The Dodge action doubles your defense, so what happens when you gain a bonus to your defense from, say, a shield? The book implies that such defense bonuses should be added to your dodge total after you double your base defense score. I'm making that order of operations official.
For example, Kathy has a Wits of 2 and a Dexterity of 3, giving her a base Defense of 2. She's hefting a big shield alongside her sword, granting a +2 bonus and giving her a total Defense score of 4. To weather a nasty incoming blow, she declares a Dodge action. This doubles her base defense to 4, which then gains the +2 bonus from her shield giving her a Defence of 6 for the round.