cast member James Pickens Jr. said Heigl was leaving the show, along with T.R. Knight, both of whom declined comment at the time. Knight missed the studio party because he'd previously scheduled the day off, an ABC spokeswoman said.
