Courts may not allow blood samples as evidence in drunken-driving cases if the accused is not arrested at the scene and does not grant permission for the test, the Utah Supreme Court has ruled.

In a unanimous decision released Thursday, the court reversed the conviction of an unnamed juvenile convicted of drunken driving and causing a head-on collision, and sent the case back to juvenile court for a new trial.The juvenile was arrested after an accident near Heber City in 1985. At the hospital, a Utah Highway Patrol trooper asked doctors to take a blood sample from the juvenile for alcohol analysis.

However, the trooper neither arrested the juvenile nor told him for what purpose the blood sample was to be used. The trooper also did not tell the juvenile he had a right to refuse the test.

"Because (the juvenile) neither impliedly nor actually consented to the testing procedure, his blood test results were erroneously admitted at trial," said the decision written by Associate Justice Christine Durham.

A lower court had ruled that the juvenile did give an implied consent to the test. The Utah Court of Appeals later reversed that decision but sustained the conviction of the juvenile because of other evidence.

Supreme Court justices said the appeals court was wrong in doing so, because the evidence had not been considered by the trial judge. They sent the case back to juvenile court where a judge will consider the evidence.