False-positive intercellular cement substance antibodies due to group A/B red cell antibodies: frequency and approach.

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.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').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).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.