What laboratory tests are used to diagnose bullous pemphigoid?

Laboratory Tests for Diagnosing Bullous Pemphigoid

The diagnosis of bullous pemphigoid requires a combination of clinical features, histopathology, and immunopathological tests, with direct immunofluorescence microscopy being the most critical test for definitive diagnosis. 1

Essential Diagnostic Tests

Direct Immunofluorescence Microscopy (DIF)

• DIF is the most critical and essential test for diagnosing bullous pemphigoid, with a sensitivity of 90.5% 2
• Biopsy specimen should be obtained from perilesional skin (not from the blister itself) 1
• Positive finding: Linear (n-serrated) deposits of IgG and/or C3 along the dermoepidermal junction 1
• Sometimes IgA and IgE may show similar pattern of deposition 1
• C3 is almost invariably deposited at the BMZ of DIF-positive patients and typically shows greater fluorescence intensity than immunoglobulins 2

Histopathology

• Specimens should be taken from early bullae arising on erythematous skin and placed in formalin solution 1
• Typical findings include subepidermal bullae containing eosinophils and/or neutrophils 1
• Associated dermal infiltrate of eosinophils and/or neutrophils or marginalization of eosinophils along the dermoepidermal junction 1
• In nonbullous forms, findings may be nonspecific, such as eosinophilic spongiosis 1
• Histology alone has high specificity (99%) but lower sensitivity than DIF 2

Serological Tests

Indirect Immunofluorescence (IIF)

• Detection of circulating IgG antibasement membrane autoantibodies using NaCl-separated normal human skin (salt-split skin technique) 1
• Positive finding: IgG antibodies binding to the epidermal side (sometimes both epidermal and dermal) of the split 1
• IIF has high specificity but lower sensitivity than DIF 2

Enzyme-Linked Immunosorbent Assay (ELISA)

• Detection of anti-BP180 (BPAG2/type XVII collagen) IgG autoantibodies 1
• If negative for BP180, test for anti-BP230 (BPAG1-e) IgG autoantibodies 1
• Multivariant ELISA systems have been developed as practical screening tools 3
• Anti-BP180 ELISA values >27 U/mL indicate increased risk of relapse 4

Additional Immunopathological Tests

• Immunoblotting: Search for reactivity with BP180 (BPAG2) and/or BP230 (BPAG1) 1
• Biochip: IIF with purified BP180 recombinant protein spotted on a slide 1
• Fluorescence overlay antigen mapping: Assessment of relative location of IgG deposits compared with other proteins within the cutaneous basement membrane zone 1
• Immunohistochemistry: Linear deposits of C3d and C4d along the basement membrane zone can be demonstrated using the same tissue sample obtained for light microscopy studies 1

Diagnostic Algorithm

1. Clinical assessment using validated clinical criteria for BP:

   • When three of four clinical characteristics are present (age >70 years, absence of atrophic scars, absence of mucosal involvement, absence of predominant bullous lesions on neck and head), diagnosis is highly likely in patients with positive DIF 1

2. First-line laboratory tests:

   • DIF of perilesional skin (most critical test) 1
   • Histopathology of early intact bulla 1

3. Second-line laboratory tests (for confirmation):

   • IIF on salt-split skin 1
   • ELISA for BP180 and BP230 antibodies 1

4. Additional tests when diagnosis remains uncertain:

   • Immunoblotting, biochip, or other specialized immunopathological tests 1

Common Pitfalls and Caveats

• Nonbullous forms of BP may present with nonspecific histopathological findings, making DIF crucial for diagnosis 1, 5
• Approximately 40% of BP cases present with atypical clinical features, which may lead to misdiagnosis if immunofluorescence tests are not performed 2
• The combination of clinical data plus one positive immunopathologic test provides the best combination of sensitivity and specificity (98%) 2
• The class of basement membrane zone immunoreactants may vary according to the site of biopsy 2
• Monitoring anti-BP180 IgG by ELISA at days 0,60, and 150 is recommended for disease activity assessment 4
• Complete blood count should be performed to check for peripheral eosinophilia, which is common in BP 6, 7

Laboratory Monitoring During Treatment

• Complete blood count, erythrocyte sedimentation rate, and C-reactive protein 1
• Creatinine, blood electrolytes, fasting glucose 1
• Liver function tests (transaminases, γ-glutamyltransferase, alkaline phosphatase, bilirubin) 1
• Serum albumin 1
• Regular monitoring of anti-BP180 IgG by ELISA can help assess disease activity and predict relapse 4