Attention deficits associated with methamphetamine-dependence history were no longer statistically significant after researchers excluded participants with positive methamphetamine urine screens. This sensitivity analysis changes how the results can be described: dependence history and evidence of recent use were not interchangeable.
Pocuca and colleagues enrolled 205 participants classified by HIV status and methamphetamine-dependence history. They used Conners’ and five-choice continuous performance tests to evaluate sustained attention and vigilance, while accounting for viral-load suppression. Two participants lacked viral-load data, leaving 203 in the analytical sample. The study was observational, not randomized methamphetamine administration.
The initial analyses linked methamphetamine history to poorer sustained attention and vigilance, including missed targets. Sixteen participants in the dependence-history group had positive urine screens. Removing these participants eliminated the significant methamphetamine effects on the principal attention and vigilance outcomes. The researchers did not find the hypothesized synergistic HIV-by-methamphetamine effect. HIV-related reaction-time slowing was a separate finding.
A positive urine screen supplied information about recent exposure; it did not directly measure brain concentration or impairment during the test. Excluding that subgroup changes both exposure composition and sample size, so loss of statistical significance alone cannot prove that acute methamphetamine caused the initial differences. Neither does it establish that longer-term effects are absent. Viral-load suppression concerns viral measurement, not a methamphetamine matrix conversion. For forensic interpretation, specify whether the opinion concerns use history, current detection, reaction speed or sustained attention. These endpoints require their own evidence and cannot be inferred from a cohort label alone.
For a separate example of the limits of use-history comparisons, see our current-user driving study. The design and measured outcome determine which conclusions can be carried into an individual case.
Original investigation: doi:10.1016/j.drugalcdep.2020.108245.