An association between a urine screen and a blood-cell count can identify an interpretive possibility without identifying the cause of an individual’s abnormal result. The distinction matters when a positive screen is used to stop investigating other explanations.
What did the historical comparison show?
Richards and colleagues’ 2014 retrospective study analyzed 1,206 emergency-department visits for psychiatric medical clearance. There were 877 visits with urine screens negative for both amphetamines and cocaine, 240 amphetamine-positive/cocaine-negative, 72 cocaine-positive/amphetamine-negative and 17 positive for both.
Mean white-cell counts were 8.4 ± 2.6, 9.4 ± 3.3, 7.1 ± 1.8 and 10.0 ± 4.2 × 10³/µL, respectively; spreads are standard deviations. Counts above the laboratory’s 11.0 × 10³/µL upper boundary occurred in 57/240 amphetamine-only visits (23.8%) versus 130/877 controls (14.8%, p=0.001).
The study used qualitative amphetamine-class screening, excluded selected medical presentations including elevated temperature, and lacked medication reconciliation, exposure timing and white-cell differential counts.
The screening class was not a methamphetamine-specific dose
The exposure category included amphetamine derivatives rather than a measured methamphetamine concentration. A class-positive group therefore should not be relabeled as people with a known methamphetamine dose, route or time of use.
Likewise, the outcome was a circulating cell count. It did not directly measure a particular brain inflammatory process, establish infection status in every future patient or identify which cell population accounted for the increase. Those questions require their own observations.
Why the cocaine comparison does not prove a protective effect
The lower mean count in the cocaine-only group is also an association in selected visits. It does not establish that cocaine prevents leukocytosis. Differences in the people, their presentation, timing or other exposures can affect a comparison even when its p value is small.
For the same reason, the higher count in the amphetamine-positive group does not isolate a pharmacological cause. A retrospective comparison without complete information on relevant alternative explanations cannot assign the entire difference to the drug category.
The useful case-review approach is to preserve the original units and category definitions, then examine what was actually documented in the person’s record. A positive urine screen is one observation to interpret alongside the examination and other findings. It cannot establish that an elevated white-cell count is harmless or permit an individual causal diagnosis from the group average.
Source
Richards JR, Farias VF, Clingan CS. Association of leukocytosis with amphetamine and cocaine use. ScientificWorldJournal. 2014;2014:207651. doi:10.1155/2014/207651. Full text. Historical observational evidence.