title: False-positive intercellular cement substance antibodies due to group A/B red cell antibodies: frequency and approach.
abstract: OBJECTIVE
  Antibodies to the intercellular cement substance of skin (ICSA) are characteristic of pemphigus vulgaris, and are commonly detected by indirect immunofluorescence (IIF). However, false-positive staining may arise when blood group antibodies, directed against A and B red cell antigens, bind to epitopes of similar distribution in the substrate. We sought to determine the frequency of such false-positive ICSA staining and to establish optimal conditions for its elimination.
  
  
  METHODS
  IIF was performed on 100 de-identified routine serum samples of known blood group (A, B or O), along with serum from four pemphigus patients. Blocking was performed on samples which displayed typical ICSA staining using either a mixture of soluble A and B antigens in solution ('soluble A/B antigens'), or red blood cells from a group AB donor ('AB-RBCs').
  
  
  RESULTS
  ICSA staining was detected in 12 of 100 (12%) routine samples at a titre of between 1:10 and 1:160, and was most frequent in (but not restricted to) samples of blood group O (10/54, 19%). Blocking with soluble A/B antigens eliminated staining in 10 of 12 (83%) samples, whilst blocking with AB-RBCs eliminated staining in 11 of 12 (92%); one sample failed to block using either method, suggesting true ICSA reactivity. Blocking had no effect on the positive pemphigus samples, the titres of which were significantly higher (1:160 to 1:640).
  
  
  CONCLUSIONS
  Staining of the intercellular cement substance arising from group A/B red cell antibodies is common, particularly but not exclusively in patients of blood group O, making blocking mandatory in the routine evaluation of samples with such staining, particularly in a hospital-based population. Blocking using either soluble A/B antigens or AB-RBCs appears to be of comparable efficacy, although the former method is more convenient for a diagnostic laboratory.
authors.affiliation: Immunology Unit, Institute of Clinical Pathology and Medical Research, Westmead Hospital, Westmead, Sydney, Australia.
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Lee
authors.forename: Roger
authors.initials: R
authors.lastname: Silvestrini
authors.forename: David A
authors.initials: DA
authors.lastname: Fulcher
mesh_terms: D000017:ABO Blood-Group System / Q000276:immunology*; D000918:Antibody Specificity; D001323:Autoantibodies / Q000276:immunology*; D001324:Autoantigens / Q000276:immunology; D015815:Cell Adhesion Molecules / Q000276:immunology*; D003429:Cross Reactions; D005189:False Positive Reactions; D019084:Fluorescent Antibody Technique, Indirect; D006801:Humans; D010392:Pemphigus / Q000175:diagnosis* / Q000276:immunology
chemical_list: D000017:ABO Blood-Group System; D001323:Autoantibodies; D001324:Autoantigens; D015815:Cell Adhesion Molecules