A DUI blood test usually take at least 3 weeks to be completed from the time of the offense by the San Diego Crime Lab. The testing only uses a portion of the blood that was drawn, there remains plenty of blood for retesting if desired by a defense attorney on behalf of their client. The sample does have preservative in it so it doesn't degrade.
The retesting is commonly referred to as a "blood split" because the defense attorney gets a court order to have the blood sample "split" with a portion given to the defense forensic expert for retesting. The Crime Lab's results tend to be fairly accurate, but can always be retested, typed, DNA tested, preservative tested or drug screening conducted. There may be a small amount of evaportation over time which could very slightly reduce the alcohol reading. Whether retesting is appropriate in any case is up to the defense attorney and their client. Hiring an expert and retesting can be relatively expensive depending on what testing or drug screening is to be done. Blood tests are the most accurate, but they only tell you what the alcohol or drug level was at the time of the blood draw. The blood draw is usually an hour or more after the stop and it could be rising or falling from the time of driving based on the drinking pattern.
For a free consultation by very experienced DUI Lawyers, call Christoph Law Offices at 760-941-5720. We have handled over 3000 DUI cases over more than 30 years located across from Vista Courts. We can answer your questions and give you an overview of the DMV and Court proceedings along with possible defense issues.