Serology Testing Is Widely Accepted : Forensics: Defense efforts to cast doubts generally fail. Blood results are simple, reliable and easily interpreted, experts say.
Blood analysis of the type used to attempt to place O.J. Simpson at the scene of his ex-wife’s slaying has been in use for more than a decade and its validity is widely accepted, experts said Friday.
It is this confidence in the serology test that made the defense counsel’s attempts to discredit the results so difficult. Even though defense lawyer Gerald F. Uelman’s arguments were “clever,” said forensic specialist William C. Thompson of UC Irvine, they were unpersuasive and did not change the outcome of the preliminary hearing.
For the record:
12:00 AM, Jul. 10, 1994 For the Record
Los Angeles Times Sunday July 10, 1994 Home Edition Part A Page 3 Column 4 Metro Desk 1 inches; 32 words Type of Material: Correction
Simpson case--In a story about blood analysis in the O.J. Simpson case, The Times incorrectly identified how frequently the 2+2- form of the blood protein PGM appears in the population. This form occurs in 1.77% of the population.
Virtually all the issues Uelman raised have been argued in previous cases and have generally failed to sway either judges or juries, Thompson said.
The one issue that might have cast doubt on the results, the possibility of a “contaminant bloom” in the LAPD forensic laboratory, was not allowed into testimony by Judge Kathleen Kennedy-Powell.
Serology is widely accepted because the tests are simple, reliable and easily interpreted, said Dr. Barry A. J. Fisher, scientific services director for the Los Angeles County Sheriff’s Department. Results obtained by serology have been validated in a large number of laboratory studies and are readily reproducible--key factors that control admissibility of scientific evidence in courtrooms.
“This is a very straightforward testing procedure, very basic,” he said. “The defense will always try to raise doubts about it . . . but unless there is specific information that a test is done improperly or the individual who did it does not have adequate training, evidence of this sort will be admitted.”
Serology relies on the fact that several proteins found in blood can be present in any one of two or three forms, much like hair can exist in different colors. The most common and widely known proteins are those that determine blood type, the so-called ABO grouping. But experts have identified as many as 19 other lesser-known proteins that provide similar information.
In the lab, criminalists select about half a dozen proteins and determine which form of each is present in a blood sample, according to Jack Mertens, who is in charge of serology at the Federal Bureau of Investigation’s forensic laboratory. Published data indicate the frequency with which each form of the protein occurs in the population at large.
Simpson’s blood type is A. About 34.7% of the general population is Type A. Similarly, the form of a protein called ESA in his blood was found to be Type 1, which is found in about 79% of the population. Finally, they found that the form of another protein in his blood, called PGM, was 2+2-. That is a less common form, occurring in only 17.7% of the population.
The chance that all three markers are present in any blood sample--obtained by multiplying the three percentages together--is just 0.43%. Only one in every 200 people in the overall population has those three markers, placing Simpson in a very small group.
Uelman raised four major issues about the serology. One is that these three markers are not necessarily independent, but might be linked. It is possible, he said, that Type A blood might be much more likely to be PGM 2+2-. In that case, a much larger group of people would share the same three markers, reducing the likelihood that the blood spot was Simpson’s.
But, Thompson said, several studies have indicated that no such link occurs and there is no good reason to suspect one.
Uelman also argued that racial biases could have affected percentages. For its reference population, the LAPD laboratory uses blood samples obtained from arrestees and crime victims in Los Angeles. That group is 40% to 45% black, Uelman argued, while the overall U.S. population is only 7% black. If PGM 2+2- is twice as common in the white population as in the black population, he argued, then the number of people sharing the three markers would be twice as large--one in 100 instead of one in 200.
He also argued that contamination in the LAPD laboratory could affect the results. In particular, he implied that a so-called contaminant bloom associated with DNA fingerprinting had occurred in the LAPD lab sometime recently. That question was not allowed because DNA evidence was not presented at the preliminary hearing, but it is likely to arise at the trial.
Contaminant blooms are associated with the PCR testing of DNA for genetic fingerprinting. In PCR, enzymes are used to multiply the amount of DNA present in a sample millions of times. There have been cases, Thompson noted, in which replicated DNA has been released into the laboratory air. Such DNA would contaminate other samples, making accurate testing impossible.
DNA contamination is probably irrelevant for serology, Thompson said, because researchers are analyzing proteins in blood, not DNA. But evidence of DNA contamination could indicate that the laboratory does not follow good operating practices, which could call other results obtained in the laboratory into question.
Finally, Uelman noted that the blood spots obtained at the crime scene could contain blood from two people, which could change the interpretation. But as prosecutor Marcia Clark noted, at least one of the two hypothetical people in such a scenario shared the same markers as Simpson.