There are essentially two types of breath tests in Maryland. There is a portable breath test generally performed on the street prior to the arrest, commonly known as the PBT or preliminary breath test and there is the "real" test which is given at the station on a much bigger and allegedly more accurate machine. This machine is generally known as a breathalyzer by the public while its technical name is EC/IR.
Starting at the PBT or street breath test, this test is designed to give your arresting officer information as to what your breath alcohol limit is at that time. This test is used by the police to give probable cause for a potential arrest. They will ask you to blow in this hand held device and generally speaking they will not tell you what you blew; instead they keep it a secret and add the results to your field sobriety test results thereby giving them probable cause to make the arrest. You need to know that pursuant to Maryland Statute 16-205.2 you do not need to blow in this machine and happily there is no negative consequence for your refusal to do so. Thus it is probably better in most cases not to submit to this test. The results of this test if you do agree to blow are not admissible in court or at the MVA so the test is truly engineered as a probable cause test to help the police with their arrest decision.
Back at the police station you will be asked to take the "real" test. The police are required by law to read you certain rights prior to asking you to blow in the machine. These rights are not your Miranda rights but rather rights specific to this blow test. The rights basically tell you what will happen if you do blow in the machine and what will happen if you refuse to blow in the machine. Some times the cops are in hurry and forget to read you this form before the test. If that happens, your number may be suppressible in court and you may be able to avoid administrative penalties at your MVA hearing. If they do read you the form you must then decide if it is better to blow in the machine and let the cops get a BAC number on you which can make defending your DUI in court more difficult, or it is better to refuse to blow and suffer the consequences of a refusal to blow. At present the MVA consequence for a refusal to blow is 120 suspension of your Maryland driving privilege or the Interlock in stalled in your car for 1 year at your expense.
This can be a complicated decision to make. The decision involves consideration as your driving needs for work and for a license. So in some cases having a conviction for DUI may be the worst possible thing that can happen to a person, in that case, a refusal to blow in the machine and suffering the administrative consequences may be the best course of action. In other cases, protecting ones license may be the most important thing, in which blowing in the machine and registering a number may be the best course because it will limit the time of any suspension. In either case, conferring with counsel at your earliest opportunity is important; however the police are not required to let you call counsel at the police station. Albeit some police may let you.