Re-analysis of suspected DWI blood samples for ethanol: A Texas case study

The gradual loss of ethanol over time in stored blood specimens under a variety of conditions has been well documented. An analysis of over 160 blood specimens from suspected impaired drivers was recently accomplished with the knowledge that the samples had previously been analyzed. These two analyses were performed independently, using different methods and instrumentation and by different individuals. Although in most cases there were two tubes available in each case, the tube used for the initial analysis was also used for the second analysis. Reported results from both laboratories were obtained and evaluated retrospectively. Over an average interval of approximately 13 months (range 34-1002 days), the average change of ethanol concentrations was a loss of 0.006 g/dL, with a maximum loss of 0.023 g/dL, and a maximum increase of 0.004 g/dL. The median difference was a loss of 0.005 g/dL. The percentage of samples that reported second concentrations equal to or less than the original reported concentrations (to the thousandths decimal place) was 96.4%. No correlation was observed between the net loss and the initial BAC value, but the amount of time between analyses did impact the extent of the loss of ethanol as determined by the second analysis. Our results indicate a smaller loss of ethanol, and hence stronger correlation between analytical events, than what has been experienced in similar work. Based on our analysis and review of previously opened and analyzed blood specimens, a previously opened blood tube can yield a strong correlation to the original analysis and may therefore be appropriate if a second tube is not available or compromised in some way.