The walk and turn along with the one leg stand would likely be noted as unable to perform. However, the horizontal nystagmus gaze test, which is very accurate, as very few people have a natural nystagmus, would be performed. If the suspect fails, he can then be placed under arrest for suspicion of DUI. The officer could also administer a portable breath test, or PBT. While a PBT is not normally admissible in court, it is still enough to give the officer probable cause to arrest for suspicion of DUI in some states/jurisdictions. At that point, the suspect would be taken to the station and administered the BAC test, which is of course, admissible in court.