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

In another experiment, we studied paired urine and serum samples from 9 patients in the emergency department using 5 different amphetamine immunoassay plat- forms (Table 3). Serum sample results from all these patients were positive for either promethazine or chlorpromazine, and negative for amphetamines using LC-PDA. Each urine sample from these patients tested positive for amphetamines by EMIT-MAM, and LC-PDA did not detect any amphetamines in the urine of the promethazine users. The EMIT-MAM assay uses a 1000 �g/L d-methamphetamine cutoff calibrator. All 9 samples were higher than the EMIT-MAM calibrator and the results were reported as positive. The EMIT-A assay detects d-amphetamine at about the same concentration as d-methamphetamine, which is 1000 �g/L. Eight of the 9 urine samples had EMIT-A enzyme rates indistinguishable from the blank calibrator (

COMMENT

Promethazine is used in a variety of clinical situations and its potential cross-reactivity in drugs of abuse assays is clinically relevant.7-9 Promethazine use appears to be increasing according to findings at this institution. Promethazine was detected in 23 (1.8%) of 1290 serum toxicology samples obtained from patients presenting to the emergency department during a recent 47-day period. A 20-�/L limit of detection for promethazine using LC-PDA allows detection of the drug in serum samples from patients taking typical therapeutic doses. As a consequence, our study population resembles a typical one from an emergency department, not a group of patients with known toxic serum promethazine concentrations. More than one third of our patient population with a positive finding for serum promethazine tested positive for amphetamine in urine using EMIT-MAM. In a previous study, 18 inmates received 50 mg of promethazine every day and their urine was subsequently screened by an earlier- generation EMIT-MAM assay for drugs of abuse.4 Three inmates (17%) had amphetamine detected in their urine. No false-positive results were seen when less than 50 mg/day promethazine was administered. In this study, we demonstrated that false-positive results by EMITMAM are caused by promethazine use. The higher falsepositive frequency observed in this study may be explained by the differences in the subject population. All subjects in this study were patients admitted to the emergency department in whom serum and urine toxicology testing was ordered, and no limit was placed on the amount of promethazine the patients had received.

A previous study by Smith-Kielland et al4 indicated that preparations of promethazine up to 3000 mg/L tested negative by the EMIT-MAM assay for amphetamine. In addition, the Syva package insert states that promethazine shows a negative response. This suggests that a promethazine metabolite(s), not promethazine itself, is responsible for the false-positive results described in Table 2. Chlorpromazine metabolites cross-react in the EMIT-MAM assay. 3,4 Promethazine and chlorpromazine possess similar phenothiazine chemical structures, supporting the hypothesis that a promethazine metabolite(s) interferes with the EMIT-MAM assay. The metabolism of promethazine has not been thoroughly investigated in humans, but it is extensively metabolized. The major metabolites are promethazine sulfoxide, norpromethazine, and monodesmethyl promethazine sulfoxide.10 Primary standards of promethazine metabolites are not currently available so we were unable to definitively determine which of the promethazine metabolites are responsible for the cross-reactivity in the EMIT-MAM assay. Manufacturers cannot test for all potential cross-reactants in drugs of abuse assays, and laboratories and clinicians should be encouraged to report false-positive results in these assays. We also suggest that manufacturers expand the information included in package inserts to include whether a compound exhibits significant cross-reactivity compared with a 'blank' solution. We believe this extra information will aid investigators studying suspected false-positive results.