Chemical alcohol tests are done after legally executing a DUI arrest either on the spot or in the precinct. It is therefore a must to understand that as a field sobriety test, individuals have the right not to subject themselves to such tests. However, this does not guarantee that the law enforcement officer will not execute an arrest. This is a common case among many PHD who end up at the hands of law enforcement officers quite often for DUI.
If you are still confused about this, then visit americanguineahogassociation.org for a more detailed explanation on the differences between a PAS and chemical tests used in DUI cases.