Typically the police need a warrant to access medical records, which requires them to show a judge there is reasonable cause for them to suspect you were driving drunk. However, if they should decide to press charges, they would likely be able to get a judge to allow them to search the hospital records to see if the blood test was positive for alcohol. The officers' observations at the scene (your appearance, the smell of alcohol, your ability to track, etc.) and any statements you made to them would form a basis for granting a warrant. It may also occur that hospital personnel will be more likely to draw blood after an officer tells them that the patient is suspected of driving drunk.
The fact that you were not ticketed at the scene is not evidence that you were sober. In addition to facing possible DU or DWI charges, the other persons injured in the accident may file a civil personal injury claim against you. Most states now allow two years from the date of the accident to file suit, though some allow less. In civil litigation the other side will also be allowed to subpoena the hospital records to show you were driving drunk, or at least impaired, even if you would not be subject to prosecution. If they do, you may have to revisit all these issues again.
If you are facing DUI or DWI charges, it's in your best interest to consult with an experienced DUI/DWI attorney in your area.