Could Dehydration Be The Cause For Positive Drug Test Results?

Hello, My son is just getting over the flu. He is on promethazine w/codeine, tamiflu, prednisone and cefprozil. He is also in a drug treatment program for marijuana. His last drug test came back positive for a 6,000 level of opiates. also his the level increased slightly to 58. Could these results be a false positive due to dehydration and his illness? Also, what level would be considered lethal?