May-June 2014

A letter from the Department of Public Safety crime lab

D. Pat Johnson

Deputy Assistant Director, Law Enforcement Support Division

As reported in the November-December 2012 edition of this journal, DPS Crime Laboratories anticipated a change in its laboratory reports during 2013. One change dealt with the sampling of drug evidence and was instituted by March 1, 2013. Implementation of the other change was delayed until January 1, 2014. It relates to the uncertainty of several measurements made while examining evidence, as follows.
    Regarding controlled substance evidence, if in the form of a solid (powder, plant matter, etc.), it is weighed using a laboratory balance. There is an uncertainty in this weight, which is now reported with some drug exhibits in a case (only those exhibits where the weight is critical to the penalty), and this uncertainty is shown as plus or minus a weight (indicated in bold, below). For example:
Exhibit No. 1: Plastic bag of white powder
Result: Contains Cocaine         
7.05  grams (+/-0.02) grams net weight  

    Regarding the analysis of a blood sample for alcohol on a DWI case, there is an uncertainty in determining the quantity or concentration of alcohol in the blood. For example:
Exhibit No. 2: Blood in gray top tube from John Doe
Result of Analysis: 0.115 (+/-0.005) grams of alcohol per 100 milliliters of blood (99.7% confidence level)

    Other examples of measurements that may include an uncertainty are the amount of a drug detected in a blood sample and the length of a shotgun barrel. Note that this uncertainty of measurement is being reported by DPS crime laboratories to satisfy requirements of the organization that accredits our laboratories. Please call the laboratory serving your county if you have any questions.