title: Using molecular similarity to highlight the challenges of routine immunoassay-based drug of abuse/toxicology screening in emergency medicine.
abstract: BACKGROUND
  Laboratory tests for routine drug of abuse and toxicology (DOA/Tox) screening, often used in emergency medicine, generally utilize antibody-based tests (immunoassays) to detect classes of drugs such as amphetamines, barbiturates, benzodiazepines, opiates, and tricyclic antidepressants, or individual drugs such as cocaine, methadone, and phencyclidine. A key factor in assay sensitivity and specificity is the drugs or drug metabolites that were used as antigenic targets to generate the assay antibodies. All DOA/Tox screening immunoassays can be limited by false positives caused by cross-reactivity from structurally related compounds. For immunoassays targeted at a particular class of drugs, there can also be false negatives if there is failure to detect some drugs or their metabolites within that class.
  
  
  METHODS
  Molecular similarity analysis, a computational method commonly used in drug discovery, was used to calculate structural similarity of a wide range of clinically relevant compounds (prescription and over-the-counter medications, illicit drugs, and clinically significant metabolites) to the target ('antigenic') molecules of DOA/Tox screening tests. These results were compared with cross-reactivity data in the package inserts of immunoassays marketed for clinical testing. The causes for false positives for phencyclidine and tricyclic antidepressant screening immunoassays were investigated at the authors' medical center using gas chromatography/mass spectrometry as a confirmatory method.
  
  
  RESULTS
  The results illustrate three major challenges for routine DOA/Tox screening immunoassays used in emergency medicine. First, for some classes of drugs, the structural diversity of common drugs within each class has been increasing, thereby making it difficult for a single assay to detect all compounds without compromising specificity. Second, for some screening assays, common 'out-of-class' drugs may be structurally similar to the target compound so that they account for a high frequency of false positives. Illustrating this point, at the authors' medical center, the majority of positive screening results for phencyclidine and tricyclic antidepressants assays were explained by out-of-class drugs. Third, different manufacturers have adopted varying approaches to marketed immunoassays, leading to substantial inter-assay variability.
  
  
  CONCLUSIONS
  The expanding structural diversity of drugs presents a difficult challenge for routine DOA/Tox screening that limit the clinical utility of these tests in the emergency medicine setting.
authors.affiliation: Department of Pathology, University of Pittsburgh, Pittsburgh, PA, USA. mdk24@pitt.edu
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Krasowski
authors.forename: Anthony F
authors.initials: AF
authors.lastname: Pizon
authors.forename: Mohamed G
authors.initials: MG
authors.lastname: Siam
authors.forename: Spiros
authors.initials: S
authors.lastname: Giannoutsos
authors.forename: Manisha
authors.initials: M
authors.lastname: Iyer
authors.forename: Sean
authors.initials: S
authors.lastname: Ekins
mesh_terms: D004632:Emergency Medical Services; D005189:False Positive Reactions; D006801:Humans; D013287:Illicit Drugs / Q000302:isolation & purification*; D007118:Immunoassay; D008968:Molecular Conformation; D012680:Sensitivity and Specificity; D015813:Substance Abuse Detection / Q000379:methods*; D019966:Substance-Related Disorders / Q000175:diagnosis*
chemical_list: D013287:Illicit Drugs