Abdominal fat pad aspiration biopsy for tissue confirmation of systemic amyloidosis: specificity, positive predictive value, and diagnostic pitfalls.

Abdominal fat pad fine-needle aspiration biopsy (FNAB) is considered the method of choice for confirmation of systemic amyloidosis. Due to our impression that positive results are rare in our FNA service, we retrospectively analyzed our results. Forty-five samples collected from 45 patients over 3 yr were reviewed. Of the 7 patients with positive Congo red-stained FNAB specimens, all 7 (100%) had documented amyloidosis. Of the 33 patients with negative Congo red fat samples, 28 (85%) were disease-free. Of the 5 patients with inadequate samples, 2 (40%) were later diagnosed with disease. We demonstrate excellent specificity (100%). The positive predictive value, documented in only a few previous studies, is likewise excellent (100%). Sensitivity is low (58%) and inadequacy is high (11%). Improvement in technique, such as concurrent cell block preparation, may help avoid inadequate specimens. Difficulties in confirming amyloidosis also include interpretation of the Congo red stain (pale-stained amyloid fibrils and collagen birefringence).