Background

Certain autoimmune bullous dermatoses are mediated by autoantibodies of the IgG4 subclass. We determined the diagnostic impact of adding IgG4 to our conventional direct immunofluorescence (DIF) panel.

Methods

For all cases submitted to our referral laboratory for DIF over one month (n=630), we performed IgG4 DIF and collected consecutive biopsy specimens showing definite or indeterminate linear or cell-surface deposition of IgG, IgG4, and/or C3. On retrospective blinded review, we classified the pattern and whether findings were definite, indeterminate or negative. Background staining was reported if substantial.

Results

Seventy DIF specimens met inclusion criteria. Of 22 (31.4%) specimens equivocal for linear or cell-surface deposition, 9 (40.9%) had definitive IgG4 findings, either linear (3 of 14 equivocal linear cases; 21.4%) or cell-surface (6 of 8 equivocal cell-surface cases; 75.0%). 40.9%). Background deposition was substantial in 14 cases (20.0%) for IgG but in none for C3 and IgG4.

Conclusion

IgG4 allowed for classification of over 40% of cases that were otherwise equivocal by IgG and C3. IgG4 staining showed lower levels of non-specific background staining than did IgG or C3. IgG4 appears to contribute the most value in cases with cell-surface deposition and with equivocal linear IgG deposition and negative C3 results.

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