Labs must keep it for atleast 1 year. However, there may be other issues, such as: enough blood drawn to be retested; preservative levels being sufficient to prevent fermentation, etc.
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For a misdemeanor offense it is mandatory that the sample be kept for one year. You will need to file a motion for chain of custody to have the sample retested, often colloquially referred to as a 'Blood Split' motion. You should certainly be speaking with a qualified criminal defense attorney in your local.