Usually, when someone is arrested for drunk driving, the prosecutor and the state rely upon a chemical breath test, like the Alcotest, to prove intoxication. There are, however, some circumstances when the police must depend on a blood test to establish the defendant's blood alcohol content (B.A.C.).
For example, if the police respond to an automobile crash where the operator/accused is injured, even if those injuries are not real serious, then that person is usually taken to the local emergency room under police custody. And this is where the state's case against the operator for DWI becomes increasingly complicated and sometimes difficult to prove.
If the defendant's attorney demands and places the state on notice, the prosecutor must establish the chain of custody and the defendant has the right to confront (at trial) anyone who "touched" the blood evidence. At a minimum, those include the police officer, the nurse, the evidence officer and the state police chemist.
Under the Confrontation Clause, a defendant has the right to cross examine or question anyone who accuses them of violating the law. Essentially, the chemist who analyzes the blood and establishes a certain B.A.C. is "accusing" that person.
And the nurse must also testify that he or she followed certain medical protocols in drawing the blood, mostly to establish that alcohol swabs were not used to prep the skin, which some would argue increases the chance of an elevated B.A.C. reading.
The problem in prosecuting these cases is this: it is harder to get four people to show in court at the same time then it is for one police officer who happens to be the arresting officer and the breath test operator. While all of the individuals except the nurse are state employees, every link in the chain must perform or the entire chain (and the case) fails.
State employees and police officers almost always appear in court; however, the nurse is a different story. Emergency rooms are busy places, hospitals typically do not pay their nurses to attend court and "days off" are highly valued.