"He needs some further tests and he hopes to be back soon," said team spokesman John Herrera.
Kiffin was initially taken to the Queen of the Valley Medical Center on Monday morning for observation but wound up being kept overnight. His condition had not changed by Tuesday so additional tests were ordered.
As they had on Monday, offensive coordinator Greg Knapp and defensive coordinator Rob Ryan presided over practice, though the team believes Kiffin will return in time for the game against the 49ers.
"I would sure think so," Herrera said. "All we can do is take it day by day."
Oakland’s first practice Tuesday -- the team also has a night practice slated to begin at 7 p.m. -- was very similar to the first workout the team held without Kiffin, particularly the quietness.
Running back LaMont Jordan said Kiffin had set a good enough tone in practice before getting sick that the practice wouldn’t lag without him.
"The most important thing he’s done is he’s come in and been consistent," Jordan said. "The way he was on Day 1 is the way he was the last time we saw him. He’s made it fun to play football again."