Forensic blood tests not infallible

June 16, 1994|By Los Angeles Times

LOS ANGELES -- Laboratory comparisons of bloodstains found in O.J. Simpson's car and driveway to blood from his former wife, killed Sunday night, could yield potentially important evidence in the investigation, say experts.

But the techniques used to obtain such a rapid analysis are not definitive, they noted, and results of a more conclusive DNA test will not be available for at least two months.

The conventional tests -- based on comparisons of proteins in the blood -- can show with up to 99 percent certainty that two blood samples are identical, and a large number of convictions have resulted from their use, Jack Mertens, supervisor of the DNA Unit at the FBI's forensic laboratory in Washington, said yesterday.

Blood analysis can proceed through as many as five separate stages, Mr. Mertens said. The first critical test is to determine if, in fact, a stain is blood. That can be done in as little as 10 minutes by swabbing a small portion of the stain with chemicals that will produce a visible reaction with hemoglobin, which is present in all blood.

If the sample is blood, the next step is to dissolve it in a liquid, a process usually accomplished by allowing the sample to soak overnight. Once the blood is suspended, researchers apply antibodies to determine if it is human or animal blood. That takes about four hours.

Historically, the next step would have been to determine the blood type or group, a process that takes only a couple of hours.

But recently, Mr. Mertens said, many labs skip this step because it yields little information. For example, if the blood is found to be type AB, a rare group, then only one person in every 10 in the general population would match it. But if it is type O, nearly half the population would match.

To refine matching, researchers have started looking at enzymes and other proteins in the blood. As many as 19 different proteins are used in this process: they occur in each person's blood in different combinations. This analysis takes about four hours.

If the pattern of proteins in blood from Mr. Simpson's car, for example, is the same as the pattern in Nicole Simpson's blood, the odds are 99 in 100 that the blood in the car is hers.

"That's good, but it's not great," Mr. Mertens said. "It's most useful in the initial screening process when they don't match," meaning that two specimens come from different individuals. Such results could show that an individual did not commit a crime.

The process should yield results within about two days, according to Mr. Mertens.

DNA tests take longer because of their complexity, but they are usually worth waiting for. If DNA testing shows that blood from Mr. Simpson's car matches Nicole Simpson's, the odds are at least 999,999 in 1 million that the blood in the car is hers.

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