Published: Journal of Analytical Toxicology, ISSN 0146-4760, Volume 31, Number 9, November/December, pp.549-554

Intensity of the Internal Standard Response as the Basis for Reporting a Test Specimen as Negative or Inconclusive
Ray H. Liu[1], Chih-Hung Wu[1], Yi-Jun Chen[1], Chiung-Dan Chang[1], Jason G. Linville[2], and Dennis V. Canfield[3],
[1]Department of Medical Technology, Fooyin University, Kaohsiung Hsien 831-01, Taiwan;
[2]Department of Justice Sciences, University of Alabama at Birmingham, Birmingham, Alabama 35294-4562; and
[3]Bioareonautical Sciences Research Laboratory, Federal Aviation Administration, Oklahoma City, Oklahoma 73125-5066

Under normal circumstances, a test specimen is reported as “negative” when the response of the analyte is absent. However, if the intensity of the internal standard (IS) is low, indicating interference factors, the test could be considered “inconclusive.” A quantitative hypothesis, A = (R × I × S)/L, serves as the “cutoff” for the acceptable signal-to-noise (S/N) ratio for the IS in making “negative/inconclusive” decisions, where A is the acceptable S/N ratio for internal standard; R is the relative response of the IS and the analyte (same concentration); I is the concentration of the IS; S is the (minimal S/N ratio); and L is the limit of detection. The hypothesis was empirically tested using the 9-carboxy-11-nor-D9-tetrahydrocannabinol (THC-COOH) analyte, THC-COOH-d3 IS, with ibuprofen and hydrogen peroxide (H2O2) as interference factors. Urine specimens containing 0–5 ng/mL of THC-COOH were spiked with various quantities of ibuprofen or H2O2, followed by liquid–liquid extraction, derivatization, and gas chromatography–mass spectrometry (GC–MS) analysis under selected-ion-monitoring mode. Among the “adulterated” test specimens evaluated (those with an S/N for the internal standard below the acceptable IS S/N “A”) the quantitative criterion was indeed found to provide a useful guide for making negative/inconclusive decisions. This equation could be programmed into the instrument software to flag results as being inconclusive when they do not meet the criteria described in this paper.

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