title: Evaluation of enzyme immunoassay and radioimmunoassay methods for the measurement of plasma oxytocin.
abstract: OBJECTIVE
  There is increased interest in measuring peripheral oxytocin levels to better understand the role of this peptide in mammalian behavior, physiology, and disease. The purpose of this study was to compare methods for plasma oxytocin measurement using a commercially available enzyme immunoassay (EIA) and radioimmunoassay (RIA), to evaluate the need for sample extraction, and to assess the immunospecificity of the assays.
  
  
  METHODS
  Oxytocin was measured in extracted and unextracted human plasma samples (n = 39). Oxytocin and its degradation products were separated by high-performance liquid chromatography and gel filtration chromatography and then assayed by EIA or RIA to identify oxytocin immunoreactive peaks.
  
  
  RESULTS
  Without extraction, plasma measured by EIA was more than 100-fold higher than in extracted plasma, and the correlation between oxytocin levels in extracted and unextracted plasma was minimal (Spearman ρ = -0.10, p = .54). Using the RIA, most samples (>90%) were below the level of detection with or without extraction. After chromatographic fractionation of sample extracts, multiple immunoreactive products were found to be present in addition to oxytocin, which casts doubts on the specificity of the assays.
  
  
  CONCLUSIONS
  Changes in oxytocin levels have been reported in social and behavioral challenge studies. This study indicates that sample extraction is necessary to obtain valid assay results. Changes in oxytocin degradation products are likely to contribute to the previously observed responses in circulating oxytocin levels to behavioral and social challenge. There is a critical need for valid and reliable methods to measure oxytocin in biologic samples.
authors.affiliation: Behavioral Medicine Research Center and Department of Psychology, University of Miami, Coral Gables, Florida, USA.
authors.forename: Angela
authors.initials: A
authors.lastname: Szeto
authors.forename: Philip M
authors.initials: PM
authors.lastname: McCabe
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Nation
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Tabak
authors.forename: Maria A
authors.initials: MA
authors.lastname: Rossetti
authors.forename: Michael E
authors.initials: ME
authors.lastname: McCullough
authors.forename: Neil
authors.initials: N
authors.lastname: Schneiderman
authors.forename: Armando J
authors.initials: AJ
authors.lastname: Mendez
mesh_terms: D000818:Animals; D003411:Arvicolinae / Q000097:blood; D002417:Cattle; D002853:Chromatography, Liquid / Q000379:methods; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques / Q000379:methods; D057230:Limit of Detection; D008251:Macaca / Q000097:blood; D008297:Male; D051379:Mice; D010121:Oxytocin / Q000097:blood* / Q000276:immunology; D011817:Rabbits; D011863:Radioimmunoassay / Q000379:methods; D051381:Rats; D011933:Reagent Kits, Diagnostic / Q000592:standards*; D012680:Sensitivity and Specificity
chemical_list: D011933:Reagent Kits, Diagnostic; D010121:Oxytocin