| Summary: | 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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