Clinical Chemistry 43: 635-637, 1997;
(Clinical Chemistry. 1997;43:635-637.)
© 1997 American Association for Clinical Chemistry, Inc.
Interference with testing for lysergic acid diethylamide
Detlef Ritter1,2,a,
Cherise M. Cortese2,
Linda C. Edwards1,
Judith L. Barr2,
Hyung D. Chung1,2 and
Christopher Long2
1
Pathology and Laboratory Medicine, 113JC, John Cochran Veterans Affairs Medical Center, 915 North Grand Blvd., Saint Louis, MO 63106.
2
Department of Pathology, Saint Louis University
Health Sciences Center, 3635 Vista at Grand, P.O. Box 15250, Saint
Louis, MO 63110-0250.
a Author for correspondence. Fax 314-268-5110; e-mail ritterdg{at}sluvca.slu.edu
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Abstract
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We found a high rate (4.2%) of positive results for lysergic acid
diethylamide (LSD) by Emit in 1898 urine samples that were submitted
primarily from psychiatric patients for drugs-of-abuse (DOA) testing.
Specimens that tested positive for LSD by Emit subsequently tested
negative for LSD with two RIAs. Furthermore, LSD was not detected in
randomly selected Emit-positive urine samples by gas
chromatography–mass spectrometry. Normal urine samples tested positive
for LSD by Emit when they were supplemented with therapeutic
medications that were prescribed for patients with positive urine LSD
results by Emit. These therapeutic drugs interfered specifically with
the Emit assay for LSD, since other Emit DOA tests were not affected by
these medications at the tested concentrations.
Key Words: indexing terms: LSD • immunoassay • drugs of abuse • drug monitoring • gas chromatography–mass spectrometry
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Introduction
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Testing for drugs of abuse (DOA) is an important diagnostic tool
that may contribute to the establishment of the diagnosis in the
intoxicated patient.1
Several
types of DOA tests have been used in the clinical laboratory, such as
chromatographic methods and antibody-based assays. More recently,
immunoassays have become a popular tool because they are easy to
perform and guarantee short turnaround times. Unfortunately,
immunoassays are associated with such caveats as lack of sensitivity or
lack of specificity (1). Results that are positive by
immunoassay may be confirmed by more complicated and time-consuming
techniques such as thin-layer chromatography or gas
chromatography–mass spectrometry (GC-MS). Frequently, clinical
decisions have to be made before preliminary results by immunoassay can
be confirmed by another method. Therefore, the interpretation of test
results requires the knowledge of substances that may interfere with
the testing (1). We have evaluated the new homogenous Emit
immunoassay for lysergic acid diethylamide (LSD) from Behring
Diagnostics. The assay was associated with a high false-positive rate
when we tested urine samples that were submitted by patients from our
hospital population. Further investigation revealed that certain
prescribed medications interfered with the LSD Emit test in vitro, but
not with Emit tests for other DOA.
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Materials and Methods
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Procedures followed were in accordance with the ethical standards
of our institutional review board. Consecutive urine specimens that
were submitted for DOA testing had been collected over 6 weeks. Samples
were stored at 4 °C and shielded from light. LSD was measured in the
urine with the Emit II® assay from Behring Diagnostics
(Cupertino, CA) on a Syva-30R automated assay analyzer (Behring
Diagnostics). LSD was also analyzed with the RIA from Diagnostics
Products Corp. (Coat-A-Count; Los Angeles, CA) and the RIA from Roche
Diagnostic Systems (Abuscreen; Branchburg, NY). Randomly selected urine
samples were sent to a reference laboratory for the detection of LSD
parent compound by GC-MS. Ions were monitored at m/z ratios
of 253, 293, and 395 on a Hewlett-Packard 5890 GC-5970 MSD instrument
(Palo Alto, CA) as described previously (2). The
manufacturers of the two RIAs and the Emit assay reported a cutoff
concentration of 0.5 µg/L for a presumptive positive LSD result. The
threshold concentrations for positive LSD results by the immunoassays
were determined in our laboratory by serial dilution of a LSD
calibrator (Radian Corp., Austin, TX). The cutoff concentration for LSD
by GC-MS was reported to be 0.2 µg/L. Current therapeutic medications
from patients who tested positive for LSD by Emit were identified by
review of the medical charts. These drugs were obtained from the
hospital pharmacy and dissolved in normal urine samples that had
previously tested negative for LSD.
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Results and Discussion
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During the 6 weeks, 1898 urine samples were submitted for DOA
testing. Specimens originated from various psychiatric clinics, from a
methadone maintenance program, from psychiatric inpatients, and from
the psychiatric emergency room (ER). DOA testing was also requested on
specimens from ER patients, on specimens from patients who underwent
treatment for acute surgical or medical conditions, and on urine
samples that were received from a shelter program for homeless
veterans. Seventy-nine of the 1898 submitted urine samples (4.2%)
tested positive for LSD by Emit (Table 1
).
The surprisingly high rate of positive LSD Emit test results prompted
us to compare the test with other commercially available assays. Of the
79 LSD-positive specimens, 8 were of insufficient quantity for further
testing and 71 were reanalyzed with two RIAs. All of the 71 urine
samples (3.7% of all screened urine specimens) that had tested
positive for LSD by Emit subsequently tested negative with both RIAs.
