Higher THC Recovery From Urine Was an Extraction Finding
The assay recovered a higher fraction of fortified THC from urine than from whole blood. Those percentages described laboratory extraction performance, not the fraction of a person’s cannabis dose excreted in urine.
The matrices had different validation results
Frei and colleagues reported THC extraction efficiency of approximately 59% for whole blood and 82% for urine in their validation tables. The methods used different pretreatment for the two matrices before automated extraction and GC–MS/MS analysis. Their calibration and quality-control work evaluated the resulting analytical measurements.
Recovery has a laboratory denominator
Extraction efficiency compares analyte recovered through preparation with the corresponding analytical reference. A percentage loss during extraction is not automatically the same percentage error in a final concentration when the validated calibration and internal-standard procedure accounts for the workflow. It is also not a measurement of renal clearance or oral bioavailability.
Avoid a biological conversion rule
The two recovery percentages do not provide a urine-to-blood concentration factor or allow reconstruction of an administered THC dose. They indicate that matrix and preparation affect analytical recovery. A case review should use the actual laboratory’s matrix-specific validation rather than importing these values as universal corrections. Exposure, excretion and impairment remain separate questions from recovery of a fortified analyte during extraction.
Related analysis: the automated cannabinoid assay.
Research cited
Frei and colleagues (2022). Quantitative determination of five cannabinoids in blood and urine by gas chromatography tandem mass spectrometry applying automated on-line solid phase extraction. DOI: 10.1002/dta.3241.
