Kitson made a story-book return with a third minute goal to pave the way for Reading's progress into the fifth round.
Kitson, out of action since the opening day of the season, put Reading ahead early on and Birmingham were then left to chase the game.
It certainly wasn't in City's script after the way they had demolished Newcastle 5-1 in the previous round.
But Birmingham's defence was given an early taste of the threat generated by Ki-Hyeon Seol. The South Korean appeared to have little difficulty slicing his way through Birmingham's rearguard in which Martin Taylor was always struggling.
Seol tested Maik Taylor with an early effort but it was his slick pull-back from the by-line which carved open Birmingham's defence in only the third minute.
An eager Kitson latched onto the pass, turned quickly and slotted his low shot to the right of Maik Taylor to silence the home fans.
City were foiled by some good defensive work by the visitors with Australian Adam Federici, making only his second appearance, distinguishing himself with a string of fine saves.
But he had to count himself lucky when he prevented Cameron Jerome from equalising. With his arms stretching upwards he saved Jerome's downward header from a Seb Larsson free-kick with his legs!
Later he saved confidently from Fabrice Muamba, who was a powerful player in midfield for Birmingham.
Reading, far from full strength, conspired to frustrate the on-rushing Birmingham attacks and it was a surprise that the Royals increased their lead just before the break.
Once again it was Martin Taylor who was left struggling. A left-wing cross by Bobby Convey left him floundering and the Lita was quick enough to spot the opening to score Reading's second goal four minutes before the break.
But the big Birmingham defender certainly atoned three minutes into the second half when he put the Blues back into the game with his first goal of the season.
The ball ran loose after Bruno N'Gotty had hammered a shot at the Reading defence. Martin Taylor gained possession and scored with an accurate, low angled shot into the far corner of the net.
After this boost Birmingham strived for the equaliser but the closest they came was from a swerving Gary McSheffrey drive which Reading's star man, Federici, pushed around the post.
With Birmingham chasing the game the threat of a third goal for Reading was always on the cards and it duly materialised in the 82nd minute when the seemingly innocuous Lita pounced a second time to hammer home an angled shot to conclusively settle the tie.
Even then Birmingham were far from finished. Jerome flashed in a header which Federici again saved brilliantly.
But the Aussie had no chance in the final minute when Larsson lashed home his free-kick to make the score a little more respectable.