THC Measured Per Forehead Sweat Pad Cannot Be Converted Into a Blood Concentration
A result expressed as nanograms of THC per sweat pad measures the amount recovered from that pad. It does not provide a blood concentration. A study involving drivers investigated in Italy illustrates why collection units, confirmation and participant selection must remain visible when interpreting forehead-sweat evidence.
The collection sampled the skin surface
Gentili and colleagues studied 66 drivers stopped for risky driving and regarded as suspicious on behavioral grounds during roadside controls in late 2015. Police rubbed a DrugWipe 5A collection pad over the forehead approximately four to six times and read the screening result after ten minutes.
After a positive screen for a drug class, a second device was used to collect material for laboratory confirmation. The researchers developed headspace solid-phase microextraction with gas chromatography–mass spectrometry to analyze that material. THC required a separate extraction and chromatographic run from the other target drugs.
This was a human casework and analytical-method study. It was not a controlled cannabis-dosing experiment, a random sample of motorists or an assessment of a patch worn continuously for several days.
Why nanograms per pad are not nanograms per milliliter
The method’s THC detection and quantification limits were 0.09 and 0.27 ng per pad, respectively. The researchers explained that the collection technology did not measure the sweat volume collected. A pad amount therefore lacks the measured fluid-volume denominator needed to express a sweat concentration.
A blood concentration would require a separate specimen and a validated blood measurement. The study did not establish a conversion equation from forehead-pad THC to blood THC. Dividing the pad result by an assumed volume would introduce information that was not measured, and still would not establish how a sweat value relates to blood.
Screening and confirmation did not always agree
The laboratory analysis found at least one drug above its quantification limit in 60 of the 66 investigated samples. The paper reported eight THC-only samples, fourteen containing THC and cocaine, and two containing THC, cocaine and MDMA. Those counts describe this selected investigation group.
It also reported seven THC-positive screens without THC confirmation by its laboratory method. The authors called these false positives relative to that confirmation procedure. Because the procedures used separately collected forehead samples, the disagreement should not be treated as a direct chemical reanalysis of the identical material.
The study did not experimentally identify the cause of each discordance. Nor does confirmation of parent THC on a surface, by itself, establish its precise route of deposition or the person’s functional ability.
What the results can support in a case review
The method demonstrated that THC could be recovered and measured from the specified collection pads under the tested conditions. That supports an analytical finding in that matrix. The selected sample and screen-triggered confirmation pathway cannot establish general roadside sensitivity, population prevalence or an impairment threshold.
A review should preserve the collection method, units, screening and confirmation cutoffs, interval between collections and laboratory identification records. A pad result alone cannot supply an absorbed THC dose or an exact time of use.
For the different question of accumulated THC during a longer wear interval, see sweat-patch findings during monitored abstinence. A forehead wipe and a weekly patch should not be treated as interchangeable specimens.
Primary research
Stefano Gentili, Claudia Mortali, Luisa Mastrobattista, Paolo Berretta and Simona Zaami. Journal of Pharmaceutical and Biomedical Analysis (2016), 129:282–287. Determination of different recreational drugs in sweat by headspace solid-phase microextraction gas chromatography mass spectrometry: application to drugged drivers.
