Abstract
False positive urine drug screening (UDS) results can cause significant confusion for clinicians. Case reports have noted that people on quetiapine can exhibit false positive test results for tricyclic antidepressants (TCA) in assays of their urine or serum [1–3]. This report describes three patients who had false positive UDS results for TCA who were on quetiapine along with other different psychotropic medications.
A 26 year old male with a diagnosis of treatment resistant paranoid schizophrenia and history of cannabis abuse, was on risperidone consta 62.5 mg intramuscularly every fortnight and quetiapine 600 mg oral per day. On a random UDS assay he showed positive for TCA on two occasions. He was not prescribed any TCA and he denies their use.
A 43 year old woman with a diagnosis of paranoid schizophrenia and history of cannabis abuse was on flupenthixol decanoate 70 mg Intra-muscularly every fortnight, quetiapine 800 mg oral daily and diazepam on tapering dosage (ranges from 5 to 15 mg per day). On two occasions her UDS showed positive for TCA in addition to benzodiazepine and cannabis. She was never prescribed TCA by the treating team and denied any use of the same.
A 35 year old woman diagnosed with rapid cycling bipolar affective disorder, intellectual disability, epilepsy and hypothyroidism with no history of illicit substance abuse, was on sodium valproate 2000 mg oral per day, quetiapine 400 mg oral per day and thyroxine 150 μg oral daily. On two separate occasions during her inpatient stay, her random UDS revealed positive for TCA. Akin to the previous cases she was also never prescribed any TCA and the only medications at the time of UDS tests were those mentioned.
In this series although there were other medications, the common medication in all three cases was quetiapine. To my knowledge there have been no reports of false positive UDS for TCA in patients taking flupenthixol decanoate, risperidone or sodium valproate. There is evidence of false positive UDS for TCA in patients on quetiapine [1,2].
The qualitative drug screen test done in our laboratory is TOX/See™. It is a lateral flow chromatographic immunoassay based on the principle of competitive binding.
Quetiapine has a structural similarity to tricyclic antidepressants. One in vitro study [4] reported that quetiapine can cross-react with quantitative and qualitative plasma TCA immunoassays in a concentration-dependent fashion. Therapeutic use, or overdose may result in a false positive plasma TCA immunoassay results [4] and other side effects [5]. Other drugs known to cause false positive TCA results are carbamazepine, diphenhydramine, cyclobenzaprine, cyproheptadine and thioridazine [3,4]. Cerullo et al. [2] recommended the use of these false positive findings as a useful qualitative test for quetiapine adherence that may avoid expensive serum level tests.
This report cautions us as to the potential for false positive tests in UDS and not to be guided by this finding alone.