Cut-Off Levels for Positive Alcohol Tests
One of the more frequent guidance we offer our customers is establishing cut-off levels for positive alcohol tests. It’s tempting for some users to regard any non-zero result as positive. After all, when a sober person blows into an Alco-Sensor breath alcohol instrument the result is always .000. Why not regard a result of .001, .002, .003, etc. as positive?
While it might make life a little simpler if one could treat any non-zero result as a positive alcohol test, there are several reasons why we do not recommend that users treat readings from .001 to .009 as positive tests.
First of all we point out what a tiny amount of alcohol that a reading from .001 to .009 represents. Picture a ruler marked in centimeters and millimeters. Now imagine that between each of the tiny millimeter lines there are 10 more lines. Those 10 imaginary lines between each millimeter on the ruler are the equivalent of the third digit of the Alco-Sensor. The imaginary lines ruler represent 100 micrometers; the third digit on the Alco-Sensor represents 1/1,000 BAC. So a reading between .001 and .009 is like trying to read one of the micrometer lines that is between the end of the ruler and the first millimeter line of the ruler.
Secondly, we point out that it is standard laboratory practice to establish a cut-off level to identify a positive test. For example, when laboratories test for drugs, they do not accept the presence of drugs at any level as a positive test. Instead, scientists establish cut-off levels that are high enough to have a high degree of confidence that the individual used the drug. Cut-off levels must also account for the margin of error inherent in every measuring device. A negative drug test result does not mean that the specimen contained zero drugs; it means that drugs were not present at or above the cut-off level established for that drug.
For workplace and other evidential testing applications we recommend using a 0.020 cut-off level; results of 0.019 and below are regarded as a negative test. This is the cut-off level established by the Department of Transportation (DOT) for workplace testing, and thus can be readily defended.
For substance abuse treatment programs and other non-evidential applications we are comfortable recommending a 0.010 cut-off level when using an Alco-Sensor. Results of .009 and below are treated as a negative test.
See our related article on zero tolerance alcohol testing for a more in depth discussion