Linearity in Blood Alcohol Testing

     Linearity is determined by testing known samples of various concentrations, which should ensure that the tests of other samples of unknown concentrations should yield accurate results.  Linearity enables an analyst to have confidence in the results of a particular test based on the results of other tests.  For example, if a known alcohol concentration of .10 is determined to be .10 and a known concentration of .20 is determined to be .20, this greatly increases the likelihood that a blood alcohol reading of .15 is accurate.

     In most cases the linearity of a gas chromatograph is checked at the beginning of each set of tests (a “run”) by injecting calibrators of varying amounts in the GC.  By plotting the amount of each calibrator versus their relative instrument responses, a linear relationship may be established.  The concept of linearity is associated with the “range” of the instrument, which is the interval between the highest and lowest concentrations that have been determined to be not only linear but accurate and precise.  Accuracy means that the testing device has correctly determined the true result, while precision is the ability of the instrument to replicate the test result.  Therefore, even if a blood alcohol test is “precise,” that merely means that the testing instrument has printed the same result more than once.   As defense attorneys we concentrate on accuracy – whether it can be proven that a blood alcohol test result has determined the correct result.

     It is generally agreed that good laboratory practice requires the use of six calibrators spanning the range of 50 to 150% of the expected range of results the analyst expects to encounter in typical cases.   In other words, the concentration of the calibrators should be such that they bracket the anticipated concentration of the specimen.   The Laboratory Guidelines of the Society of Forensic Toxicologists recommends “at least three calibrators.”  If any result exceeds the range, the substance being tested should be diluted and retested.  If the concentration of the specimen is less than that of the lowest calibrator, in most cases an additional calibrator below the expected range of the analyte in the sample should be set up. 

Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website

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