How does the Patricia Stallings case shed light on the murder of Meredith Kercher?

Part XVI in the Knox/Sollecito case

Patricia Stallings was convicted, then exonerated of poisoning her infant son Ryan with ethylene glycol, the major component of antifreeze. When Ryan was unable to keep food down, Mrs. Stallings took Ryan to Cardinal Glennon Children’s hospital accidentally, which happened to be associated with a toxicology unit. Ryan had metabolic acidosis (the pH of his blood was too low) and the laboratory associated with St. Louis University reported 180 milligrams per liter of ethylene glycol.

Ryan was put into foster care, but Mr. and Mrs. Stallings were allowed weekly visits. Four days after one such unsupervised visit, during which Mrs. Stallings gave Ryan a bottle, Ryan became ill again, having metabolic acidosis and died three days later. The same lab reported finding 911 milligrams per liter of ethylene glycol in Ryan’s serum and also in his baby bottle, which had been put into a dishwashing machine.

What are the odds of the tests being wrong?
There were three separate determinations of ethylene glycol, the original serum, the serum after the second feeding, and the bottle itself. In addition, one or both of the serum samples was also retested by a commercial lab. How could this be just chance? The prosecutor told the jury not to worry about motive, but just to concentrate on the fact that there was no rational explanation for these results except deliberate poisoning. He implied in so many words how unlikely it was for the labs to be wrong.

An alternative explanation begins to emerge
Patricia Stallings was arrested after a half-empty bottle of antifreeze was found in the couple’s basement. Mrs. Stallings was pregnant with her second son at the time of her arrest; her son David was immediately placed in foster care. David began to exhibit the same symptoms as Ryan, but he was taken to a children’s hospital. The hospital diagnosed David with a genetic disease known as methylmalonic acidemia (MMA)*. In this disease, one enzyme in a particular biological pathway is defective, causing a metabolic traffic jam, in which other compounds build up in the bloodstream. Although the odds of any one infant having this disease are about 1 in 48,000, David’s diagnosis meant that there was a one in four chance that Ryan had the disease as well. This fact was not allowed to be presented at her trial however, possibly because the judge and the defense attorney misunderstood each other with respect to what the defense attorney needed to do in order to bring the issue up.

Professor of biochemistry William Sly was one of the citizen-scientists who became involved in the case. Molecular geneticist Piero Rinaldo also reexamined the evidence, with some surprising results. James Shoemaker was involved in gathering evidence prior to the trial and subsequently. Both William Sly and Robert Ritter, Mrs. Stallings’ second attorney, learned of the case from the television show "Unsolved Mysteries.”

How could the analyses of ethylene glycol have been done better?
The gas chromatography (GC) technique measures how much time it takes for substances to pass through a column of stationary liquid (a waxlike substance) over which inert gas flows. This period is the retention time of a substance, and this length of time depends on its chemical properties. Two compounds may have the same retention time but usually do not.

The first analysis of ethylene glycol showed that the unknown compound in Ryan’s serum had a retention time within 30 seconds of a standard of ethylene glycol. A difference of 30 seconds is larger than is typically seen for an identical compound run twice, but let us assume generously that 30 seconds is within experimental error between one GC run and the next. The investigators should not have concluded that they had identified ethylene glycol even under this assumption, rather they should have performed additional tests.

One way to distinguish between two compounds that have identical or almost identical retention times is to run each compound through a mass spectrometer as it emerges from the end of the column. The mass spectrometer measures how heavy a compound is and in some cases how heavy the fragments of the compound are when the compound disintegrates. The combined technique gas chromatography-mass spectrometry (GC/MS) is usually very discriminating between one compound and the next, but it was just coming into widespread use about the time of the Stallings case in 1989.

A second way to distinguish two compounds is to run the gas chromatography experiment three times with three chemically distinct stationary phases. If the retention times of the two compounds, the unknown and the standard, are the same in all three trials, then the two compounds are probably identical. A third way is to spike a portion of the sample with the unknown compound with the reference. This eliminates the factors that cause run-to-run variation in retention times. The labs in question apparently did not do any of these three additional tests.

Shoemaker, Sly, and their coworkers examined a stored sample of Ryan’s blood using gas chromatography/mass spectrometry and showed that the substance believed to be ethylene glycol was actually propionic acid, one of the substances that would be expected to be elevated if Ryan had the same genetic disorder as his brother David. They also spiked a sample of Ryan’s blood with ethylene glycol and showed that it did not have the same retention time as the unknown compound, now identified as propionic acid.

The analysis of Ryan’s blood after the unsupervised visit suggested a very high concentration of ethylene glycol. Yet ethylene glycol is highly water-soluble and clears the body with a half-life of 3.0 hours. The half-life is the time it takes for the concentration of a substance to go from some value x to x/2. Therefore, Ryan would have had to consume 300 liters of ethylene glycol during his unsupervised visit to account for his blood plasma concentration four days later, an absurd volume.

Dr. Rinaldo found evidence of methylmalonic acid in Ryan’s serum. Along with propionic acid, methylmalonic acid would be expected to be in elevated concentrations in a person suffering from MMA. He also reexamined the GC data from the two labs. He found that the commercial lab saw an unknown compound in Ryan’s serum and merely assumed it was ethylene glycol without even running a standard.

He was unable to find evidence of ethylene glycol in the tests that were run on the bottle, but the bottle itself had gone missing. The positive result for ethylene glycol on the baby bottle should have raised a red flag on its own. Ethylene glycol in the bottle would have been washed and rinsed away in the dishwasher. Drs. Shoemaker and Sly believed that the small amounts of ethylene glycol found in some of the samples were due to contamination.

