Diagnosis of Amyloid Using Congo Red
Congo red is the accepted standard for detection of amyloid. Detection is easier on sections thicker than usual. A simple and convenient method is that described by Stokes. Between crossed polariser and analyser, the red colour of amyloid stained by Congo red is changed to a variety of other colours, called anomalous colours. These can be explained by the principles of optical physics. Only green is usually reported, because this colour is widely but not entirely correctly assumed to be essential for the diagnosis, but this idea is based on a misunderstanding of the physics. Combinations such as green and yellow or even blue and yellow are more commonly seen than green on its own. Other combinations of anomalous colours appear when the polariser and analyser are progressively uncrossed. All these colours are genuine, respectable and helpful findings, not artefacts or technical errors to be ignored. The changes in colour are a useful way to confirm amyloid.
KeywordsAmyloid Anomalous colours Congo red Polarisation microscopy
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