In a criminal trial, blood tests can give juries and judges the probabilities, but not absolute proof, that a defendant committed a crime.
They can eliminate someone as a suspect. But they are not refined enough to prove that a blood sample found at a crime scene could only belong to one person in the world.
Police have so far performed three types of tests on blood in the O.J. Simpson murder case.
The tests have been in use for more than a decade and their validity is widely accepted.
Based on what they know now, a police blood specialist testified Friday, only 0.43 percent of the population has blood with the same qualities as Simpson's.
Similar blood was found near the bodies of Nicole Brown Simpson and Ronald Goldman. The tests prove that the blood did not belong to either of the victims.
First, scientists looked at standard blood types. These are the familiar letters - A, B, AB and O. They are indications of the ability of red blood cells to produce certain types of sugars.
Scientists determine the blood type by looking for antibodies to the various sugars. Someone with Type A blood, for example, has antibodies to the sugars in Type B blood.
Nicole Simpson and O.J. Simpson both had Type A blood. About 40 percent of whites and 27 percent of blacks have this type.
Ronald Goldman had Type O blood, as do about 45 percent of whites, said Dr. Don Siegel, one of the medical directors of the blood bank at the Hospital of the University of Pennsylvania. …