Cognitive bias
Once some of these facts emerged, D.A. McElroy joined with Mrs. Stallings’ new lawyer in a motion of ineffective counsel. First Mrs. Stallings was granted a new trial, and later the charges were dropped.

The first analysis to show ethylene glycol was done in a toxicology laboratory, and they might have been trained to focus on poisonings, not inherited metabolic disorders. Dr. Bob Smith called this an “expectation effect,” and he later referred to the general forensic phenomenon as “investigator bias.” The commercial lab that retested on of the serum samples may have been guilty of confirmation bias. It seems likely but not a certainty that the second lab knew that ethylene glycol poisoning was suspected. In this context it is interesting that Dr. Shoemaker later sent normal plasma spiked with propionic acid to three laboratories and two of the three reported finding ethylene glycol.

The lab that first identified the unknown compound might or might not have been following the standard protocols of the day, but the protocol was insufficiently rigorous in the sense that the identification was only one piece of information, the retention time, as discussed above. However, the commercial lab was deficient by any measure. Clearly, neither lab did a thorough job of examining all information pertaining to Ryan’s serum.

Roger Koppl discussed many of the problems that beset forensic science in the United States. Some of the problems he illuminates, such as dependency bias, were not factors in the Stallings case but may have been at work in the Knox/Sollecito case. He wrote, “the typical forensic worker has psychological incentives to reach findings based on his
close psychological ties with the law enforcement community he serves.” He also treated confirmation and other cognitive biases (p. 13).

“Larry S. Miller demonstrates an excellent example of cognitive bias at play. He asked a group of 14 students trained in hair analysis, all of whom met the basic requirements for expert testimony on human hair identification in courts of law, to examine four cases each. For each student, two cases were presented the usual way: They were given two samples and told that one was from the crime scene and the other from the suspect. The other two cases were presented through a forensic lineup. The known sample from the imaginary crime scene was compared to five suspect-known hair samples. In all 56 cases, there were in reality no true matches. The first group of cases yielded an error rate of 30.8 percent; the second group an error rate of only 3.8 percent.12

Miller’s study illustrates how evidence is often presented to the forensic scientist in a bias-inducing manner. The samples are labeled as coming from the defendant or from the victim and are frequently accompanied by a synopsis of the investigation indicating the reasons that the investigators believe the suspect is guilty. This protocol cues the forensic worker to the expected or correct result.”

The Stallings case in the context of the Knox/Sollecito case
The Johnson/Hampikian open letter criticized both the DNA evidence surrounding the knife and the bra clasp. Some observers of the Knox/Sollecito case have asked what are the odds that both of the DNA results could be the result of contamination or secondary transfer. One might ask what are the odds of at least four identifications of ethylene glycol being wrong, and yet they were in Stallings case. It may be that assigning a probability to the chances of both the bra clasp and the knife being the result of contamination or secondary transfer by standard rules is misleading. When two events are independent, the probability of both occurring are the product of the probabilities of each. So if event A has a 10% probability, and event B has a 10% probability, then the probability of both is 1%. However, when some form of cognitive bias is in play, then this calculation does not apply; in other words, the two events are no longer independent.

Some other parallels can be found. In both cases the investigators seemed oblivious to the contradictory nature of their findings. The second determination of ethylene glycol in Ryan’s serum was so high as to rule out ethylene glycol poisoning four days earlier. The lack of blood on the kitchen knife should have told the forensic DNA analysts that Meredith’s DNA profile was the result of contamination. In both cases the motive for the crime was dubious. Patricia Stallings was thought to be guilty of Munchausen’s by proxy syndrome on the basis of scant evidence. Amanda Knox was accused of trying to get back at Meredith Kercher, because of tensions from rooming together.

References
Michelle Hoffman, “Scientific Sleuths Solve a Murder Mystery,” (1991) Science 253, 931.

Roger Koppl, “CSI for Real: How to Improve Forensics Science”

Dr. Bob Mead, “Accused and Convicted—a case study”

Journal of Pediatrics, The. 1992 Mar;120(3):417-21. Misidentification of propionic acid as ethylene glycol in a patient with methylmalonic acidemia. Shoemaker JD, Lynch RE, Hoffmann JW, Sly WS.

Bill Smith, “Not Guity! How the system failed Patricia Stallings”

Appendix: Methylmalonic acidemia* (MMA)
A metabolic pathway is a series of chemical conversions, in which an enzyme catalyzes each reaction. Enzymes are the products of genes, and an alteration of a gene from its normal form may produce an enzyme that does not function properly. The compound propionyl coenzyme A is the thioester of propionic acid conjugated with coenzyme A (CoA), a thiol. When propionyl coenzyme A is produced in the body, it is converted into succinyl coenzyme A in a pathway that is three steps long: Propionyl CoA carboxylase is the enzyme that changes propionyl CoA into D-methylmalonyl CoA, and this compound is converted into L-methylmalonyl CoA by methylmalonyl CoA epimerase. Finally, this compound is converted into succinyl CoA by the vitamin B12-dependent enzyme methylmalonyl CoA mutase. The gene for this last enzyme is defective in methylmalonic academia, and the inheritance pattern is autosomal recessive. When this happens, the previous compounds build up in concentration. At some point the bond linking propionic acid or methylmalonic acid and coenzyme A is hydrolyed, and the free acids are found in the bloodstream. Among the symptoms of this disease are lethargy, seizures, and vomiting. Metabolic acidosis and ketosis are also observed. Some forms respond to supplementation with vitamin B12.

References to Appendix
Healthline
Nelson, DL and Cox, MM, "Lehninger Principles of Biochemistry," 5th edition.

*update 6/3/10, 4:30 PM. Corrected several spelling errors/typos.