The first is the role of the Medical Review Officer. A positive laboratory result is not treated as proof of substance abuse on its own, because it may not be — certain pain-relief medicines containing codeine can trigger a false positive for opiates. The MRO's job is to establish whether the result came from legitimate medical treatment before any disciplinary consequence attaches. That step protects the individual without weakening the safety rule. The second is that refusal to be tested is treated on par with a positive result. Without that equivalence the entire regime would be voluntary in effect, since anyone anticipating a positive result would simply decline. It is worth noting that the drug test here is standard procedure after any safety-related incident and runs parallel to the AAIB's investigation — sources have also pointed to possible hydraulic and control system snags, so the test result does not by itself establish the cause of the altitude loss.