Toxicology is far from robust and based on flawed evidence

The science of measuring drug levels in the blood after death is far from robust and based on flawed evidence - leading to likely miscarraiges of justice and conspiracy theories, say forensic scientists in this week's BMJ.

For living patients doctors can determine the toxicology (drug concentration levels) of a patient through straightforward tests, involving factors such as how drugs were administered and number of doses taken.

But for dead subjects such information is almost never available, say the authors, so conclusions about drug levels are often incomplete. In addition the way blood behaves after death - what happens when it stops circulating for instance - confuses toxicology measurements. Chronic drug use also muddles matters, inducing 'tolerance' which can be factored into toxicology measurements for living patients but cannot be measured in dead bodies. Despite these concerns however, forensic scientists continue to draw conclusions based on comparisons with living subjects.

These problems and others make it impossible to determine accurately whether the cause of death was solely caused by drug concentrations. Furthermore, if drug levels at the time of death are impossible to determine, pathologists cannot make judgements on the amount of drugs taken into the body before death. Yet these projections are often produced in court as evidence.

This continued reliance on flawed scientific methods can be highly misleading, say the authors, and almost certainly results in miscarraiges of justice. Such unreliable evidence also contributes to false perceptions of conspiracy and cover up. An example of such confusion is that following the death of Government weapons expert David Kelly, where differences of opinion have been expressed in public over the interpretation of the toxicology results, say the authors.