A Stronger Cannabis Signal Could Mean Lower Raw Fluorescence in a Competitive Urine Assay

forensic toxicology working group members October 31, 2026 Forensic Toxicology

In the portable microfluidic urine assay, increasing drug concentrations reduced the raw test signal. The competitive design explains why a lower fluorescence response can correspond to a more positive cannabinoid result.

The sample competed with labeled material

Ko and colleagues used fluorescent antigen-conjugated beads and immobilized drug-specific antibodies. With a drug-free specimen, labeled material could bind and produce a strong test response. Drug in the specimen competed for antibody binding, reducing the measured test fluorescence.

The reported index changed the signal’s direction

The reader compared test and reference responses and then normalized the specimen ratio to the cutoff ratio. Because the specimen ratio appeared in the denominator, a lower raw ratio could yield a higher cutoff index. Reading the instrument’s raw fluorescence as though it increased directly with urinary THC-COOH would invert the intended interpretation.

Signal interpretation requires the actual workflow

The design is useful for understanding laboratory output and troubleshooting, but does not make every low signal a valid positive. Reference performance, preparation and the decision rule still matter. This specific assay measured urinary THC-COOH rather than parent THC in blood. Its antibody signal and calculated index are analytical outputs; neither directly measures a consumed dose, timing of use or functional impairment. When reviewing an instrument report, identify whether the number is raw fluorescence, a test/reference ratio or the normalized classification index before comparing it with another method.

Related analysis: what the urine platform measured.