title: Anti-signal recognition particle autoantibody ELISA validation and clinical associations.
abstract: OBJECTIVE
  The aim of this study was to develop and validate a quantitative anti-signal recognition particle (SRP) autoantibody serum ELISA in patients with myositis and longitudinal association with myositis disease activity.
  
  
  METHODS
  We developed a serum ELISA using recombinant purified full-length human SRP coated on ELISA plates and a secondary antibody that bound human IgG to detect anti-SRP binding. Protein immunoprecipitation was used as the gold standard for the presence of anti-SRP. Serum samples from three groups were analysed: SRP(+) myositis subjects by immunoprecipitation, SRP(-) myositis subjects by immunoprecipitation and non-myositis controls. The ELISA's sensitivity, specificity, positive predictive value and negative predictive value were evaluated. Percentage agreement and test-retest reliability were assessed. Serial samples from seven SRP immunoprecipitation-positive subjects were also tested, along with serum muscle enzymes and manual muscle testing.
  
  
  RESULTS
  Using immunoprecipitation, we identified 26 SRP(+) myositis patients and 77 SRP(-) controls (including 38 patients with necrotizing myopathy). Non-myositis control patients included SLE (n = 4) and SSc (n = 7) patients. Anti-SRP positivity by ELISA showed strong agreement (97.1%) with immunoprecipitation (κ = 0.94). The sensitivity, specificity, positive predictive value, and negative predictive value of the anti-SRP ELISA were 88, 100, 100 and 96, respectively. The area under the curve was 0.94, and test-retest reliability was strong (r = 0.91, P < 0.001). Serial samples showed that anti-SRP levels paralleled changes in muscle enzymes and manual muscle testing.
  
  
  CONCLUSIONS
  We developed a quantitative ELISA for detecting serum anti-SRP autoantibodies and validated the assay in myositis. Longitudinal assessment of SRP levels by ELISA may be a useful biomarker for disease activity.
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA aggarwalr@upmc.edu.
authors.forename: Rohit
authors.initials: R
authors.lastname: Aggarwal
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Chester V
authors.initials: CV
authors.lastname: Oddis
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Danielle
authors.initials: D
authors.lastname: Goudeau
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Noreen
authors.initials: N
authors.lastname: Fertig
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Ilinca
authors.initials: I
authors.lastname: Metes
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Chad
authors.initials: C
authors.lastname: Stephens
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Zengbiao
authors.initials: Z
authors.lastname: Qi
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Diane
authors.initials: D
authors.lastname: Koontz
authors.affiliation: Department of Medicine, Division of Rheumatology and Clinical Immunology, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: Marc C
authors.initials: MC
authors.lastname: Levesque
mesh_terms: D000328:Adult; D001323:Autoantibodies / Q000097:blood*; D015415:Biomarkers / Q000097:blood; D016022:Case-Control Studies; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000201:enzymology; D009220:Myositis / Q000097:blood / Q000175:diagnosis* / Q000276:immunology; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D018271:Signal Recognition Particle / Q000276:immunology*; D013997:Time Factors
chemical_list: D001323:Autoantibodies; D015415:Biomarkers; D018271:Signal Recognition Particle
keywords: ELISA; anti-signal recognition particle (SRP) autoantibody; idiopathic inflammatory myopathy; immunoprecipitation; quantitative measure