Utility of abdominal skin plus subcutaneous fat and rectal mucosal biopsy in the diagnosis of AL amyloidosis with renal involvement

Skin fat biopsy of the abdominal wall is a simple and safe method for detecting amyloidosis, and rectal mucosal biopsy is also frequently used for screening for the disease; however, the sensitivity of these approaches has not been fully studied. The aim of this study was to evaluate the efficacy of skin fat biopsy combined with rectal mucosal biopsy as a screening procedure for the diagnosis of systemic immunoglobulin light-chain (AL) amyloidosis. We retrospectively analyzed 224 AL amyloidosis patients confirmed by renal biopsy, including a test group of 165 patients and validation group of 59 patients. Surgical skin fat biopsy from the abdominal wall and rectal mucosal biopsy under endoscopy was performed to obtain specimens. Congo red staining and immunofluorescence staining with antibodies against light chains were performed to type the disease. Pathology reports were reviewed to assess the diagnostic sensitivity of skin fat biopsy and rectal mucosal biopsy. Diagnostic specificity was not examined in the present study, because no healthy volunteers and only few patients with other diseases had performed immunofluorescence staining on skin fat and rectal specimens. Of the 165 patients in the test group, Congo red staining of skin fat and rectal mucosal specimens was associated with a sensitivity of 89.3% and 94.8%, respectively. The sensitivity increased to 98.9% by combining both biopsy methods. Immunofluorescence stains were positive in 81.1% of patients undergoing skin fat biopsy and 84.7% of patients undergoing rectal mucosal biopsy. Immunofluorescence stains yielded positive results in 86.7% of cases combining skin fat biopsy with rectal mucosal biopsy. The diagnostic results also performed well in the validation group. Surgical skin biopsy including the subcutaneous fat pad can be performed safely at the bedside and is useful for diagnosing AL amyloidosis. Combining skin fat biopsy with rectal mucosal biopsy may identify amyloid deposits in almost all patients, and a negative result of both biopsies makes the diagnosis very unlikely.