Tetanus is a condition where you have continuous muscle spasms. You don't need a clinical test, the patient will be rigid and all his muscles will be in spasm. You can culture C tetani which is the bacteria that is a major cause of tetanus (tetanus itself is a symptom described about, can be caused by more than just the tetanus bacteria), but like someone else said, by the time those results come back it would be too late. There are antitoxins that can counteract the tetanus toxin and those would just be given empirically (meaning you treat based on a presumed diagnosis as opposed to one confirmed through lab tests).