ELISA-4-amyloid: diagnostic accuracy of an ELISA panel for typing the four main types of systemic amyloidosis in subcutaneous abdominal fat tissue samples.

Background: Reliable typing of amyloid is essential. Amyloid extraction from tissue enables immunochemical typing of the precursor protein using an enzyme-linked immunosorbent assay (ELISA).

Objective: To assess the diagnostic accuracy of a panel of ELISAs for typing the four main types (AA, ATTR, AL-kappa and AL-lambda amyloid).

Methods: From 1996 to 2023 subcutaneous abdominal fat tissue aspirates were obtained from 1339 amyloidosis patients and 868 controls. Amyloid was visually graded 0-4+ in Congo red-stained smears. Amyloid extracted from tissue by Guanidine was typed using a panel comprising four ELISAs.

Results: All amyloid protein concentrations in extracts correlated with amyloid grade in smears. Typing sensitivity was low (23.3%) in samples with grade 1+/2+ amyloid. Overall typing sensitivity of the panel was 81.6% for all easily visible amyloid (grade 3+/4+): high for AA (98.8%) and ATTR (96.8%) and fair for AL-kappa (66.7%) and AL-lambda (75.9). Overall typing specificity was 98.0% and the overall positive predictive value was 98.0%.

Conclusions: We describe a highly specific ELISA panel for routine typing of the main amyloid types in fat tissue. Until more sensitive typing techniques will become generally available, typing easily visible amyloid in fat tissue using this ELISA panel is reliable, affordable and straightforward.