Several factors could have accounted for the discrepant results that
were obtained with the three assays. First, a lower threshold
concentration for LSD by Emit could have resulted in the detection of
LSD by Emit, whereas LSD may have been missed by the RIAs. We
determined the minimal concentrations resulting in positive LSD test
results for the Emit, Coat-a-Count RIA, and Abuscreen RIA to be 0.43,
0.31, and 0.58 µg/L, respectively. Furthermore, LSD was not detected
in the urine samples with the Abuscreen RIA even at a lower threshold
concentration of 0.2 µg/L. These results demonstrate that the
positive test results by Emit were not due to different threshold
concentrations of the assays.
The discrepant LSD results between Emit and RIA may also have been
caused by interfering substances, resulting in false-negative
measurements by RIA or false-positive results by Emit. Visual
inspection of urine samples and determination of urine pH and
creatinine failed to indicate that the samples were adulterated
(3)(4). To evaluate the accuracy of the
immunoassays, we tried to detect LSD in urine samples with a
chromatographic method. Ten of the 71 urine samples with discrepant LSD
results between Emit and RIA were randomly selected and submitted for
detection of LSD by GC-MS. LSD was not found in any of the 10 urine
samples. On the basis of these results, a negative LSD result by GC-MS
would have been expected in at least 53 of the 71 urine samples that
tested positive for LSD by Emit (one-sided 95% exact confidence
interval: 53–71). Because urine samples tested negative for LSD by
three alternative assays, we concluded that the positive LSD results by
Emit were due to unidentified interfering substances.
Because the highest rate of false-positive LSD results were obtained
from patients who had been seen by psychiatric, medical, or surgical
services, we suspected the patients' prescribed medications as a cause
of the drug interference. Most of the prescribed drugs have previously
been shown to be excreted into the urine as metabolites rather than as
unchanged compounds (5). Since drug metabolites were not
readily available, we evaluated whether the parent compounds of
therapeutic drugs would interfere with the LSD Emit test in vitro by
supplementing normal urine with 47 prescription drugs. The following 26
medications did not have an effect on LSD measurements by Emit at the
tested concentrations of one mg/mL: acetaminophen, albuterol,
allopurinol, alprazolam, atenolol, caffeine, carbamazepine, clonazepam,
diazepam, flunitrazepam, glyburide, hydrochlorothiazide, ibuprofen,
indomethacin, librium, lidocaine, lorazepam, methaqualone,
methocarbamol, nicotine, nifedipine, perphenazine, phenobarbital,
procainamide, ranitidine, or temazepam. In contrast to these 26
medications, 21 drugs caused the reading of the LSD Emit to exceed the
threshold value (Table 2
). These interfering drugs were diluted to determine the minimal
interfering concentration. During the serial dilution the interfering
drugs showed a semilogarithmic dose–signal relation. Some of the
implicated prescription medications may not be detected in urine at the
concentrations that have been found to interfere with the assay in
vitro (Table 2
). These medications cannot be excluded as a cause of
interference because metabolites rather than parent drugs may have
interfered with the assay. To evaluate the specificity of the
interference, we tested most of these supplemented urine samples for
other DOA by Emit. All of the treated urine samples tested negative for
amphetamines, barbiturates, benzodiazepines, cannabinoids, cocaine
metabolites, methadone, opiates, phencyclidine, or propoxyphene (Table 2
). These results demonstrate that the interfering drugs are highly
specific for the Emit assay that detects LSD.
We estimated the number of interfering therapeutic medications that
were prescribed for patients with positive LSD results by Emit. The 79
urine samples that tested positive for LSD by Emit had been submitted
by 48 patients. Two of the 48 patients (4%) were not taking any known
prescription drugs at the time of testing. Three of the 48 patients
(6%) were prescribed only medications that did not interfere with the
LSD Emit assay in vitro at the tested concentrations. The 43 remaining
patients (90%) were taking one or several of the therapeutic drugs
that were identified to interfere with the Emit LSD assay in vitro
(Table 3
). These results suggest that the patients' prescribed
medications or their metabolites may have been responsible for most of
the false-positive LSD assay results by Emit. Other medications may
also have interfered with the LSD Emit assay, since drug metabolites
were not tested.
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Table 3. Interfering medications that were prescribed for 43
patients who tested positive with the LSD Emit
assay.
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Our study demonstrates that the new LSD Emit assay was
associated with a high rate of false-positive results (3.7%) overall,
with a false-positive rate that exceeded 10% in patients from
psychiatric clinics or medical clinics. On the other hand, a lower
false-positive rate of 0.5% was found in subjects from the veterans
shelter, and the false-positive rate of the assay could be acceptable
in a more general population. Although it is stated in the LSD Emit
package insert that samples from patients taking chlorpromazine may
produce positive results with the assay, we found that many different
medications resulted in false-positive assay results in vitro,
including antipsychotic drugs, antidepressants, anxiolytic drugs,
antiemetic medication, analgesics, cardiovascular medications, and
antibiotics. Our results suggest that therapeutic drugs or their
metabolites may be considered a cause for the observed interference
with the LSD assay. In conclusion, positive LSD test results by Emit
should be confirmed by an alternative method similar to the two-step
protocols used in employment-based DOA testing
(6)(7). In clinical situations where
confirmatory tests are not readily available, such as in the ER, the
current version of the LSD Emit assay cannot be recommended as a
screening assay for intoxicated patients because positive results seem
to be inconclusive.
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Acknowledgments
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We thank Joseph Hoffmann for his critical review of the
manuscript. We also are indebted to Behring Diagnostics and Roche
Diagnostic Systems for providing us with a free test kit for LSD.
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Footnotes
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1 Nonstandard abbreviations: DOA, drugs of abuse; GC-MS,
gas chromatography–mass spectrometry; LSD, lysergic acid diethylamide;
and ER, emergency room. 
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Abstract
Introduction
