title: Autoantibody detection with indirect immunofluorescence on HEp-2 cells: starting serum dilutions for systemic rheumatic diseases.
abstract: Antinuclear antibodies (ANA) are determined, among other reasons, to identify samples which need a second test to detect the associated specificities. Our aim was to evaluate the clinical and economic impact generated by using an initial dilution for ANA of 1:160. We analyzed all samples for which ANA, anti-ENA and anti-dsDNA were requested over a 1-year period. ANA were detected by indirect immunofluorescence. Anti-ENA were analyzed with a combination of techniques. Anti-dsDNA were detected by radioimmunoassay. Cost analysis was performed by calculating the difference between two cut-offs (ANA 1:40 and 1:160). A total of 13,233 samples were processed for ANA, of which 59.9% were positive with the 1:40 cut-off and 39.2% with the 1:160 cut-off. At ANA titer 1:40, 0.2% of the samples were anti-ENA-positive and 2.2% were anti-dsDNA positive. Only ANA dilutions of 1:160 and higher showed significantly increased positive predictive value for anti-ENA (1.5 versus 0.2, p=0.029) and anti-dsDNA (8.3 versus 2.2, p<0.001) compared to the 1:40 titer. With the 1:160 cut-off, 16.6% fewer ANA tests, 41.8% fewer anti-ENA determinations and 36.4% fewer anti-dsDNA tests would have been needed. The average saving was 0.87 cost-units per sample (1 unit=2.06euro). We conclude that setting the starting dilution for ANA at 1:160 avoids unnecessary studies, increases the positive predictive values of ANA for anti-ENA and anti-dsDNA, and generates clinical and economic benefits.
authors.affiliation: Immunology Unit, Nuestra Señora de la Candelaria University Hospital, Santa Cruz de Tenerife, Spain.
authors.forename: Delia Almeida
authors.initials: DA
authors.lastname: González
authors.forename: Antonio Cabrera de
authors.initials: AC
authors.lastname: León
authors.forename: Alfredo Roces
authors.initials: AR
authors.lastname: Varela
authors.forename: Miriam García
authors.initials: MG
authors.lastname: García
authors.forename: Marta de Sequera
authors.initials: Mde S
authors.lastname: Rahola
authors.forename: María Del Cristo Rodríguez
authors.initials: Mdel C
authors.lastname: Pérez
authors.forename: Ana González
authors.initials: AG
authors.lastname: Hernández
authors.forename: María José Falcón
authors.initials: MJ
authors.lastname: Falcón
authors.forename: Buenaventura Brito
authors.initials: BB
authors.lastname: Díaz
mesh_terms: D000974:Antibodies, Antinuclear / Q000097:blood*; D045744:Cell Line, Tumor; D003365:Costs and Cost Analysis; D019084:Fluorescent Antibody Technique, Indirect; D006801:Humans; D011237:Predictive Value of Tests; D011863:Radioimmunoassay; D012016:Reference Values; D012216:Rheumatic Diseases / Q000097:blood / Q000191:economics / Q000276:immunology*; D012698:Serologic Tests / Q000191:economics / Q000379:methods; D017186:Titrimetry
chemical_list: D000974:Antibodies, Antinuclear