Postmortem blood alcohol concentrations (BACs) tend to be less reliable and stable than BACs obtained from drinking drivers for the following reasons:
Postmortem blood is not sterile, over half of postmortem blood samples were found to contain bacteria and fungi (WOA70202)
Postmortem blood can have a much higher glucose concentration of 7 to 10X that of antemortem blood (WOA70201)
Postmortem diffusion of alcohol from the gut can falsely elevate the BAC (WOA70301).
A high glucose concentration and bacteria/fungi/yeast found in postmortem blood makes an ideal environment for the production of alcohol as the sugar is converted eventually into alcohol causing a falsely high BAC:
Glucose = Acetaldehyde = Alcohol
Yeasts, the most efficient fermenters, can convert approximately 100 mg of glucose into 40 to 50 mg of alcohol. Bacteria and fungi typically can only convert 100 mg of glucose into 10 to 20 mg of alcohol.
During fermentation, other volatiles such as n-propanol and acetaldehyde are produced which can be used as markers of putrefaction. Other samples such as urine and vitreous humor are more stable than blood and can also indicate putrefaction. This is illustrated in the following postmortem case in which I was the forensic toxicologist (WOA70211).
A 31 year old man was found dead several hours after he apparently committed suicide via a fall from a great height. At autopsy the next day, peritoneal cavity blood and urine were collected. The blood was placed in a jar without preservatives and the urine in a tube with 1% NaF. Nineteen days later, the samples were received at the laboratory and a headspace gas chromatographic analysis showed a BAC of 0.096 g/100mL.
This result raised all sorts of questions. Where did the victim drink? Was the BAC related to the fall? Did he stumble due to the BAC and the fall was an accident and not a suicide?
Fortunately, analysis of the urine and a more detailed analysis of the blood showed:
It has been suggested that the concentration of alcohol produced postmortem is about 20 to 25X that of the n-propanol produced (WOA70208, WOA702U2), which would indicate that all the alcohol was produced postmortem. Combined with the zero urine alcohol concentration and elevated blood acetaldehyde concentration, it can reasonably be concluded that the victim was alcohol-free at the time of death. This case also illustrates that, especially for postmortem alcohol analysis, t-butanol should be used as the internal standard and not n-propanol.
My favourite researcher and writer in this field is the Australian pathologist V.D. Pleuckhahn, who published several important studies nearly 50 years ago on postmortem blood alcohol levels. His conclusions, however, are as valid today (WOA70202):
Blood alcohol levels at autopsy are valid up to 48 hours after death when simple principles are observed in the collection and storage of samples
Alcohol levels in samples of blood taken from the intact heart are as significant as levels of blood from the femoral veins
False blood alcohol levels greater than 0.200% can be generated in autopsy blood samples which are not correctly stored.
High blood alcohol levels may develop during putrefaction and levels up to 0.200% do not necessarily indicate that alcohol was imbibed before death.
Significant false blood alcohol levels do not develop during incineration in absence of putrefaction.
Pleuckhahn also stated that the more specific GC method be employed for postmortem alcohol analysis rather than the chemical methods typically used at the time.
What complicates the postmortem blood alcohol issue even further is that, not only can putrefaction increase the BAC, but microorganisms can also decrease the BAC by up to 50% (WOA70207).
In my next blog on the reliability of postmortem BACs, I will discuss the effects of postmortem diffusion of alcohol on BAC.