What Causes Positive Drug Test Results For Marijuana?

My husband is a 57 year old male with diabeties and he has just tested positive for marijuana when his blood drawn to check his A1C. He does not smoke marijuana and his blood work never tested positive in the past. We started drinking smoothies about 2-3 months ago with 1 tblspn of ground flaxseed every morning. He also takes approx 12 ibprofin daily. Could this be the culprit?