Unlike some states you can and usually will be prosecuted for DUI even if you refuse a blood or breath test.
After you have been pulled over and an officer suspects that you are driving under the influence of drugs (controlled or over the counter) or alcohol the police will have you get out of your car and perform a series of field sobriety tests. Afterwards the officer will ask you if you would be willing to submit to a blood or breath (officer's choice) test. Years ago drivers in Pennsylvania would refuse these tests fearing that the result would incriminate them. When the officer went to court without a conclusive test it gave the driver room to argue that they were not operating under the influence. In Delaware this is still the case. But a little over 16 years ago the law changed in Pennsylvania.
Now, police can successfully prosecute you in the absence of this test. So long as the officer can testify that based on their training, education and experience they formed the opinion that you were 1) under the influence of drugs or alcohol, 2) to such an extent that it would render you incapable of safe driving, you can be found guilty of DUI. To make matters worse, many refuse the test when their BAC level was low enough that it would qualify them for a lesser sentence. By refusing the test 3 things happen. First, as stated above, you can and will be prosecuted anyway. Second, you will be automatically placed in the highest tier for intoxication and will receive the highest penalty. Finally, just by refusing the test you will have your license suspended for a year above any suspension that you receive for the DUI conviction.
The bottom line is that you should take the test. If you have been arrested for DUI and refused the test, seek an experienced attorney immediately to help minimize the damage and a possible conviction and sentence.