In this study, we compared the value of pathological alpha-synuclein (αSyn) seed amplification assay (SAA) in gastric and duodenal biopsies with skin biopsies in Parkinson disease (PD) patients with different disease duration. The accuracy of αSyn SAA was 87.7% in skin, 67.4% in duodenum, and 80.0% in gastric biopsies, with significantly higher sensitivity in advanced PD (skin: 81.8%; gastric: 88.9%; duodenal 58.8%). Misfolded αSyn was detected with higher sensitivity in advanced PD across all matrices, likely reflecting the progression of αSyn pathology. The seeding activity was lower in the duodenal than in the gastric wall, indicating differences in αSyn burden.
