Investigations
1st investigations to order
direct immunofluorescence microscopy
Test
Perform a skin biopsy of perilesional skin for direct immunofluorescence microscopy in all patients with suspected dermatitis herpetiformis.[2][20][21] This is the definitive investigation for diagnosing the condition.[2][11][16][20]
The optimal biopsy site is perilesional, uninvolved skin because direct immunofluorescence microscopy performed on lesional biopsies rather than perilesional skin often leads to false-negative results.[2][22] If a patient with high suspicion of dermatitis herpetiformis tests negative at direct immunofluorescence microscopy, request a repeat skin biopsy; false-negative results after repeated biopsies are very rare.[23]
Result
granular IgA deposits at the dermal papillae or along the dermal-epidermal junction
histopathological examination
Test
Perform a skin biopsy of lesional skin for histopathological examination in all patients with suspected dermatitis herpetiformis.[2][21] Take a 4-5 mm lesional punch biopsy.[2] This test is not specific, but can help to identify differential diagnoses.
Result
accumulation of neutrophils in the dermal papillae and subepidermal blister
immunoglobulin A-tissue transglutaminase (IgA-tTG)
Test
Order in all patients with suspected dermatitis herpetiformis.[2]
IgA-tTG can give information on the presence of bowel damage (a normal result does not exclude dermatitis herpetiformis). IgA-tTG detected by enzyme-linked immunosorbent assay (ELISA) is present in up to 95% of patients.[25]
Result
titre above normal range for laboratory
Investigations to consider
human leukocyte antigen (HLA) typing
Test
Order in patients with suspected dermatitis herpetiformis in case of diagnostic uncertainty in order to exclude the disease. The test has a high negative predictive value.[2] Virtually all patients with dermatitis herpetiformis have a positive HLA-DQ2 (around 95%) or HLA-DQ8 (around 5%).[10]
Result
positive HLA-DQ2 or HLA-DQ8
FBC
Test
Perform a CBC with differential, with or without ferritin levels, to assess for anaemia in patients with confirmed dermatitis herpetiformis and to obtain baseline levels prior to commencement of treatment.[2]
Result
variable - low Hb and microcytic hypochromic red cells may be present in patients with bowel involvement
gastroscopy with duodenal biopsy
Test
Consider referral of patients with confirmed dermatitis herpetiformis to a gastroenterologist, who may order gastroscopy with duodenal biopsy to assess bowel involvement.[2]
Result
villous atrophy in the duodenal and/or jejunal mucosa
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