I know this came up last game, so I went ahead and updated the defense rules to include countering with items. Here's the excerpt from the rulebook.
Daily or adventure actions within items that are currently being used by your character may be used to counter as described above. For example, you can use a sword with a daily Fumble action to counter a Fumble while you're holding it. However, you cannot use the sword's action to counter if it is sheathed or sitting nearby.
Most single-use and charged items, however, cannot be used to counter. For example, a character may not use an or a to counter a Slay. The only exceptions are , which are specifically designed for this purpose.
Questions? Ask away.
"Pay no attention to the man behind the curtain."
- The Wizard of Oz