For vasculitis, biopsies for DIF should be taken from lesions that are less than 24 hours old for highest yield, as the level of tissue immunoreactants tends to decline over time.28 This guideline does differ from hematoxylin and eosin specimens sent for evaluation of vasculitis, which ideally should be lesional tissue over 72 hours old. When evaluating for lupus (including subacute cutaneous lupus, discoid lupus, and systemic lupus), DIF is more likely to be positive in well-established, active lesions.
Which Test Should I Order?
The answer to this question depends, but the use of all 3 tests has a specificity close to 100% when evaluating for autoantibody-associated diseases.23 For autoimmune blistering disease, DIF is considered the diagnostic standard. The sensitivity of DIF for diagnosing BP is in the range of 82% to 90.5%, while specificity is 98%.29-31 Other autoimmune blistering diseases, such as pemphigus or dermatitis herpetiformis, have even higher sensitivities and specificities. Direct immunofluorescence often is used as a screening test, but false negatives do occur.32,33 Although rare, false positives also can occur, especially in cases of infection, and should be suspected when there is a lack of clinicopathologic correlation.34 If DIF is negative but clinical suspicion remains high, IIF should be ordered to directly evaluate a patient’s serum for autoantibodies.
In acute cutaneous lupus, subacute cutaneous lupus, and discoid lupus, DIF of active lesions may be helpful if histopathologic examination of a cutaneous lupus erythematosus lesion is nondiagnostic. However, histopathologic examination of formalin-fixed tissue remains the standard for these diagnoses. In vasculitis, while DIF is not used for diagnosis, it is useful to evaluate for IgA deposition. This is important in adults, as IgA deposition has been associated with a greater risk for developing end-stage renal disease.35
Final Thoughts
This is an overview of the tests available for diagnosing autoimmune blistering diseases. Residents should keep in mind that these tests are just one part of the puzzle when it comes to diagnosing these diseases. Results of DIF, IIF, and ELISA testing should be considered in conjunction with patient history and physical examination as well as histopathologic examination of lesional tissue when evaluating for dermatologic diseases with autoantibodies.