A reporting cutoff determines how a laboratory classifies a result; it is not automatically the lowest concentration the instrument can detect or quantify. A postmortem urine EtG/EtS method published by Al-Asmari and colleagues illustrates this distinction, while also showing why printed validation figures need checking before they are applied to a case.
The method measured metabolites, not ethanol itself
The 2010 study developed a hydrophilic interaction chromatography method coupled to electrospray ionization ion-trap tandem mass spectrometry. It measured ethyl glucuronide (EtG) and ethyl sulfate (EtS) in urine. Headspace GC–FID separately measured ethanol in blood and urine.
The procedure used 0.1 mL urine, deuterated internal standards, acetonitrile treatment, centrifugation, evaporation and reconstitution before a 5-µL injection. EtS eluted at approximately 1.9 minutes and EtG at six minutes. Separation within seven minutes was not the entire analytical cycle: the authors reported a routine run time of 20 minutes including other chromatographic steps.
Three different thresholds answer different questions
A detection limit concerns recognizing an analyte under defined identification criteria. A quantification limit concerns measuring it with specified performance. A reporting cutoff is a decision threshold used to label results. These concepts should remain distinct even when a laboratory happens to use the same number for more than one purpose.
This study used reporting cutoffs of 1 mg/L for EtG and 0.1 mg/L for EtS. The authors selected cutoffs above their stated lower quantification limit to address interpretive concerns about low-level exposure. Those historical study choices are not universal current requirements, legal rules or validated thresholds for impairment.
The publication’s numerical limits are internally inconsistent
The text and Table I report a lower quantification limit of 0.001 mg/L for both metabolites. However, Table I prints detection limits of 0.13 mg/L for EtG and 0.2 mg/L for EtS—higher than that reported quantification limit. This ordering is inconsistent with using quantification as a more demanding form of reliable detection.
The correct response is not to silently change the units or guess a decimal point. Before using those figures to evaluate a low result, obtain the actual laboratory validation and reporting procedure. The publication also describes the main calibration range as 0.05–10 mg/L in its results. A claimed lower limit should be evaluated with the corresponding low-concentration experiments, rather than inferred solely from the main calibration curve.
Identification depended on more than one peak
The authors used analyte-specific internal standards and retention information. EtG produced multiple fragments. For EtS, which yielded only one major product ion in their procedure, they used the product ion and remaining precursor signal as part of the identification approach.
This is not equivalent to saying that every LC–MS/MS assay uses two independent product ions. Screening, identification and quantitative acceptance criteria should be recovered from the actual method; the instrument name alone does not establish that a particular result satisfied them.
Performance in validation does not answer every case question
The results described calibration R² values above 0.999 and recoveries around 98–99%; the individual recovery entries varied by concentration. Table III reported intraday precision of 1–8% RSD and interday precision of 2–10% RSD across the listed levels. RSD describes repeatability or reproducibility, not certainty about the origin of ethanol.
The procedure was applied to 79 autopsy urine cases. Their blood came from several venous sites, with 1–2% potassium fluoride; autopsies occurred from 24 hours to several days after death. Those case observations do not make every validation experiment representative of heavily decomposed urine.
A negative report should therefore be read with its cutoff, underlying signal, quantification status and specimen conditions. Our discussion of unquantifiable postmortem results illustrates a related reporting distinction with a different analyte. Neither a biomarker threshold nor successful quantification establishes impairment or cause of death.
Primary source
Al-Asmari AI, Anderson RA, Appelblad P. Direct Determination of Ethyl Glucuronide and Ethyl Sulfate in Postmortem Urine Specimens Using Hydrophilic Interaction Liquid Chromatography–Electrospray Ionization-Tandem Mass Spectrometry. Journal of Analytical Toxicology. 2010;34(5):261–272. doi:10.1093/jat/34.5.261.
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