To commit the offence of PCA, you must drive a motor vehicle on a road while having present a blood alcohol reading exceeding the prescribed concentration of alcohol. Your blood alcohol concentration is calculated by submitting to a breath test designed to draw air from your lungs. A conversion then occurs to relate the breath test to a blood alcohol reading. The law is not concerned with your ability to drive, or how affected you appeared. It is only concerned as to whether you drove while over the limit.
However, if you are charged with DUI, then the police must prove that you drove a vehicle on a road, or attempted to do so, whilst so much under the influence of alcohol and/or drugs that you could not exercise effective control of your vehicle. Police usually rely upon observations that support faculty impairment such as slurred speech, being unsteady on your feet, bloodshot eyes, slow body movements and poor driving, etc. The offence of DUI carries a minimum loss of licence for at least 12 months for a first offence and 3 years for a second offence. Imprisonment is an option but is not common for this offence unless you have prior convictions.
If you refuse or fail a breath test or reasonable direction of a police officer in respect of the breath analysis procedure the same penalties as DUI normally apply, irrespective of whether you had been drinking or not. You can only rely upon a medical reason for refusing if you submitted to a blood test. In addition, before the police can prove this charge they have to give you a prescribed warning about the consequences and then give you another opportunity to comply.
In all cases above there are also significant fines that are imposed if you are found guilty of PCA, DUI or refusing/failing a breath analysis.