Standardizing initial dilution titers of antinuclear antibodies for the screening of systemic lupus erythematosus

Background: Systemic Lupus Erythematosus (SLE), a systemic autoimmune disease, is diagnosed by correlating clinical features with a positive Antinuclear Antibody (ANA) test. Detection of ANA by Indirect Immunofluorescence (IIF) is used for screening SLE and is dependent on initial dilution titers which require population-specific standardization. With lack of exclusive commercial kits for the South Indian population, it is necessary to standardize initial screening dilution titers for ANA to distinguish SLE and other rheumatic diseases from the healthy. Methods: Newly diagnosed SLE patients between 18 and 60 years along with healthy controls from South India over 8 months (September 2015-April 2016) were selected for this prospective study. Serum samples were subjected to ANA-IIF in dilution titers of 1:40, 1:80, and 1:100 as per kit recommendations. IIF intensity and staining patterns were correlated clinically and statistically analyzed. Results: ANA positivity in dilutions of 1:40 and 1:80 was seen in 2.1% of healthy controls and negative at 1:100. About 97.9% of SLE patients were positive at 1:100 dilution; speckled pattern being the most common (52.1%), followed by homogeneous (37.4%). The patterns were best appreciated in 1:100 dilution with high significant measure of agreement of kappa between the two pathologists for both patterns and intensity at all three dilutions. Conclusion: 1:100 is the best screening dilution to distinguish SLE patterns from normal healthy individuals, and its main advantage is the delineation of various ANA patterns when positive, especially when mixed at this lower dilution.