Hi ShambhuHegde,
First off, a 680/Q50 is a strong score (it's well above the 80th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. Depending on the Schools you plan to apply to, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
https://gmatclub.com/forum/ask-admission ... tants-124/That having been said, when these types of score drops occur, the two likely "causes" involve either something that was unrealistic during practice or something that was surprising (or not accounted for) on Test Day. If you can answer a few questions, then we should be able to figure this out:
When you took your CATs:
1) Did you take the ENTIRE CAT each time (including the Essay and IR sections)?
2) Did you take them at home?
3) Did you take them at the same time of day as when you plan to take your Official GMAT?
4) Did you ever do ANYTHING during your CATs that you couldn't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Did you ever take a CAT more than once? Had you seen any of the questions BEFORE (re: in an online forum or in a practice set)?
GMAT assassins aren't born, they're made,
Rich