Characterization of Amyloidosis in the Gastrointestinal Tract With an Emphasis on Histologically Distinct...

Amyloidosis can involve the gastrointestinal (GI) tract, and deposition can present with varied histologic patterns that make recognition challenging. This retrospective observational study aimed to characterize the deposition patterns in the GI tract and evaluate key quality metrics, including discrepant cases, to improve recognition and provide insight for accurate diagnosis. Sixty-two patients (195 biopsies) with amyloid involvement of the luminal tract were reviewed. Amyloid subtyping by mass spectrophotometry was available for 59 patients. Immunoglobulin light chain (AL) was the most commonly identified subtype (60%), followed by serum amyloid A (AA; 19%) and transthyretin (ATTR; 16%). 150/195 biopsies (77%) were positive for amyloid deposition, with an average of 2.4 positive biopsies per every 3.1 taken per patient. The sites with the highest yield were duodenum (37/37, 100%) and colon (63/74, 85%). Gastric biopsies were most likely to involve the lamina propria (41/45, 91%, P < 0.001), with the background mucosa showing reactive epithelial changes in almost half of the biopsies (20/45, 44%). Several distinct histologic patterns of interstitial deposition were identified, including muscularis mucosae deposition (n = 40, 27% of positive biopsies), peri-Brunner gland (n = 6, 17% of duodenal biopsies), mass-forming (n = 4, 2.7% of positive biopsies, including 3 suspected cases with localized involvement), collagenous colitis-like (n = 3, 4.8% of positive colonic biopsies), and globular (n = 19, 12.7% of positive biopsies). Congo Red was ordered in 81% of cases in which it was requested clinically, with a positivity rate of 30%. Of the 34 cases in which an amyloid workup was requested (but Congo Red was not performed), 14 were positive on reevaluation. Several missed cases had deposition in multiple biopsies, and almost half were missed by subspecialist GI pathologists. Nine misinterpretations were from the stomach, with seven initially diagnosed as chemical or reactive gastropathy. Additional discrepant cases were identified from the duodenum (n = 2) and colon (n = 3), with the vascular-only deposition pattern (n = 3), muscularis mucosae-only deposition (n = 3), and globular pattern (n = 1) identified. Given the challenges of identifying amyloid on hematoxylin and eosin staining, Congo Red ordering percentage should be 100% in clinically suspicious cases unless deposition is definitively seen on hematoxylin and eosin staining.