A larger amount of drug on positive sweat patches does not necessarily mean a larger percentage of patches will be positive. Barnes and colleagues’ historical controlled methamphetamine study illustrates this distinction: the higher-dose phase produced greater methamphetamine mass on positive weekly patches, but a smaller proportion of the available weekly patches tested positive. The apparent contradiction disappears when the denominators, missing data and reporting rules stay attached to the results.
Who received which drug?
Eight volunteers lived on a secure research unit during a ten-week protocol that included a two-week washout. All eight received four oral 10 mg doses of sustained-release S-(+)-methamphetamine hydrochloride within one week. Five later received four oral 20 mg doses after an interval of at least three weeks that included placebo administration. Two participants did not proceed for medical reasons and one for personal reasons.
These were cumulative dosing-week totals of 40 mg and 80 mg of the administered formulation, rather than single 40 mg or 80 mg doses. Some doses were not on consecutive days because of baseline heart rate or scheduling. The findings concern this oral sustained-release regimen; they should not be silently reassigned to smoked, injected or intranasal methamphetamine.
Were 682 patches equivalent to 682 independent participants?
No. The researchers analyzed 682 sweat patches from eight people. Short-term patches covered intervals from two hours upward, while weekly patches accumulated material over a longer period. Duplicate weekly patches were placed on the back and abdomen, and their results were reported individually.
Thus repeated and duplicate patches are observations within participants, not additional independent volunteers. A patch-level percentage cannot be described as the percentage of all people who would be detected after a particular dose. Available observations were also affected by missing patches, skin irritation, collection issues and analytical exclusions. The paper reports that 48 short-term and 20 weekly patches were lost between collection and analysis.
What did the weekly dose comparison show?
| Measure | Four 10 mg doses | Four 20 mg doses |
|---|---|---|
| Methamphetamine at or above the assay quantification limit | 13/14 patches: 92.9% | 5/8 patches: 62.5% |
| Median methamphetamine on positive patches | 63.0 ng/patch | 307 ng/patch |
| Range on positive patches | 16.8–175 ng/patch | 199–607 ng/patch |
| Methamphetamine-positive under the historical proposed reporting rule | 12/14 patches: 85.7% | 5/8 patches: 62.5% |
The higher-dose median was larger, but fewer available weekly patches were positive. The percentage difference was not statistically significant, with the authors reporting p>0.05. Small patch counts, different participant availability, missing data and individual variation limit interpretation. These figures do not establish that a larger dose generally makes methamphetamine harder to detect.
Why did the reporting cutoff change the answer?
The GC-MS method quantified methamphetamine and amphetamine at 2.5 ng/patch. The investigators also applied a proposed SAMHSA rule discussed at the time: a methamphetamine result of at least 25 ng/patch with amphetamine present at or above the method’s detection limit. This was a historical proposed criterion, not a statement of current law or present laboratory policy.
After the first 10 mg dose, 12 of 32 short-term patches had measurable methamphetamine, but none met that reporting rule because amphetamine was not detected in those patches. An analytically measurable parent drug and a reportable positive under a rule requiring its metabolite are therefore different classifications.
Can a later positive patch date a new exposure?
At the assay quantification limit, some patches applied after the dosing week still contained drug. During the first postdose week, methamphetamine was measurable in eight of 13 available patches after the lower doses and seven of seven after the higher doses. One patch collected during the second postdose week after the higher doses contained 15.2 ng methamphetamine. No postdose weekly patch met the historical proposed methamphetamine-positive rule.
Those observations show why patch application and removal dates, wear duration and the actual reporting criterion matter. They do not create a universal detection window or establish that a positive patch necessarily represents a new dose taken during that wear interval.
What does the measured unit mean?
Nanograms per patch express accumulated analyte mass on the collector. They are not nanograms per milliliter of blood or a measured administered dose. The study also found that summed short-term patch amounts often exceeded corresponding weekly amounts, raising questions about losses during longer wear. It did not establish a universal recovery factor that could convert one patch result into total intake.
The evidence supports careful exposure monitoring under the conditions tested. It does not establish an individual’s time of ingestion, dose, current impairment or driving ability from patch mass alone.
Scientific source
Barnes AJ, Smith ML, Kacinko SL, Schwilke EW, Cone EJ, Moolchan ET, Huestis MA. Excretion of methamphetamine and amphetamine in human sweat following controlled oral methamphetamine administration. Clinical Chemistry. 2008;54(1):172–180; published online November 2, 2007. doi:10.1373/clinchem.2007.092304. Historical controlled human administration study; full text recovered for the paper identified in the supplied research collection.
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