Miscarriages of justice are "almost certainly" taking place because of a mistaken belief that it is possible to calculate from blood analysis at a postmortem examination how many tablets somebody swallowed before they died, a group of eminent scientists and doctors says today.
An article by the group in the British Medical Journal was written after the death of the Iraq arms expert David Kelly and the Hutton inquiry which concluded that Dr Kelly killed himself by cutting his wrists and taking painkillers.
The evidence at the inquiry has "led to the exchange of acrimonious views, including allegations of conspiracy and murder," they note. The Hutton inquiry heard from a forensic toxicologist that Dr Kelly could have swallowed between 29 and 30 tablets of a strong painkiller called Coproxamol which he had been prescribed for back pain.
But, say the authors of the BMJ editorial, the measurement of toxic substances in the blood after death is a very inexact science.
Blood that is not circulating after death is not the same as before death, said Robert Forrest, professor of forensic toxicology at Sheffield University and one of the authors. "After death, drugs which are bound in tissue move back into blood."
They write that drug concentrations are likely to have changed before blood samples can be taken. "For many drugs, including those found in David Kelly, concentrations may increase by as much as tenfold," they say.
To make things even more difficult, different concentra tions of a drug will be found in different parts of the body. "The problem is that some pathologists take samples from anywhere they can find it," said Professor Forrest.
Three samples taken from different parts of the body of an elderly woman suspected of taking an overdose of the antidepressant amitriptylene might vary from 0.3mg per litre to 1mg to 10mg or even more per litre. "How do you interpret that? You need to know an awful lot of information. Trying to do a back calculation of how many tablets somebody has taken is pretty awful," he said.
The danger was that an elderly woman who died of a heart attack would be certified as having taken her own life.
These uncertainties need to be acknowledged, say the doctors and scientists who are members of the International Toxicology Advisory Group.
"If the blood concentration at the time of death cannot be known with certainty, then how is it possible to extrapolate the time and amount of drug ingested before death? The simple answer is that such extrapolations are prone to considerable error and generally should be viewed as unreliable and not evidence based."
"Despite these limitations, such calculations are frequently and wrongly produced during court proceedings, even though the problems we outline have been widely known for many years.
"Postmortem measurements of drug concentration in blood have scant meaning except in the context of medical history, the sequence and circumstances surrounding death and necropsy findings.
"The paucity of evidence-based science, coupled with the pretence that such science exists in regard to postmortem toxicology, leads to the abuse of process, almost certainly to the miscarriage of justice and possibly even to false perceptions of conspiracy and cover up."
