Reduced Interference by Phenothiazines in Amphetamine Drug of Abuse Immunoassays

Archives of Pathology & Laboratory Medicine, Dec 2006 by Melanson, Stacy E F, Lee-Lewandrowski, Elizabeth, Griggs, David A, Long, William H, Flood, James G

Blood and urine samples obtained from patients in the emergency department are sent to the laboratory via a pneumatic tube transportation system and processed immediately on receipt by the laboratory. On completion of analysis (usually 1-3 hours), all urine and serum samples are stored frozen at -20�C.

Toxicology analysis for all serum and select urine specimens (as described later in an adjunct study) was performed using liquid chromatography with photodiode array detection (LCPDA). 6 The detection limits for phenothiazines, amphetamines, and amphetamine-like compounds using LC-PDA are as follows: amphetamine (20 �g/L), chlorpromazine (20 �g/L), methamphetamine (35 �g/L), MDMA (70 �g/L), ephedrine (60 �g/L), phentermine (20 �g/L), promethazine (20 �g/L), and pseudoephedrine (70 �g/L).

Urine toxicology screens were performed on a Hitachi 911 analyzer (Roche Diagnostics, Indianapolis, Ind) using the EMITMAM assay. If the enzyme rate observed using a patient's urine sample exceeds the rate generated using the 1000 �g/L d-methamphetamine calibrator, the sample is considered presumptive positive for amphetamines. The assay also has a limit of detection of 1000 �g/L for d-amphetamine.

In an adjunct study, we selected consecutive patients from the emergency department who were found to have either promethazine (n = 3) or chlorpromazine (n = 6) in their serum and a positive result from an EMIT-MAM urine amphetamine screen. Their serum and urine samples were stored at -20�C for further toxicology analysis. The LC-PDA analysis of the urine (promethazine) or the LC-PDA analysis of serum (chlorpromazine) was performed to determine whether the patients had taken amphetamines. None of the samples had amphetamines detected. All 9 samples producing false-positive urine amphetamine results were also analyzed using 5 other immunoassay platforms, in addition to the EMIT-MAM assay previously described (Table 1).

The EMIT II Plus amphetamine assay (EMIT-A) (Syva Corp) was performed on a Hitachi 911 analyzer. Similar to the EMITMAM, the EMIT-A uses a 1000 �g/L d-methamphetamine calibrator. Urine sample results with enzyme rates above this cutoff are considered positive. The Triage TOX Drug Screen (Biosite Incorporated, San Diego, Calif) and TesTcard 9 (Varian, Inc, Palo Alto, Calif) point-of-care drugs of abuse immunoassays for amphetamine and methamphetamine were performed according to the manufacturers' instructions. The Triage amphetamine and methamphetamine assays use calibrator cutoff concentrations of 1000 �g/L. In the TesTcard 9 system, 300 �g/L of d-amphetamine and 500 �g/L of d-methamphetamine are used as calibrator cutoff concentrations for the amphetamine and methamphetamine assays, respectively.

RESULTS

We retrospectively reviewed the results of all patients who presented to the Massachusetts General Hospital emergency department during an 11-month period who had promethazine detected in their serum. From these patients, we found 22 who also had a urine toxicology screen performed; these 22 patients were included in this study (Table 2). Using LC-PDA, no amphetamine or amphetamine- related compounds were detected in the serum of any of the patients. Eight (36 %) of the 22 patients taking promethazine had positive test results for urine amphetamine (Table 2 [results in bold]). Results greater than 100% of the cutoff calibrator were considered positive. In addition, 4 (29 %) of the 14 negative results were more than 50% higher than the amphetamine-free blank calibrator used in the assay (Table 2 [results in italic]), suggesting some effect of promethazine on assay results. Sixty- four percent of patients taking promethazine showed cross-reactivity greater than 20% above the blank calibrator rate using the EMIT-MAM assay.