Why a Cannabis Urine Assay Used Lower Sensitivity for Its Highest Concentrations
An assay designed to detect trace urinary cannabinoids deliberately used a less responsive measurement channel for high concentrations. This was a way to avoid detector saturation and extend the usable range, rather than evidence that high-exposure samples needed less careful analysis.
Two analytical ranges addressed different problems
The method measured THC, CBD, CBN, 11-hydroxy-THC and THC-COOH using UHPLC–tandem mass spectrometry. Positive electrospray ionization supplied sensitive measurements at low concentrations. A deliberately detuned negative-ion channel covered a higher range, approximately 12.5 to 800 ng/mL.
The ranges overlapped, allowing the investigators to compare measurements from both channels in fortified samples. Agreement in that overlap supported combining the approaches within one analytical workflow.
Sensitivity and quantitation range are different properties
A low detection limit describes the ability to distinguish a small signal from background. It does not establish that the same settings can accurately quantify an arbitrarily large amount. Saturation can compromise a high-concentration result even when the instrument readily detects it.
The study’s dual-channel design reduced the need to prepare and analyze some samples again. That operational benefit depended on calibration, controls and demonstrated agreement; it was not a license to report concentrations beyond a validated range.
Free and total measurements used different preparation
The researchers also distinguished unconjugated cannabinoids from measurements obtained after enzymatic and alkaline hydrolysis. Hydrolysis was intended to include conjugated material but introduced additional preparation steps and analyte losses.
The unconjugated and total methods consequently had different detection limits and recovery characteristics. A report should identify which preparation was used before two numerical results are compared.
What this validation cannot establish
The validation relied heavily on fortified human urine. Its high analytical sensitivity did not, by itself, classify a person’s exposure as active smoking or secondhand exposure, establish the exact last-use time, or measure impairment.
For an individual report, request the analyte, preparation, ionization channel, calibration range and handling of values outside that range. Those details determine what the number means analytically. They cannot be replaced by a general claim that mass spectrometry is highly sensitive.
Related: Interpreting THC concentration and impairment.
Primary research
Analysis of Cannabinoids and Their Metabolites in Human Urine. DOI: 10.1021/acs.analchem.5b02603.
