High-sensitivity diagnosis of AA amyloidosis using Congo red and immunohistochemistry detects missed amyloid deposits

Biopsy diagnosis of early amyloid-A (AA) amyloidosis has often been difficult. Examination of 57 consecutive biopsy specimens from 42 patients with inflammatory pediatric diseases permitted comparison of the precision of biopsy amyloid diagnosis in six different laboratories (labs), which applied the following methods: Congo red alone (four unspecialized labs combined as Lab 1), Congo red and electron microscopy (Lab 2), or Congo red and immunohistochemistry using monoclonal antibodies (Lab 3). Lab 3 reexamined the diagnoses made by Lab 1 and Lab 2. Of the 42 patients, 17 patients with 32 biopsies were selected for this study based on the presence of amyloid in at least one biopsy. Whereas massive or no amyloid was concordantly recognized by all labs in 18 biopsies from nine patients, discordance was demonstrated in 14 biopsies from eight patients. Comparison of Labs 1-3 revealed amyloid in 12 rectal and 18 renal biopsies evaluated by Lab 3, whereas Lab 2 missed amyloid in two of 18 renal biopsies and Lab 1 missed amyloid in 11 of 12 rectal biopsies. Most amyloid was missed when only minute amounts of amyloid were present. Had our technique (Lab 3) been available at the time of biopsy, amyloid could have been diagnosed years earlier, thereby sparing the patient further biopsies and allowing initiation of earlier treatment before organ damage could occur.