The congo red stain revisited.

The Congo red stain has undergone several modifications since it was first used by Bennhold in 1922 in order to increase the specificity for staining amyloid. Most of the laboratories in the United States use the method of Puchtler which uses alkaline Congo red solution. Some of the variables associated with the procedure were investigated by us. Our results showed the following: (1) amyloid showed green birefringence at all levels between 4 to 12 mu thick sections with better visualization of small deposits with increased thickness. Best results were obtained with 8 mu thick sections; (2) omission of the pretreatment with alkaline alcoholic solution of sodium chloride (NaCl) did not affect the sensitivity of the method; (3) the use of polar mounting media had no effect on amyloid and collagen birefringence; (4) 50 percent saturation of the Congo red staining solution with NaCl caused strong staining of collagen, elastic fibers and eosinophilic granules. In addition, collagen showed green birefringence and dichroism and its differentiation from amyloid became difficult; and (5) using the staining solution fully saturated with NaCl, no positive staining was seen with tissues other than amyloid. Collagen and elastic fibers showed red fluorescence which was of less intensity than amyloid. It is our conclusion that the method of Puchtler for detecting amyloid gives better results if the staining solution is fully saturated with NaCl. The pretreatment step may be deleted without compromising the quality of staining. Improved staining of amyloid enhances the specificity of green birefringence, dichroism, and red fluorescence.