In toxicology, unraveling the unexpected positives
Amy Carpenter Aquino
December 2022—In toxicology testing, cross-reactive compounds, incomplete medical records, immunoassay performance, calibrator drift, and human error all play into unexpected positives. “Always be critical of your testing. Be hard on it,” Danyel H. Tacker, PhD, D(ABCC), said in an AACC session in July.
Patient interference should be on the radar, but drug and supplement cross-reactivity is the most likely culprit in urine drug screens, said Dr. Tacker, medical director of clinical chemistry and mass spectrometry laboratories at West Virginia University and J.W. Ruby Memorial Hospital. In a session on toxicology investigation, she tackled the unexpected positives and Nicholas Heger, PhD, NRCC, co-speaker, the unexpected negatives (see CAP TODAY, https://bit.ly/3AFIKrZ). Toxicology testing is a “continuous quality control process,” Dr. Tacker noted, one from which the laboratory “can’t look away.”
But it’s sometimes something else. Patients transferred to WVU Medicine from one of many independent clinics that don’t use Epic or document medications the same way may assume their data transferred too, but it hasn’t. “Then we get this unexpected positive and you end up doing all this work and you find out, yes, it was supposed to be there. These unexpected results can be that wolf in sheep’s clothing,” she said.
In the first case she described from her clinical experience, the laboratory began to use a new fentanyl screening test that quickly resulted in a cluster of false-positives in behavioral medicine substance use disorder clinic patients who had used fentanyl before or could have had a contamination. Patients were adamant about compliance with the program’s rules for nonuse, and confirmatory compliance checks were consistently negative.