As Roy said, lab tests should not take that long. Call the lab before you deliver sample, tell them when you are coming in, label container, and ask when the results should be available. (Usually, you have to get them through the doc.'s officde.) Yes, leaving the sample out for more than a couple of hours could affect the results.
If the lab doesn't call you, call them. Be polite but persisitant. Make friends with the people in the lab; if you do, you will be "served" faster.
You would be amazed at how little doc.'s offices and labs communicate. Double check with both. It might not help, but it makes you part of your own recovery - a survivor rather than a victim.
I emailed a C. diff. expert to check on the "sterile" vs "clean" container.