Background Pemphigus is frequently diagnosed via indirect immunofluorescence (IIF) or skin biopsy with direct immunofluorescence staining. However, the accuracy of IIF to detect anti-intercellular substance autoantibodies (anti-ICS Ab) has not been evaluated in Asian populations. Method IIF was performed on 177 patient samples. Demographic data and underlying diseases were analyzed. Histopathology and direct immunofluorescence results were also reviewed. The false-positive group included 45 patients without pemphigus but with positive ICS Ab results. The other groups included true-negative (116 patients without pemphigus and negative ICS Ab results), true-positive (14 pemphigus patients with positive ICS Ab results), and false-negative (two pemphigus patients with negative ICS Ab results) results. Univariate and multivariate analysis were performed to exam the factors associated with false positivity. Results Anti-ICS Ab detected using IIF has been shown in 87.5% of patients with confirmed pemphigus. The specificity of IIF on pemphigus was 72.1%. The positive and negative predictive rates were 23.7% and 98.3%, respectively. The false positive rate was 25.4% (n = 45/177). The titers of the false positive results were 1:20, 1:40, and 1:80, in 28, 10, and 7 patients, respectively. The false-positive results also diminished during sequential follow-ups in some patients after treatments. There was higher rates of patients with blood type O (66.7% vs. 36.3%; p = 0.02) and bullous pemphigoid (31% vs. 15%; p = 0.05) in the false-positive group, compared with the rest of the population. In the adjusted analysis, blood type O was found to be significantly associated with a false-positive IIF results when compared with blood types A and B. Conclusion This study provided practical statistics for the accuracy of anti-ICS IIF tests. Due to the high false-positive rate and low positive-predictive rate, the results of anti-ICS IIF for pemphigus diagnosis should be carefully interpreted.
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