Amyloidosis is a rare disease that results from accumulation of inappropriately folded proteins. These misfolded proteins are called amyloids. When proteins that are normally soluble in water fold to become amyloids, they become insoluble and deposit in organs or tissues, disrupting normal function. The type of protein that is misfolded and the organ or tissue in which the misfolded proteins are deposited determines the clinical manifestations of amyloidosis.Diagnosis of amyloidosis requires tissue biopsy. Biopsy is assessed for evidence of characteristic amyloid deposits. The tissue is treated with various stains. The most useful stain in the diagnosis of amyloid is Congo red, which, combined with polarized light, makes the amyloid proteins appear apple-green on microscopy. Also, thioflavin T stain may be used.
Tissue can come from any involved organ. But in systemic disease first line site of biopsy is subcutaneous abdominal fat, known as a fat pad biopsy, because it's simple to acquire and less invasive than biopsy of the rectum, salivary gland or internal organs. An abdominal fat biopsy is not completely sensitive and so, sometimes, biopsy of an involved organ (such as the kidney) is required to achieve a diagnosis. For example, in AL amyloidosis only 85% of people will have a positive fatpad biospy using Congo red stain. By comparison, rectal biopsy has sensitivity of 74-94%.
Cited from: Wikipedia.