A postmortem GHB result slightly above an interpretive cutoff does not become certain evidence of ingestion because it is printed to two decimal places. The laboratory needs a method suitable for that matrix and concentration, and the biological interpretation needs the specimen history. Numerical resolution, analytical uncertainty and uncertainty about the source of GHB are different issues.
What kind of source raised this concern?
Pirani and Busardò addressed these issues in a methodological letter published online in 2020. It was commentary on another publication, not a new autopsy series, dosing experiment or independent validation. It therefore supplies a reasoned critique and questions to investigate; it does not provide a new sample size or experimentally established error rate.
The letter supported using multiple matrices while emphasizing missing information about storage, decomposition, analytical validation and the brain region sampled. Its central practical point was that adding specimens does not compensate for an inadequately characterized measurement process.
A cutoff, a quantification limit and uncertainty answer different questions
An interpretive cutoff is a proposed decision threshold used to assess the possible origin of GHB. A quantification limit concerns the concentration at which a method can produce quantitatively acceptable results. Neither is itself a measurement-uncertainty interval.
The letter recommended sensitive, properly validated methods with a quantification limit below the proposed cutoff. A method that only begins quantifying at the decision point provides little information about results immediately below it. Reporting a precise-looking number does not resolve that limitation.
Likewise, a printed value only slightly above a threshold does not show whether a repeat measurement or an uncertainty assessment would remain above it. That question requires the laboratory’s actual performance data and decision approach. An uncertainty interval cannot be invented from the number of decimal places.
What useful validation evidence looks like
A separate primary study by Andresen-Streichert and colleagues illustrates the kinds of details that can be reported. Their derivatization GC–MS method had a detection limit of 0.5 mg/L, quantification limit of 0.6 mg/L and linear calibration range of 1–100 mg/L. They reported accuracy of at least 91% and imprecision of 9% or less at tested controls.
Those are validation results, not a ready-made uncertainty interval for another laboratory’s case result. They also do not validate every possible postmortem matrix or concentration. The study used different control matrices for blood, urine and the other fluids and examined whether aqueous calibration produced comparable results to serum or saline calibration.
That distinction matters when reviewing the letter’s concern about non-matrix-matched calibration. The concrete question is whether a laboratory has demonstrated suitable performance in the relevant matrix. The mere word “water” on a calibrator preparation sheet does not quantify a case-specific error.
Analytical confidence does not settle biological origin
Even a well-supported quantitative result cannot, by analytical precision alone, distinguish ingested GHB from GHB formed after death or during storage. The letter emphasized refrigeration history, decomposition and specimen selection for this reason. Those uncertainties require biological and case evidence beyond the instrument output.
For brain specimens, the sampled region should be identified. For blood, the collection site should be identified. The interval before collection and subsequent storage time and temperature should remain separate. A universal uncertainty percentage cannot account for all those processes.
What to request for a near-threshold result
Useful records include the method’s matrix scope, calibration and controls near the concentration of interest, quantification limit, interference assessment, measurement uncertainty and handling history. Reviewers should also ask what the proposed threshold was designed to classify and under which specimen conditions.
These questions do not establish that a reported result is wrong. They define what is needed to assess its strength. Drug detection, a quantitative concentration, an inference of ingestion and an attribution of impairment or death remain separate conclusions. For a specific analytical comparison, see GHB measurement with different internal standards.
Primary sources
Pirani F, Busardò FP. The Difficult Interpretation of GHB Values in Postmortem Samples: What Strategies Should Be Followed? Journal of Analytical Toxicology. 44(9):e11–e12. Published online November 18, 2020. doi:10.1093/jat/bkaa130.
Andresen-Streichert H, Jensen P, Kietzerow J, et al. Endogenous gamma-hydroxybutyric acid (GHB) concentrations in post-mortem specimens and further recommendation for interpretative cut-offs. International Journal of Legal Medicine. 2015;129:57–68. doi:10.1007/s00414-014-1051-8.
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