Detection of amyloid in abdominal fat pad aspirates in early amyloidosis: Role of electron microscopy and Congo red stained cell block sections

Background: Fine-needle aspiration biopsy (FNA) of the abdominal fat pad is a minimally invasive procedure to demonstrate tissue deposits of amyloid. However, protocols to evaluate amyloid in fat pad aspirates are not standardized, especially for detecting scant amyloid in early disease. Materials and Methods: We studied abdominal fat pad aspirates from 33 randomly selected patients in whom subsequent tissue biopsy, autopsy, and/or medical history for confirmation of amyloidosis (AL) were also available. All these cases were suspected to have early AL, but had negative results on abdominal fat pad aspirates evaluated by polarizing microscopy of Congo Red stained sections (CRPM). The results with CRPM between four reviewers were compared in 12 cases for studying inter observer reproducibility. 24 cases were also evaluated by ultrastructural study with electron microscopy (EM). Results: Nine of thirty-three (27%) cases reported negative by polarizing microscopy had amyloidosis. Reanalysis of 12 mixed positive-negative cases, showed considerable inter-observer variability with frequent lack of agreement between four observers by CRPM alone (Cohen′s Kappa index of 0.1, 95% CI -0.1 to 0.36). EM showed amyloid in the walls of small blood vessels in fibroadipose tissue in four out of nine cases (44%) with amyloidosis. Conclusion: In addition to poor inter-observer reproducibility, CRPM alone in cases with scant amyloid led to frequent false negative results (9 out of 9, 100%). For improved detection of AL, routine ultrastructural evaluation with EM of fat pad aspirates by evaluating at least 15 small blood vessels in the aspirated fibroadipose tissue is recommended. Given the high false negative rate for CRPM alone in early disease, routine reflex evaluation with EM is highly recommended to avert the invasive option of biopsying various organs in cases with high clinical suspicion for AL.