Can the Medications I was taking cause a False-Positive Drug Screen?

Dear Ask The Doctor: I am a Registered Nurse,Charge Nurse with 28 yrs in the field. Recently 4 of us were asked to give urine for a drug screen due to a medication documentation discrepancy. Mine came back positive for both Demerol as well as THC, both of which I have not had contact with. I suffer from kidney and liver disease and at the time was taking the antibiotic Cipro 2000mg a day, I had been on that for 3 days, Azo-Standard, 2 tabs 3times daily and several other OTC meds for back and kidney pain including Goody powders and Excedrin Migraine. I also take Geritol and use Afrin almost every day. I was diagnosed 8 years ago with the kidney and liver disease and have multiple large cysts on both organs, hematuria and infections frequently. January 16, 2011 I saw my doctor for a migraine and was given Demerol 75mg and Phenergan 50mg IM. Can any of these cause a false drug screen? I am very upset about these results as I have never had or been accused of drug abuse and was subsequently terminated for violation of the hospital's drug policy. My license is being questioned as of now and I cannot convince these people that I have not used these drugs! I plan to seek legal advice to hopefully reverse the accusations and get my job back. The hospital is telling other employees as well as outside people about this and my previous good name is now history. Your advice on how to handle this situation would be greatly appreciated. Thank you very much G. Swanner, RN

Dear Gina: Excretion of urinary begins within hours after exposure to THC and in 72 hours usually only 50% of the initial dose is eliminated, while it takes about 2 to 4 days for demerol to clear completely from the body. Looking at the pharmocokinetics of these drugs, their positivity in the urine is highly indicative of the drugs having been in the system for at least 4 to 6 days. Demerol taken on january 16th will have been completely eliminated by now, unless you had to continue the medication for treatment under the supervision of your treating doctor.
Phenergan have no such effects. Sadly, I could not find a reason for the test to be false positive accoridng to the biological and pharmalogical basis. There is a possiblity of technical or a manual error, on which grounds you may be able to defend yourself.