I wouldnt wait until they sue--rather, I would send the CA a certified DV letter. make them validate the debt if they want to collect on it. if they cannot validate it, then by federal law, they cannot collect on it. send off that letter ASAP by certified mail, return receipt requested. the USPS will send you the green receipt card back in the mail when they sign for your letter--that way you can prove that they got it.
Then, sit back and wait to see what they respond with. Also, I would definitely do as cellular said--see if the dr office will write up a letter for you stating that you have no outstanding balance with their office. Then, if the CA does decide to try to take action against you, you not only have them unable to validate, but you also have your ace in the hole--the dr office letter. with those two things, I dont think a court in the land would rule for them....